99SH

I just replaced my head-gasket a few days ago and refreshed the head while I was at it. Put the car together and it runs great. I noticed that the cooling system will hold pressure overnight, even when the coolant has cooled down to ambient.

Common sense tells me that pressure is entering the cooling system from combustion (i.e. head gasket not sealed, micro-fracture in head).

I did a leak-down test and measured <4% leak on all cylinders (did not see any bubbles in radiator during test @ 100psi). Ran the car with the radiator cap off and can see maybe 1 very small bubble every few seconds if that.

What are your thoughts? Car is running great so far, water temps are nominal.

MM3Kwolik

I doubt it's your head gasket. If anything, you MAY have a little more air in the system that needs to be bled out. The fact that it's HOLDING pressure is actually a good thing. It means your system is fully sealed and not leaking anywhere. I'd pop the cap however and make sure your radiator is filled right to the bottom of the filler neck and make sure that the overflow tank is ONLY filled to the max line at most (when cold). The radiator cap is what bleeds off system pressure, nothing else (unless there's a leak), but the pressure has to meet or exceed the PSI rating of your cap before it would bleed off that pressure. If it's below, there won't be enough pressure to lift the spring and bleed excess pressure into the overflow.

Slowcrash_101

When you say holds pressure does that mean that if you let the car sit overnight, when you remove the rad cap coolant goes spraying everywhere?

99SH

^ Yes. The hose isn't rock hard, but there is some residual pressure in there.

My gut feeling is that something is wrong. Since it's a closed loop system, the only pressure that should be present is when the coolant heats up, expands and creates pressure. When everything is cooled back to it's initial state, the temperature and therefore pressure should also drop back to where it was. If there's positive pressure in the system after it's cooled off, then that must have entered in the cooling system from an external source (e.g. headgasket).

Slowcrash_101

I would think that having a pressure leak from the head gasket would allow pressure to flow back and forth. Worst case scenario, you either need to properly bleed the air out of the cooling system, or get a new radiator cap. If your leakdown numbers are as you say they are, then it's not valves nor the head, the only thing I could think of is to do a compression test and see if two adjoining cylinders are low. That normally indicates a head gasket leak, but if you've just replaced the head gasket and it was put on correctly I don't see it being the cause.

99SH

Thanks for your input.

I'll keep monitoring it for now and hope for the best. I'll be driving a good 800 miles during the holidays, 5 hours continuous each way, so that would be a good test. The radiator & cap is only 2 months old (running mrsideways 2-pass radiator) but may replace the cap for peace of mind.

realblag

Mine has always had a little bit of pressure when fully cool. It has been that way since I got the car 7 1/2 years ago. I have no cooling or head gasket issues.

DaGou

mines always had a bit of pressure in it to.