Stupid Question

[Fazer]

I should probably know this, but when I remove the temp sensor at the bottom of the radiator, am I going to lose the coolant?

[Ken aka Oil Burner]

I'm going to go with yes; you will lose some coolant. If you do it quickly, you may not lose as much as you think. As long as you don't remove the radiator cap, there will be no way for air to immediately replace the lost coolant from the topside; sort of like a vacuum seal. Break that seal and the coolant can flow more quickly out. I've done this sort of thing on cars a time or two. If you're replacing the sender, have the new sender ready to go. Loosen the old one. Antifreeze will likely weep past the threads once it's loose. When it comes out, put your thumb over the hole. Grab the new one. Move your thumb and install the sensor as quick as you can. You should lose the least amount by this method.

Of course, if you're due for a cooling system service, now's probably as good a time as any.

[Ken aka Oil Burner]

If you try this while the engine is warm, the coolant may be under pressure. I'd recommend my method when it's completely cold; like after an overnight sit.

[sandy]

Air can get in through the rad cap. Pinch the vent line with a clamp or small vise grips.

[Ken aka Oil Burner]

Good point. Not nearly as quickly as if the cap was off, but the cap does in fact allow some bypass. If it didn't, the cooling system wouldn't be able to draw coolant back from the overflow bottle as it cools down. As Sandy said, a small clamp or a pair of vice grips pinching off the overflow hose to the tank would just about eliminate the ability for air or coolant to enter the topside of the system.

[Bret SD]

I replaced mine a couple years ago, not much coolant loss happened if memory serves.

[rws]

Air can get in through the rad cap. Pinch the vent line with a clamp or small vise grips.

how is air gonna get in...it's a closed system...the only way air can get in through a properly functioning cap is if the vent line has a leak or the overflow tank is empty

https://www.freeasestudyguides.com/engine-radiator-cap.html