Temperature Gauge Fluctuates - Non Valk

[carolinarider09]

I have a 1996 Explorer 2DR Sport Utility with over 200,000 miles, original engine.

I posted the following on the "Explorer Talk" board but after two days and 25 views no comments.  So, I thought I would ask the question here. 

Had an issue a few years ago with the engine temperature never getting very high. Figured it was a thermostat failure but never worried about it (yes I know I should have).

I had the Explorer serviced sometime last year for a transmission leak and some brake issues and well, they did a lot of work including fixing the thermostat issue.

All things were normal unitl today when i was driving it into town and noticed the temperature gauge was reading sort of low (yes it was a cold day but I had driven the Explorer about 10 miles or so). I kept an eye on it and noted that it finally climbed to the "normal" center location on the gauge.

I kept a further eye on the gauge and it looked real low. So, I watched it really close and noticed it would drop down to just above the low spot and then, after a few seconds slowly begin to climb back to and then slightly above "normal" reading. After reaching that point it would fall back down to the low position.

See the pictures below.

I checked the fluid in the radiator at a stop after I was sure it was cool and it was just right at the top of the radiator opening. I checked the Coolant Reservoir and it was empty. I purchased some fluid while on the road and filled the Coolant Reservoir up to the "normal" level.

When I got home and let it cool again, I checked the radiator and the level was below the top opening. The Coolant Reservoir was about the same.

All this seems very, very strange. Any suggestions on what might be happening. One thing that could be happening is a bad sensor. Or it could be a bad thermostat

Pictures below are about 10 seconds apart. 

Any suggestions would be welcomed.

[Jersey mike]

On a cold start, open the radiator cap, start the truck, turn your heat to hot and put the fan on like medium. Let the engine run and get to normal. You might have a have an air pocket.

If you can have someone step on the gas pedal while you watch the coolant level in the radiator that would be good too. On acceleration the coolant level should drop some and when released should go back to normal level. If there’s  an air pocket it might “bubble” a little bit or “burp” some. It might take a few times since the coolant needs to circulate needs to after a few thermostat opening and closing.

Are you blowing heat?

How are the hoses? If they’re old you may have a hose collapsing.

[carolinarider09]

On a cold start, open the radiator cap, start the truck, turn your heat to hot and put the fan on like medium. Let the engine run and get to normal. You might have a have an air pocket.

Thanks I  will do that tomorrow.  

If you can have someone step on the gas pedal while you watch the coolant level in the radiator that would be good too. On acceleration the coolant level should drop some and when released should go back to normal level. If there’s  an air pocket it might “bubble” a little bit or “burp” some. It might take a few times since the coolant needs to circulate needs to after a few thermostat opening and closing.

Thanks I have not tried that I will when I do the above.

Are you blowing heat?  Yes

How are the hoses? If they’re old you may have a hose collapsing.

They seem to be ok but will check them again. I don't remember if they were replaced during that "big" maintenance activity. 

Thanks for the response. 

[f6john]

More than likely the sensor or some electrical malfunction in the system. But I’m not savvy on Ford stuff but I know on my old 95 Impala the oil pressure “gauge” is really only an idiot light made to look like a gauge. It doesn’t work right now either, 27 years and 215,000 miles.

[scooperhsd]

If you want - look for a "Scanguage II" on Amazon (I think that is where I got mine. It plugs into the OBDII port and provide info like a higher prced OBDII scanner - I use one in my 2000 VW New Beetle to keep a watch in the Water temperature (it only has an idiot light otherwise).

[Moonshot_1]

This is a terrible symptom. It is obviously internal Climate Change.

You need to install solar panels and a windmill on the roof of the vehicle.

[Jersey mike]

On a cold start, open the radiator cap, start the truck, turn your heat to hot and put the fan on like medium. Let the engine run and get to normal. You might have a have an air pocket.

Thanks I  will do that tomorrow.  

If you can have someone step on the gas pedal while you watch the coolant level in the radiator that would be good too. On acceleration the coolant level should drop some and when released should go back to normal level. If there’s  an air pocket it might “bubble” a little bit or “burp” some. It might take a few times since the coolant needs to circulate needs to after a few thermostat opening and closing.

Thanks I have not tried that I will when I do the above.

Are you blowing heat?  Yes

How are the hoses? If they’re old you may have a hose collapsing.

They seem to be ok but will check them again. I don't remember if they were replaced during that "big" maintenance activity. 

Thanks for the response. 

I re-read your original post. You said the reservoir was empty, how long ago was it full?

Keep an eye on the coolant level in that reservoir you could be burning off coolant out the tailpipe which would be the head gasket unless you have a coolant leak somewhere. Sometimes it takes awhile to see a drop.

Another thing is make sure the reservoir hose to the radiator isn’t blocked or pinched.

Then again it just may be the sensor or just another bad thermostat.

[carolinarider09]

More than likely the sensor or some electrical malfunction in the system. But I’m not savvy on Ford stuff but I know on my old 95 Impala the oil pressure “gauge” is really only an idiot light made to look like a gauge. It doesn’t work right now either, 27 years and 215,000 miles.

Regarding the oil pressure gauge found that out the hard way on the explorer.  About one mile from home, a year or so ago, oil pressure dropped to zero (pegged low on gauge).   I almost stopped the engine but there was not sound indicating loss of oil pressure so, I drove it home.  Thats when I found out the meter/gauge on the dash was fed by a "switch".

[carolinarider09]

This is a terrible symptom. It is obviously internal Climate Change.

You need to install solar panels and a windmill on the roof of the vehicle.

 

[carolinarider09]

I re-read your original post. You said the reservoir was empty, how long ago was it full?

No idea.   I don't check it regularly. 

Keep an eye on the coolant level in that reservoir you could be burning off coolant out the tailpipe which would be the head gasket unless you have a coolant leak somewhere. Sometimes it takes awhile to see a drop.

I have been keeping an eye on it. It did not change.  Today, I opened the radiator cap when it was warm and some fluid came out about a half pint of so.  Will check the resvoir tomorrow (since I drove it tonight) and see what has happened.

Another thing is make sure the reservoir hose to the radiator isn’t blocked or pinched.

If the resvoir level is down when I check it that should mean the hose is ok.  

Then again it just may be the sensor or just another bad thermostat.

Yes on both.   I will look again at the process for replacing the thermostat.  Last I looked it did require a few steps. 

Thanks for the feedback.

[cookiedough]

I doubt  your issues,  buy my kids old 2004 pontiac vibe radiator cap had a slow leak in it loosing coolant very slowly but was noticeable with liquid residue flowing down by the cap.  My guess another bad thermostat or some leak somewhere.

My sonata only 110K past few years ONLY when very cold out if I drive it say 6 miles and temp light goes up to where it should be which does take if VERY cold out about 5 miles to get to that NORMAL HOT mid-level needle on dash,  if I go to next town 6 miles away just as I slow down to say 30 mph from 60 mph the dash temp light does drop IN NEXT TOWN LOW speeds that 1 mile until I get to to the hwy. again doing 60 mph and the dash temp light goes back up to the mid-range normal again and stays there. 

I suspect the engine slowing down and just barely getting fully WARMED up after 5 miles slowing down in town 5 miles after starting is a loss in coolant temp is normal since 1 mile later doing hwy. speed the dash temp light goes back up to normal range in the middle.  The dash light goes down from 5 bars in middle to only 2 or 3 bars never down to 1 bar which is COLD start. 

[carolinarider09]

 

[Oss]

I remember my 57 olds had a green cold light and a red hot light
I went to college in Buffalo

The cold light almost never went off from November to March

not mine but it is a 57 Olds

[carolinarider09]

Today I went out and checked the radiator fluid level.  It was full. 

I checked the level in the "Coolant Reservoir", it was down a little. 

This implies that the radiator is sealed and does allow for coolant to be fed from the reservoir to the radiator as it cools down (as needed).

I then started the engine and let it get warm.  I felt the top hose on the radiator (the one coming from the  thermostat) it was firm and getting warm as the engine warmed up.   This implies flow exists through the hose. 

This, implies that the thermostat is bad and needs to be replaced.  Which is what I sort of thought but I have never ever seen one do that before.

[Jersey mike]

Today I went out and checked the radiator fluid level.  It was full. 

I checked the level in the "Coolant Reservoir", it was down a little. 

This implies that the radiator is sealed and does allow for coolant to be fed from the reservoir to the radiator as it cools down (as needed).

I then started the engine and let it get warm.  I felt the top hose on the radiator (the one coming from the  thermostat) it was firm and getting warm as the engine warmed up.   This implies flow exists through the hose. 

This, implies that the thermostat is bad and needs to be replaced.  Which is what I sort of thought but I have never ever seen one do that before.

Like most anything mechanical, location and access is key. Some thermostats are simple and straightforward, 2 bolts out, break the seal of the gasket and just pop out the thermostat. Reinstalling is pretty easy too. Scrape off any old adhesive and gasket residue, don’t get any in the coolant, put the thermostat in, a little adhesive on the gasket and reinstall the bolts…tight but not over tight.

Be sure the thermostat hose is in good shape. If it needs replacing, this is the time to do it.

[Robert]

After replacing the thermostat share the results. A 10 second drop in temp is very strange and before I would replace the thermostat I would invest in a temp gun. Which you may already have considering your interior work. You have not said that you notice any symptoms other than the initial low coolant which is not really unusual. I have known the senders and even the dash boards to go out so the temp gun would be a big help.

[carolinarider09]

Thanks Robert, I shall do that (share the results). 

[carolinarider09]

Update:   It was easier to replace the temperature sensor and I sort of figured it was OEM.   New sensor, let it idle in the carport, same issue. 

So, next stop, thermostat.   But that's a lot more work.  No like the old days when it was right there.  Pull to bolts and remove and re-insert. 

Oh, well.  We shall see.   

[Patrick]

Sounds like the sensor/switch or stat. How does the coolant look ? If it is good then it's probably a wobbly stat since you put in another switch.

[carolinarider09]

I agree.  What ticks me off is the thermostat is only about two years old.   And it is a bit of a pain to replace. 

We shall see what I can get done.  I have the parts. I have the tools.   Do I have the patience

[carolinarider09]

Just replaced the thermostat.  Old one looked just like the new one.  Same condition, nice and shiny and clean.

The replacement process was a real pain in the "arse".    Two of the screws holding the thermostat housing in place are very hard to get to.   

After replacement, started engine and let it warm up.  Gauge got to about 1/3 off full range and then sort of leveled out.  It did show a few "changes" as it idled and got warmer.   

Took it for a drive and same conditions occurred.  But the range of change was not as great as it was the last time I drove it with the old thermostat and sensor.   

So, that was not the issue. 

What I do believe is that, whatever the issue, it is not going to cause a catastrophic engine failure.  But I don't have any other ideas of what might be causing the issue.

There is, also, another sensor that appears to be a "switch" instead of a "gauge" output.  I have not checked to see if there is a "over temperature" light on the dash.  If there is that could be what this sensor is for.   This sensor is near the thermostat housing but I have not verified (as noted above) its purpose. 

[Robert]

I have seen many temp changes like you talk about with the loss of coolant in engines that have a head gasket failure.

To check start engine from cold with the radiator cap on. Let it run a few minutes and carefully take the cap off and see if there is any pressure in the system. If a little put the cap back on with the engine running and take off again in a minute or two. If there is a good amount of pressure again then there may be a problem. You can also do this at operating temp. Take the cap off when it reaches temp, then put it back on, a minute later take it off and see if there is pressure. If yes then there is most likely a leak.

At this point it would help to buy a cheap temp gun and see what the real temp is when the thermostat does its thing also.

[98valk]

temp sensor is bad or the gage has a ground problem.

[carolinarider09]

I have seen many temp changes like you talk about with the loss of coolant in engines that have a head gasket failure.

To check start engine from cold with the radiator cap on. Let it run a few minutes and carefully take the cap off and see if there is any pressure in the system. If a little put the cap back on with the engine running and take off again in a minute or two. If there is a good amount of pressure again then there may be a problem. You can also do this at operating temp. Take the cap off when it reaches temp, then put it back on, a minute later take it off and see if there is pressure. If yes then there is most likely a leak.

At this point it would help to buy a cheap temp gun and see what the real temp is when the thermostat does its thing also.

Question.   I understand what I think you are trying to get me to do but the process is not clear.

Start engine and let it warm up a little.   This should cause the water in the engine to increase in temperature but since the thermostat is almost closed when cold there is little flow. 

But as the temperature of the water in the engine increases, so should the pressure. 

Just to cut to the end, if I let it warm up to what I will call operating temperature, then there should be some pressure behind the radiator cap when I move it to the "mid" position (not totally unlocked but in the position where the pressure can be released).   

With a head gasket leak, the leak would be internal to the engine and the water would end up on the combustion areas (i.e. where the pistons are).   This would also mean I would be losing coolant, which I don't link I am. 

But it's hard to tell (about the coolant loss).  I will run the engine again tomorrow and move the cap to the "mid" position and see if there is pressure.   

The question then is, why would the temperature fluctuate with a water leak.  I do not understand.

[Jersey mike]

Just replaced the thermostat.  Old one looked just like the new one.  Same condition, nice and shiny and clean.

The replacement process was a real pain in the "arse".    Two of the screws holding the thermostat housing in place are very hard to get to.   

After replacement, started engine and let it warm up.  Gauge got to about 1/3 off full range and then sort of leveled out.  It did show a few "changes" as it idled and got warmer.   

Took it for a drive and same conditions occurred.  But the range of change was not as great as it was the last time I drove it with the old thermostat and sensor.   

So, that was not the issue. 

What I do believe is that, whatever the issue, it is not going to cause a catastrophic engine failure.  But I don't have any other ideas of what might be causing the issue.

There is, also, another sensor that appears to be a "switch" instead of a "gauge" output.  I have not checked to see if there is a "over temperature" light on the dash.  If there is that could be what this sensor is for.   This sensor is near the thermostat housing but I have not verified (as noted above) its purpose. 

Is this a constant issue? Meaning that does the coolant gauge do this if you were driving all day or does it do it once after the initial warmup and then stop?

So, to recap.

New thermostat, fresh coolant, good hoses, new temp sensor.

Is the coolant overflow maintaining “Normal” cold coolant level?

[98valk]

https://www.explorerforum.com/forums/threads/1996-ford-explorer-with-fluctuating-temp-gauge.377216/

best ford site there is.
https://www.ford-trucks.com/forums/943832-1996-explorer-4-0-temp-gauge-movement.html

https://www.2carpros.com/questions/ford-explorer-1996-ford-explorer-engine-tempgauge-fluctuations

https://www.2carpros.com/questions/ford-explorer-1996-ford-explorer-temp-gauge-and-temp-change-rapidly

https://www.explorerforum.com/forums/threads/engine-temperature-gauge-not-working.98498/
Also-- If you ground the wire coming to the sender, it should make the gauge go up right away. Just touch the wire from the boot on top of the sender to the engine block, if it goes up, it's the sensing unit, if it stays the same, it's the gauge, or wiring to it.
I followed the wire from the thermostate to the firewall. there was a very tiny split in the boot that covers the wire. i cut the wire in half, put both ends in a but conector and put some sylicone in it and it works awesome!

[Robert]

I have seen many temp changes like you talk about with the loss of coolant in engines that have a head gasket failure.

To check start engine from cold with the radiator cap on. Let it run a few minutes and carefully take the cap off and see if there is any pressure in the system. If a little put the cap back on with the engine running and take off again in a minute or two. If there is a good amount of pressure again then there may be a problem. You can also do this at operating temp. Take the cap off when it reaches temp, then put it back on, a minute later take it off and see if there is pressure. If yes then there is most likely a leak.

At this point it would help to buy a cheap temp gun and see what the real temp is when the thermostat does its thing also.

Question.   I understand what I think you are trying to get me to do but the process is not clear.

Start engine and let it warm up a little.   This should cause the water in the engine to increase in temperature but since the thermostat is almost closed when cold there is little flow. 

But as the temperature of the water in the engine increases, so should the pressure. 

Just to cut to the end, if I let it warm up to what I will call operating temperature, then there should be some pressure behind the radiator cap when I move it to the "mid" position (not totally unlocked but in the position where the pressure can be released).   

With a head gasket leak, the leak would be internal to the engine and the water would end up on the combustion areas (i.e. where the pistons are).   This would also mean I would be losing coolant, which I don't link I am. 

But it's hard to tell (about the coolant loss).  I will run the engine again tomorrow and move the cap to the "mid" position and see if there is pressure.   

The question then is, why would the temperature fluctuate with a water leak.  I do not understand.

Pressure in the cooling system should not build up quickly.

 Once operating temp, pressure in the cooling system should really not build much at all with the cap on for 30 seconds to a  minute. If it does, more than once then there is a problem. I am talking about a good change in pressure, if its just a very small amount then that is ok.

 Coolant loss in the cooling system by a head gasket may not be noticeable, but the combustion gases going into the cooling system under higher pressure would more easily be noticed. Especially when the pressure does not relax at all and constantly builds every time you close the system

When the car is cold of course the cooling system will build pressure, slowly, and should be consistent. So when cold and you start the car for about the first few minutes there should be minimal to no pressure. If you take the cap off and there is close to full pressure then there may be problem.

By taking the cap off and checking pressure and then putting it back on and taking it off after about 20 seconds all you are doing is seeing how much pressure how fast builds in the system. We are talking essentially about vapor lock of the thermostat.

Thermostats work when water gets hot and transfers heat to the thermostat element, this at the preset temp will open the thermostat. With a head gasket leak even one that may not be apparent, the cooling system will have air or combustion gas in it. This air gets stuck around the thermostat and does not allow the heat transfer to the element in the thermostat. When the temp finally does get high enough the thermostat does open but its usually a higher temp and then the temp drops quickly because the air gets purged from the system the water circulates, temporarily and the cooling system does its job but the temp can be erratic because there is not constant contact with water and the thermostat. Once the air is purged and the thermostat closes again the cycle starts again. Since combustion gases will rebuild in the system. 

Other things I was thinking as I wrote this,

Make sure if there is an air bleed line from the thermostat housing that it is open and clear. I have seen them block closed.

Check the temp of the air coming out of the radiator when the problem happens. If the air temp changes quite a bit then this could also be an indication that there is something causing problems with circulation.

Ideally the temp out of the radiator fan at first should be air temp, then progress to warm, to fairly hot and remain consistent. If the air stays cool then suddenly gets hot or is inconstant then it will indicate that the cooling system is having a problem maintaining a consistent temp. Or if the engine is hot and the air out of the radiator is cool to cold.

A long explanation for something that takes all of 5 minutes maybe a bit longer and does not cost a dime to do. 

Fords can have a engine fan that is controlled electronically, if this is the case there should be wires going to the fan. Check to make sure the fan does come on and pulls air even if it has a centrifugal clutch be sure it pulls air when needed. On initial start up check the air, if not much air turn on the ac make sure the air does change and pull through if its an electric fan.

[Jersey mike]

I have seen many temp changes like you talk about with the loss of coolant in engines that have a head gasket failure.

To check start engine from cold with the radiator cap on. Let it run a few minutes and carefully take the cap off and see if there is any pressure in the system. If a little put the cap back on with the engine running and take off again in a minute or two. If there is a good amount of pressure again then there may be a problem. You can also do this at operating temp. Take the cap off when it reaches temp, then put it back on, a minute later take it off and see if there is pressure. If yes then there is most likely a leak.

At this point it would help to buy a cheap temp gun and see what the real temp is when the thermostat does its thing also.

Question.   I understand what I think you are trying to get me to do but the process is not clear.

Start engine and let it warm up a little.   This should cause the water in the engine to increase in temperature but since the thermostat is almost closed when cold there is little flow. 

But as the temperature of the water in the engine increases, so should the pressure. 

Just to cut to the end, if I let it warm up to what I will call operating temperature, then there should be some pressure behind the radiator cap when I move it to the "mid" position (not totally unlocked but in the position where the pressure can be released).   

With a head gasket leak, the leak would be internal to the engine and the water would end up on the combustion areas (i.e. where the pistons are).   This would also mean I would be losing coolant, which I don't link I am. 

But it's hard to tell (about the coolant loss).  I will run the engine again tomorrow and move the cap to the "mid" position and see if there is pressure.   

The question then is, why would the temperature fluctuate with a water leak.  I do not understand.

Pressure in the cooling system should not build up quickly.

 Once operating temp, pressure in the cooling system should really not build much at all with the cap on for 30 seconds to a  minute. If it does, more than once then there is a problem. I am talking about a good change in pressure, if its just a very small amount then that is ok.

 Coolant loss in the cooling system by a head gasket may not be noticeable, but the combustion gases going into the cooling system under higher pressure would more easily be noticed. Especially when the pressure does not relax at all and constantly builds every time you close the system

When the car is cold of course the cooling system will build pressure, slowly, and should be consistent. So when cold and you start the car for about the first few minutes there should be minimal to no pressure. If you take the cap off and there is close to full pressure then there may be problem.

By taking the cap off and checking pressure and then putting it back on and taking it off after about 20 seconds all you are doing is seeing how much pressure how fast builds in the system.

Thermostats work when water gets hot and transfers heat to the thermostat element, this at the preset temp will open the thermostat. With a head gasket leak even one that may not be apparent, the cooling system will have air or combustion gas in it. This air gets stuck around the thermostat and does not allow the heat transfer to the element in the thermostat. When the temp finally does get high enough the thermostat does open but its usually a higher temp and then the temp drops quickly because the air gets purged from the system the water circulates, temporarily and the cooling system does its job but the temp can be erratic because there is not constant contact with water and the thermostat. Once the air is purged and the thermostat closes again the cycle starts again. Since combustion gases will rebuild in the system. 

Other things I was thinking as I wrote this,

Make sure if there is an air bleed line from the thermostat housing that it is open and clear. I have seen them block closed.

Check the temp of the air coming out of the radiator when the problem happens. If the air temp changes quite a bit then this could also be an indication that there is something causing problems with circulation.

Ideally the temp out of the radiator fan at first should be air temp, then progress to warm, to fairly hot and remain consistent. If the air stays cool then suddenly gets hot or is inconstant then it will indicate that the cooling system is having a problem maintaining a consistent temp. Or if the engine is hot and the air out of the radiator is cool to cold.

A long explanation for something that takes all of 5 minutes maybe a bit longer and does not cost a dime to do. 

Fords can have a engine fan that is controlled electronically, if this is the case there should be wires going to the fan. Check to make sure the fan does come on and pulls air even if it has a centrifugal clutch be sure it pulls air when needed. On initial start up check the air, if not much air turn on the ac make sure the air does change and pull through if its an electric fan.

Shouldn’t or wouldn’t he see a cream colored oily residue on his oil fill cap or see an oily residue floating in the overflow reservoir of there is a head gasket issue?

There may be a “sweet smell” coming from the exhaust or engine area of antifreeze being burned or vented.

[Robert]

Shouldn’t or wouldn’t he see a cream colored oily residue on his oil fill cap or see an oily residue floating in the overflow reservoir of there is a head gasket issue?

There may be a “sweet smell” coming from the exhaust or engine area of antifreeze being burned or vented.

Yes there can be, if the head gasket leak occurs near a oil passage, but a straight up head gasket failure will only give gases.

Yes there is a sweet smell many times but sometimes its hard to distinguish since many are not connoisseur of exhaust gas and sometimes may be small enough to not be noticeable.    

If the head gasket leak is small enough also, just enough to play with the thermostat, there may not be much other indication.

Also a crack in the head or engine block will do the same thing and they only usually just put gases in the cooling system.

[Jersey mike]

Shouldn’t or wouldn’t he see a cream colored oily residue on his oil fill cap or see an oily residue floating in the overflow reservoir of there is a head gasket issue?

There may be a “sweet smell” coming from the exhaust or engine area of antifreeze being burned or vented.

Yes there can be, if the head gasket leak occurs near a oil passage, but a straight up head gasket failure will only give gases.

Yes there is a sweet smell many times but sometimes its hard to distinguish since many are not connoisseur of exhaust gas and sometimes may be small enough to not be noticeable.    

If the head gasket leak is small enough also, just enough to play with the thermostat, there may not be much other indication.

Also a crack in the head or engine block will do the same thing and they only usually just put gases in the cooling system.

Ok that’s a good clarification.

Aside from the possibilities of an electronic issue, the only other things that come to mind (for me) are this;

Possibly a blockage/rust in radiator

Possibly a bad heater core (rust or whatever) or heater hoses.

Does the truck make steady constant heat?

If I remember correctly, the radiator and trans cooler are separate but the trans cooler sits in front of radiator, possibly some debris over time accumulated in that space.

The truck doesn’t have an electric cooling fan does it?

[carolinarider09]

So, to recap.

New thermostat, fresh coolant, good hoses, new temp sensor.

Is the coolant overflow maintaining “Normal” cold coolant level?

Yes, new thermostat, new temp sensor.

Hose are good, no leaks or issues.  

No, did not change out the coolant but had to add a half gallon after thermostat replacement.  

Also it started a few weeks or maybe a month ago.  Nothing was done to the engine or the coolant system.  Just noticed it one day while driving and just normal gauge scan.  

[carolinarider09]

https://www.explorerforum.com/forums/threads/1996-ford-explorer-with-fluctuating-temp-gauge.377216/

best ford site there is.
https://www.ford-trucks.com/forums/943832-1996-explorer-4-0-temp-gauge-movement.html

https://www.2carpros.com/questions/ford-explorer-1996-ford-explorer-engine-tempgauge-fluctuations

https://www.2carpros.com/questions/ford-explorer-1996-ford-explorer-temp-gauge-and-temp-change-rapidly

https://www.explorerforum.com/forums/threads/engine-temperature-gauge-not-working.98498/
Also-- If you ground the wire coming to the sender, it should make the gauge go up right away. Just touch the wire from the boot on top of the sender to the engine block, if it goes up, it's the sensing unit, if it stays the same, it's the gauge, or wiring to it.
I followed the wire from the thermostate to the firewall. there was a very tiny split in the boot that covers the wire. i cut the wire in half, put both ends in a but conector and put some sylicone in it and it works awesome!

I will try your recommendation on how to verify the wiring.  Have a similar issue on my Massey Ferguson  Tractor with fuel gauge.  

I was not aware of the other Explorer forum.  I had used this one in the past.

https://www.explorertalk.com/threads/strange-temperature-gauge-operation.85339/#post-437726

Looks like the https://www.explorertalk.com/threads/strange-temperature-gauge-operation.85339/#post-437726 does not have an answer to the post for 2013.  Maybe I have to join to see any respsones.  

Same with this one https://www.2carpros.com/questions/ford-explorer-1996-ford-explorer-engine-tempgauge-fluctuations

One of the listings found the issue to be the thermostat and replacing it did the trick.

One other suggested a ground issue (well short) with the wire.  And that fixed the issue on his system.  Problem is this  seems far out.  But.....   Could be an issue.  

And yes it turns out there are two sensors on the coolant system right at the thermostat outlet.  I did not realize how it was connected until I looked at it a little harder yesterday.  When I purchased the new sender I got the one identified as an engine temperature sensor.  There was also one identified as a Temperature Sensor Switch which I figured was for a different engine.  

As it turns out, it appears (have not verified this yet) that a temperature switch is also installed on the coolant outlet above the thermostat.   I suspect, (but have not verified) that is might be for a "warning" light.  But.....  As an aside, the oil pressure sensor on the Explorer is a "switch".   Switch fails oil pressure goes to zero.  Replace switch/sensor oil pressure goes to mid scale.   (sort of like a warning light but fake).

It is possible that there is air trapped in the system.  Although I have no idea how that could really happen.   I say this because the system, while controlled via the thermostat, the thermostat allows a small about of flow when closed, if I remember correctly.  

It is possible that a hose is collapsed.  The upper hose appears to be in good shape but will check them both to see how the "feel".  

I am amazed that this has been documented in two forums and no real answer was found .

[carolinarider09]

Ok that’s a good clarification.

Aside from the possibilities of an electronic issue, the only other things that come to mind (for me) are this;

Possibly a blockage/rust in radiator

Possibly a bad heater core (rust or whatever) or heater hoses.

Does the truck make steady constant heat?

If I remember correctly, the radiator and trans cooler are separate but the trans cooler sits in front of radiator, possibly some debris over time accumulated in that space.

The truck doesn’t have an electric cooling fan does it?

A blockage on the radiator sort of crossed my mind but.... What I will do next is to let the engine warm up and verify there is pressure in the radiator by opening the radiator cap one half turn.   Should have hot water.   

The truck has constant heat.  I did a test the other day while driving it.  I turned the heat on max (temperature wise) and medium fan speed.  I felt the heat output (by hand no measuring device) and which the temperature gauge as it changed trying to ascertain if there was a correlation between the temperature gauge and the air temp.    I could not detect any changes but, I was a little sure of the results.   I should get a real electronic thermometer out there to verify. 

I will check the trans cooler

Cooling fan is belt driven.   

The fact that it just "happened" one day on the road is the real strange thing.

[carolinarider09]

Pressure in the cooling system should not build up quickly.

 Once operating temp, pressure in the cooling system should really not build much at all with the cap on for 30 seconds to a  minute. If it does, more than once then there is a problem. I am talking about a good change in pressure, if its just a very small amount then that is ok.

 Coolant loss in the cooling system by a head gasket may not be noticeable, but the combustion gases going into the cooling system under higher pressure would more easily be noticed. Especially when the pressure does not relax at all and constantly builds every time you close the system

When the car is cold of course the cooling system will build pressure, slowly, and should be consistent. So when cold and you start the car for about the first few minutes there should be minimal to no pressure. If you take the cap off and there is close to full pressure then there may be problem.

By taking the cap off and checking pressure and then putting it back on and taking it off after about 20 seconds all you are doing is seeing how much pressure how fast builds in the system. We are talking essentially about vapor lock of the thermostat.

Thermostats work when water gets hot and transfers heat to the thermostat element, this at the preset temp will open the thermostat. With a head gasket leak even one that may not be apparent, the cooling system will have air or combustion gas in it. This air gets stuck around the thermostat and does not allow the heat transfer to the element in the thermostat. When the temp finally does get high enough the thermostat does open but its usually a higher temp and then the temp drops quickly because the air gets purged from the system the water circulates, temporarily and the cooling system does its job but the temp can be erratic because there is not constant contact with water and the thermostat. Once the air is purged and the thermostat closes again the cycle starts again. Since combustion gases will rebuild in the system. 

Other things I was thinking as I wrote this,

Make sure if there is an air bleed line from the thermostat housing that it is open and clear. I have seen them block closed.

Check the temp of the air coming out of the radiator when the problem happens. If the air temp changes quite a bit then this could also be an indication that there is something causing problems with circulation.

Ideally the temp out of the radiator fan at first should be air temp, then progress to warm, to fairly hot and remain consistent. If the air stays cool then suddenly gets hot or is inconstant then it will indicate that the cooling system is having a problem maintaining a consistent temp. Or if the engine is hot and the air out of the radiator is cool to cold.

A long explanation for something that takes all of 5 minutes maybe a bit longer and does not cost a dime to do. 

Fords can have a engine fan that is controlled electronically, if this is the case there should be wires going to the fan. Check to make sure the fan does come on and pulls air even if it has a centrifugal clutch be sure it pulls air when needed. On initial start up check the air, if not much air turn on the ac make sure the air does change and pull through if its an electric fan.

when cold and you start the car for about the first few minutes there should be minimal to no pressure. If you take the cap off and there is close to full pressure then there may be problem.

No real pressure after the engine runs for a few minutes.  Temp gauge still pegged low.

Thermostats work when water gets hot and transfers heat to the thermostat element, this at the preset temp will open the thermostat. With a head gasket leak even one that may not be apparent, the cooling system will have air or combustion gas in it. This air gets stuck around the thermostat and does not allow the heat transfer to the element in the thermostat. When the temp finally does get high enough the thermostat does open but its usually a higher temp and then the temp drops quickly because the air gets purged from the system the water circulates, temporarily and the cooling system does its job but the temp can be erratic because there is not constant contact with water and the thermostat. Once the air is purged and the thermostat closes again the cycle starts again. Since combustion gases will rebuild in the system.

Ok, that makes some sense but..... I will have to think about it for awhile. 

What I will do over the next few days is to let the engine warm up and loosen the radiator cap at various intervals.  Only problem is I do not think I will get the system to read the "high" spot on the gauge under idling conditions. 

I will check two things, just to verify.  One.  What is the water level in the radiator when cold.  Two.  What is the water level in the radiator when hot. 

I will also check coolant temperature with an independent thermometer.

[carolinarider09]

This morning's experiment.

Went out to the Explorer.  Checked the coolant level and it was below the top of the tank, fluid just barely visible.

Expansion tank was about 1/2 full (level lower than has been based on the coloring but that coloring is probably 18 year is the making). 

Checked upper radiator hose.  It was cool and flexible. 

Started engine and let it idle for about 5 - 10 minutes.  Waited for temperature gauge to get off peg and enter the "white" zone.

Checked upper radiator hose.  It was cool and flexible (not pressurized)
Removed radiator cap, no pressure was there but coolant level seemed a little higher. 

Re-started engine and let idle until temperature gauge got to "normal".   It did seem to wobble a little in that area and it did seem to dip once.  That could be when the thermostat finally opened....

Anyway it had run for about another 10 or 15 minutes from first test.  Upper radiator hose was warm and pressurized (to the touch).   Radiator cap was warm to the touch as was the upper part of the radiator.

Opened radiator cap just little and fluid came out (it was satisfactory pressurized). 

Shut down the engine and checked it all again.  There was no change in the coolant level in the expansion tank. 

I did not check the lower radiator hose or the heater hoses. 

As a side not, it is hard to determine if the gauge fluctuates when hot and at idle.  I will have to drive it and then bring it back and let it idle to see if the condition persists at idle.   

[Robert]

Opened radiator cap just little and fluid came out (it was satisfactory pressurized). 

Shut down the engine and checked it all again.  There was no change in the coolant level in the expansion tank. 

I did not check the lower radiator hose or the heater hoses. 

As a side not, it is hard to determine if the gauge fluctuates when hot and at idle.  I will have to drive it and then bring it back and let it idle to see if the condition persists at idle.   

When you did this did you restart it and run it for a minute or so with the cap on?

But I would say you are clear on the idea that the head gasket is leaking.

Ok a seemingly stupid question does the serpentine belt have the correct routing?

[six2go #152]

This vehicle is 26 yrs. old with 200,000 miles? I think it has served you well. If it was mine, I would just drive it until it crapped out then kick it to the ditch & replace it. 

[carolinarider09]

This vehicle is 26 yrs. old with 200,000 miles? I think it has served you well. If it was mine, I would just drive it until it crapped out then kick it to the ditch & replace it.  

Thought has crossed my mind several times.   Probably would have except we got the "B" instead of the "T" and the economy tanked.  

Edited to add:  Besides I hate to lose.   

[carolinarider09]

When you did this did you restart it and run it for a minute or so with the cap on? 

Yes

But I would say you are clear on the idea that the head gasket is leaking.

Ok

Ok a seemingly stupid question does the serpentine belt have the correct routing?

I think so.  Nothing has happened that would have made it change but I will give it a look.

[Robert]

Ok a seemingly stupid question does the serpentine belt have the correct routing?

I think so.  Nothing has happened that would have made it change but I will give it a look.

Went back and reread where the problem just happened out of the blue, so I doubt belt routing is the answer but needed to check

Sometimes the serpentine belt can go on opposite to the way its supposed to go on. This could cause a opposite rotation on the pulleys, specifically the water pump and cause a overheat or erratic cooling situation since the pump would be operating in reverse.

I do have another idea, remove the instrument cluster and clean the connections or plugs that go to it. Use WD40 and clean the contact areas on the connection, plug it back in and see if that changes anything. On your truck its not hard and sometimes corrosion on the connectors can cause intermittent contacts that will cause the gauge to fluctuate.

Seen it more than once.

ON the opposite rotation issue,

A guy brought a Dodge diesel truck into a shop with a strange issue.

The fan would loosen and fall off, yes the main engine cooling fan would fall off the engine but everything else was ok. The owner had tighten it many times without luck. So he brings the truck into a shop and they look at it tighten the fan and send him on his way. Sure enough the fan again falls off.

Well after some diagnosing time and head scratching even to thinking thread locker on the fan, one guy notices that the thread rotation of the fan was such that it would loosen every time the engine would turn off. So looked at the belt diagram took the belt off routed it the way that the diagram said and no more problem.