Cylinder Pressures & Cooling issues.

Hi,
I own a 2008 jimny M13AA engine. just replaced thermostat due to over heating. thermostat now working fine, no leaks. But I have bubbles 1 or 2 a second in my overflow tank ( only after long run, might be cap opening). I have been told by a garage its head gasket. but I am not convinced. I can drive around 20 miles or so checking coolant level in overflow tank it rises and rises to nearly full and twice overflowed, and once overheated. I decided to a cylinder compression test. and here are my results.

Engine run for 15-20 mins until warm

1. 168 psi
2. 162 psi
3. 163 psi
4 162 psi
I've also run with cap off to watch for fizzing bubbles, nothing. no oil coolant mixes anywhere. Are these compression figures within range. I am stating to suspect ill fitting cap. Any thoughts. cheers.

I cant remember how a garage does it but they can put something into the expansion tank which tests to see if head gasket is leaking. It does sound like a probably cause due to the amount of water being pushed up into the expansion tank... or just a huge air lock in the system?

Just back from trying 2 different caps still got few bubbles in overflow tank. Usually if there's a head gasket leak I would have thought the pressure readings from the cylinders would have indicated this. Was it a mot stations co2 probe they used?

i dont think it was the mot test probe they used... whether it was a strip that reacted to contaminants in the water or a complete testing device i really cant remember..was quite a while ago. Your cylinder pressure readings are very good in that all 4 are within 6 psi of each other..
Another suggested test is to pressurize the cooling sytem and see if there is any pressure loss overnight

Take it for your 20mile run, park up and leave overnight, then in the morning give the top hose a squeeze to see if it is soft or hard. (It should be soft)
Undo the radiator cap, and listen for any hiss. (There should be no hissing at all)
See where the water level is, it should be totally full, with no air gap*.

If it fails any of these tests, the head gasket is gone. (*Unless the top seal (1) on the rad cap is missing/damaged)

The radiator cap has 3 seals all 3 need to be in good condition



1- seals the top outer rim of the radiator, so coolant can't come out from under the cap. (Or air get in and break the vacuum that will draw the coolant back from the expansion tank.
2- seals the lower inner rim. As the water heats up and expands this lifts up and allows coolant into the expansion tank, it also maintains a pressure that raises the boiling point of the water.
3- is the vacuum break valve, as the engine cools, the coolant contracts, and this allows coolant to come back from the expansion tank, otherwise the hoses would suck flat.

After a couple of cycles to expel all the air from the radiator and expansion pipe and remove all dissolved gasses from the water, there should never be any visible bubbles in the expansion tank. any bubbles will be hydrogen from corrosion of metal, or combustion gasses pumped past a weeping head gasket.

Great explanation Facade

Your pressures are all quite low. Did you do the test with all the plugs out and the throttle wide open?