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indybobm
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« on: September 25, 2011, 04:00:21 PM » |
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When checking compression, the service manual says to remove one plug at a time and then spin the motor to check compression. I have always pulled all of the plugs whenever I checked the compression on anything. Does it really make a difference? With all of the plugs out, would it spin too fast? Hurt something? I just want to get a baseline reading.
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VRCC # 5258
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BigAl
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« Reply #1 on: September 25, 2011, 04:04:26 PM » |
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No they jsut don't want you to screw up which plug wire goes where.
Or lose anything down in one of the holes on the suction, that each cylinder would be producing as the piston retreats down in it's bore.
Make sense.
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R J
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Posts: 13380
DS-0009 ...... # 173
Des Moines, IA
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« Reply #2 on: September 25, 2011, 04:10:52 PM » |
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When I took auto diesel mechanics, many many moons ago, 1 instructor told us to take 1 plug at a time, the other instructor told us to take all out and then do 1 hole at a time.
At the kid's shop, if we pull a compression test it is usually 1 plug out, test, put back and pull the next one.
Now that you have brought that up, the next car we get in that the compression needs checked, I'll have the tech pull all plugs, run a test, reinstall and do each one with the others in. See if there is any difference..
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44 Harley ServiCar   |
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BigAl
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« Reply #3 on: September 25, 2011, 04:14:37 PM » |
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If there is a difference then Gravity and Molecular Science Books will have to be changed immediately.
Just Kidding.
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indybobm
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« Reply #4 on: September 26, 2011, 10:04:56 AM » |
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Well, I checked compression today before while changing the plugs. I first pulled #5 and hooked up the compression tester and hit the starter button. Of course, the engine started and the compression tester read only 90 lbs. Freaked me out!.
I then pulled all of the the other plug wires so it would not start and checked compression again with the throttle wide open. This time it read 165 lbs. Relief! Check the rest with the following result:
1. 160 2. 160 3. 160 4. 160 5. 165 6. 170
Shows the importance of having the throttle wide open when checking compression.
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So many roads, so little time
VRCC # 5258
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Jess Tolbirt
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« Reply #5 on: September 26, 2011, 11:07:34 AM » |
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with the throttle closed, it doesn't get any air in there to compress...
with the engine running the gauge cant act fast enough before the exhaust valve opens..
i pull all the plugs because its easier on the starter
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Valkyrie member # 23084
Started out on old forum on day one but lost my member number.
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alph
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« Reply #6 on: September 26, 2011, 11:09:14 AM » |
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YES!! take one plug out at a time!! if you take all of the plugs out, (and i don't know why) your readings will be all over the place, were as, if you do one plug at a time, and the others are in, your readings will be much closer together on each cylinder. my first compresion test scared the living tar out of me! one cylinder would have over 20#'s difference then the one right next to it! then i talked to a good friend (that use to build engines for drag-racing guys) and he told me i was doing it wrong and showed me how to do it right. from what you've got for readings, your engine is fine. my results were very simular.
NOW STOP WORRING AND GO RIDE!!
alph.
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Promote world peace, ban all religion. Ride Safe, Ride Often!!   |
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thumper
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« Reply #7 on: September 26, 2011, 11:34:39 AM » |
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If there is a difference then Gravity and Molecular Science Books will have to be changed immediately.
Just Kidding.
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An oak tree is nothing but an acorn that stood it's ground!
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The Anvil
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« Reply #8 on: September 26, 2011, 11:41:39 AM » |
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In aviation we take all of the plugs out when doing compression checks as part of regular maintenance checks. Since cleaning the plugs and annealing the gaskets is usually part of the process it just makes it easier to put all of the plugs in a caddy and do it all at once rather than walking back and forth.
I've never seen anything get sucked into a cylinder but I won't say it can't happen.
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Boxer rebellion, the Holy Child. They all pay their rent.
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