Porsche8777

I thought the expansion tank was to hold coolant when the engine heated up and the coolant expanded. Well the coolant level in mine goes down when the engine is hot and up when the engine cools. In fact with the engine warm if you loosen the radiator cap and release the pressure the level in the tank rises. I have bled the system numerous times. It doesn't run hot and doesn't loose coolant. Any ideas on what is going on.

Mouthpiece

My understanding of the cooling system has always been that additional coolant is released into the system when the coolant heats up to the thermostat's release point and the thermostat opens. Accordingly, you should have less coolant in the expansion tank when the engine is hot and more when the engine is cool. To me, your description sounds like your system is working properly.

Of course, if I'm wrong , someone will point that out.

Techno Duck

I find mine to be the opposite. When the car is off and cool the coolant level sits at the halfway mark between 'full' and 'minimum'. When the car is at running temperature, the coolant level reaches just under the full mark.

Take care when removing that coolant cap. You can have a nasty volcano of scalding hot liquid mag-ma...coolant spew out.

Jakerx

yep...when the coolant gets hot it "expands" and the level rises and the expansion tank goes near the full mark. and yes be careful removing the lid if the car just been running *ouch*

Granite 944

I could be all wrong here, but the ways I sees it, on a good, properly working cooling system, and the level in the expansion tank starting out at the maximum mark when cold. As engine heats up the water, (it doesn't "expand"), it creates steam, thus creating pressure (pressure compresses the system coolant). Our "closed" cooling systems are setup with a 1 bar (14.5psi) cap (1.5 bar for late '85 on, and don't use late caps on early cars!). Anything over 1 bar, cap will allow this pressure to escape out of the system. The higher the pressure in a closed system, the higher the boiling point of the water/coolant. (And we sure don't want our water to boil ). A good system should run around a pressure of 7-10 psi? when running normally warmed up. This pressure makes the level in the expansion tank appear lower, thus the minumum and maximum marks on the tank (hot or cold, it should always be between these marks). If you have anything different than as explained above, you have a defective cooling system. If your level is getting higher when warm, I'd question if you had some leakage into cooling system from a bad head gasket (compression gasses). Leakage in the system, bad/leaking cap, leaking hoses,gaskets, air in system, ect. are all going to affect this operation. A coolant pressure tester, will tell all. Well.......mostly. Am I all wet with this explaination/thought?

Kokopelli

Are you saying that a gas is compressing a liquid? If so I must have missed some vital information in physics.

Porsche8777

Found the problem The bleeder bolt head came off so I guess it was cracked and leaking. Put in a new bolt and the system works normal. Level comes up with the engine hot, goes down with the engine cold. At least that is what I think is normal, seems to be several opinions on that.

Granite 944

Maybe it was the Jose Cuervo that was compressing me last night......