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Excessive pressure/air (exhaust?) in cooling system

I have excessive pressure/air (exhaust?) in my cooling system, causing loss of atmospheric heat and engine overheating. I have been through the entire cooling system, replaced everything from the pump to the heater core over the past 18 mths. This particular problem began just before I replaced the heater core (about 3-4 months ago) and has been getting steadily worse. This past weekend, I replaced the intake manifold gasket. The problem is still there, so I tried running without the thermostat installed… Still lots of air. It seems obvious that the only thing it could possibly be is a head gasket, but this motor runs great, and there is no evidence of oil in the water or water in the oil. I see white smoke when cold at first startup, but it quickly stops, and there isn’t any water actually flowing from the tail pipe. Maybe a drip or two when cold. I am kicking myself for not just going ahead and doing head gaskets when I did the intake; I was half way there! Any ideas, advice, thoughts would be greatly appreciated!

Stab 1 at the cat, head gasket.

I didn’t think of the cat, it started making a rattling noise right about when this all began. Do you think that could be it? If so, did it take out the head gasket?

Head gasket problem. You should have done them when you had the intake off.

It sure sounds like a head gasket problem.

Yeah, I knew it when I was there… The cat never really entered my mind though, aside from having an annoying rattle. I was trying to get this cooling system dealt with first, little did I know that they were related (I read up on converters and reasons for failure last night). All the pieces are starting to fall into place.


You can find out for sure whether it’s the headgasket. Pick up a cylinder leakdown test kit at the parts store. Basically, you’ll us a fitting in the spark plug hole to pump air into each cylinder (with the valves closed) and monitor the pressure to see of the cylinder holds.

But I gotta be honest, if you’re seeing bubbles coming up through the radiator cap hole with the engine running it’s a pretty safe bet that you have a breech in the head gasket. The bubbles are the combustion gasses getting blown through the breech and into the water jacket and migrating to the system’s highest point: the radiator cap hole.

A quick test for a head gasket leak is this; and keep in mind that it’s not 100% definitive but close enough.

When the engine is stone cold loosen the radiator cap and release any pressure. Start the engine and allow it to idle for 15-20 seconds. Shut it off and quickly release the radiator cap.
If you hear a hiss there is at least a reasonable chance of a head gasket breach. At that point further diagnosis is needed to verify it. (compression test, hydrocarbon test, vacuum gauge test, etc.)