What is the connection between head gasket failure and coolant system pressure?

[bustedbenz]

I'm curious about this from a technical point of view, it's one of those things that I'd know if I had a lifetime of experience with engines (or more common sense.) I do not currently suspect any problems with my car, do not have any symptoms, and am only asking this in terms of "what if?"

Just saw an explanation in another thread and decided not to hijack that one - that stated that pressure in the coolant hoses (on cars with expansion tanks) was indicative of head gasket failure allowing "exhaust gases into the cooling system" if I read it correctly.

Here's what I don't understand. I find it impossible to believe that these exhaust/cooling systems are 100%, completely, totally, around-every-crack airtight. Yes, the cooling system is supposed to be pretty much sealed... but I'm talking about even a tiny little pinhole-in-a-gasket-somewhere would be enough to admit atmospheric air pressure over a long enough period of time.

If gases are allowed to escape into the cooling system, then, once the exhaust pressure drops (When the engine stops running and sits for several days) - why doesn't the excess pressure in the cooling system merely exit the way it entered? If there's a leak in the head gasket seal that's allowing exhaust pressure into the cooling system, why doesn't it also allow exhaust pressure OUT of the cooling system once the exhaust has stopped generating pressure?

I realize this is a "fundamental engine basics" question - but this is the only way I can think of to find the answer to this one. "Pressurized cooling system" is a standard provided answer to "head gasket failure" and I'm trying to understand why the pressure stays built up instead of escaping backwards through the same system from which it entered.

[jkoebel]

Exhaust gas pressure coming from active combustion is enough to force through a blown gasket, but the limit to which the cooling system can pressurize isn't big enough to force it back the other direction?

Sort of like a one-way valve.

[Knightrider966]

Originally Posted by jkoebel

Exhaust gas pressure coming from active combustion is enough to force through a blown gasket, but the limit to which the cooling system can pressurize isn't big enough to force it back the other direction?

Sort of like a one-way valve.

Yes, exactly! Besides we're dealing with two different substances here. Air is going through the blown head gasket into the cooling system, but coolant has to work its way back into the cylinder! Not like this won't happen, but you really have to warp a head this way.

Also, a head gasket and head, when torqued down properly will be absolutely airtight, not even a pinhole. If there was you wouldn't make it very far before this turned into a tow truck situation!

[bustedbenz]

Originally Posted by Knightrider966

Yes, exactly! Besides we're dealing with two different substances here. Air is going through the blown head gasket into the cooling system, but coolant has to work its way back into the cylinder! Not like this won't happen, but you really have to warp a head this way.

Also, a head gasket and head, when torqued down properly will be absolutely airtight, not even a pinhole. If there was you wouldn't make it very far before this turned into a tow truck situation!

AHA. That's the piece that my 1:00 AM brain wasn't coming up with. Once air bubbles entered the coolant stream, they'd eventually wind up at the top somewhere, probably right under the expansion tank cap with all the other air. Therefore, the air wouldn't be available to pass through.

Thanks! That was the missing link.