I was told a way to diagnose a bad headgasket is to remove the serpentine belt, so that the water pump does not spin, and start the vehicle with the cap off. If coolant erupts from the open radiator cap, then that means you have combustion gases in the coolant system? I can see the logic in this in that the coolant should not move without the flow of the water pump. When I did this coolant erupted from my open radiator.

Now as a counterpoint, the machine shop guy and owner of my local napa store told me that this is not effective, due to the differentiation in pressure in the system due to the coolant in the radiator being cool, versus the coolant in the block heating rapidly.

Any thoughts? I'm just curious what everyones take is on this?

 

So true. My dad does this all the time. Another way is to hook air pressure up to each cylinder one at a time, when you get air in the radiator, thats usually where the gasket failure is. So you can kinda pin point it. Who ever told you that is an old school Jedi fyi..

 

Yeah, that's actually a valid test. It can also be a crack in the head that will cause that. But either way the head(s) have to come off.

I'm assuming you only ran it for maybe a minute or so like that- you don't want to do it much longer without coolant circulating.

 

Yeah I've got the heads out being pressure tested now.