Coolant boiling in overflow (not bubbling) when TJ working hard

[TJSafariBlack]

Hello all - first off, what a cool forum this is!

I have a 2006 TJ Safari that I drive in the desert. Occasionally, after pushing it quite hard, I notice the coolant boiling (not just bubbling) in the overflow. Temp gauge stays fine.

I've seen previous threads that say this is most likely the radiator cap (which I will pressure test and / or replace). Mechanic insists, however, that it must be a bad head gasket but not yet completely failed. He says the cooling system is 'building up pressure' due to the bad gasket which will blow any time soon and temperatures of the car are high. There is no white smoke, oil is fine / not mixing, and the engine isn't overheating.

He also said this as evidence of the 'bad' head gasket: 'the car's building up pressure right now while driving the temperature should be fine but idle temperatures not normal which shows a failing gasket.'

Question - is the mechanic right? Instinct tells me he's not but would appreciate your views. If heating at idle, I would think that would suggest a bad fan clutch (as the car doesn't overheat while driving normally). He says the fan clutch is working normally, but it seems odd that it would overheat at idle (something I haven't noticed by the way) and not while driving normally.

Question - from my reading, it seems like I should pressure test the coolant system and flush the coolant system. Is there anything else I should ask him to do?

Otherwise, there is a coolant leak (he says this is due to the car building up pressure due to the bad gasket), steering pressure pipe is leaking, and differential leaking. Just wondering if these are major things and if this guy can be trusted.

Appreciate you all!

 

[IOwnCalculus]

You (or the mechanic) can do a combustion gas test and rule it out pretty quickly.

 

[TJSafariBlack]

You (or the mechanic) can do a combustion gas test and rule it out pretty quickly.

Thanks. Mechanic says it's not blown just 'bad' and will blow soon. Would a combustion gas test help here? Not sure what he means with building up pressure either - would a bad gasket cause this? I thought it's meant to be pressurised.

 

[IOwnCalculus]

Head gaskets are either good or they're not. A head gasket that is doing its job will show no symptoms of any kind because it's working. A head gasket that is causing symptoms is already bad, so there's no "it's going out", it's already "gone".

The cooling system is supposed to be pressurized, but it is possible for head gaskets to fail in such a way that the only major symptom is combustion gasses leaking into the coolant (with no white smoke or oil/water mixing), which will cause the cooling system to pressurize far quicker than it would on its own. If the head gasket leak is particularly bad it could be pressurizing the system faster than the radiator cap can relieve it. This would also be visible in a matter of seconds on a combustion gas test.