I'm not at all familiar with the engines, but is it possible that some kind of breather is blocked and the tank is actually cracking from vacuum when the engine cools?

Age and mileage of car, service record and how long have you owned it?

None of which appears to matter at all, except maybe the last one, a bit, in that it might give a clue as to whether the previous owner was offloading a problem.

You have reasons for these questions, or are they just an SOP?

OP, you could probably get the cooling system pressure checked at operating temperature and see if it is way over spec, assuming you can find one, and pressure test it for leakage, engine off. You could also look for bubbles in the radiator filler, engine running, but some cars seem to do that anyway, maybe from cavitation.

Garages,have gas analysers, used for emissions testing. I havn't heard of these being used fo test for HG gasket failure, but its such an apparently obvious application you'd think someone would make adapter kits for it.

I suppose it might not be any more sensitive than the chemical kits, though, if the volume of leaked gas is small,

If it hasn't been done you probably wouldnt be able to get anyone to try it anyway, because they wont want to bust their expensive gadget.

I assume your cooling fan and thermostat both work.

You might try a crackle test on your oil, though thats by no means a sure-fire test for HG failure. I dont think there is one

Edited by edlithgow on 20/08/2023 at 02:28