Fixnstuff
Full Access Member
I missed that the first time. I remember long ago being able to see and smell the exhaust coming out of the radiator fill tube on gas engines with blown head gaskets into a water jacket, or maybe in a coolant reservoir and there is a way to test for exhaust gasses in the coolant/anti-freeze to diagnose that but I've never done it. You might want to look that up!
Seems like it would be doing that LONG BEFORE 40 minutes of driving though! Like there is so much pressure from the detonation stroke in a cylinder that it would seem that it would start doing that within a few minutes.
Looks like you'll need to do some compression testing to rule out a head gasket issue and you can find topics on doing that.
