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I recently got a used, ’01 Subaru Forester (non-turbo) for my wife and shortly after while on our first road trip in it to Bear Lake, UT the coolant overflowed from the reservoir tank and the engine overheated. At the time, I replaced the thermostat because the upper radiator hose was boiling hot and the lower radiator hose was warmish/cool. I also replaced the radiator cap just in case to help get us home. While refilling the coolant though and ever since, I’ve noticed bubbles still in the system even after purging it and if I don’t keep an eye on things, it will eventually build up enough air to stick the thermostat shut and/or boil the coolant over. The symptoms are typical of a blown head gasket or cracked cylinder but every test I do, the engine passes!
Audible Compression Test
Radiator Compression Test (@ hot and cold temps)
Chemical Block Test (@ hot and cold temps)
Cylinder Compression Test (@ hot and cold temps)
Leak Down Test (@ hot and cold temps/bottom and top of compression stroke)
Oil Sizzle TestCould it still be a head gasket or cracked cylinder issue? And if not, what else would cause air bubbles to build up in the system like this? How could I test for that?
Other pertinent facts are that the pervious owner had the head gasket replaced about 1yr/12,000mi ago as well as the radiator and water pump. I get hot air from the heater core and coolant appears to be flowing well so long as the air bubbles don’t build up at the thermostat. Currently I have the thermostat out because on non-turbo Subarus, the thermostat is upside down instead of on it’s side so air gets trapper there very, very easily. I’m almost wondering now if I’m over reacting about these little air bubbles, that it’s normal to have some in the system and that the real issue is with Subaru’s thermostat design…
I forgot to mention, I also put dye into the coolant when I refilled it and there are no visible leaks on the exterior of the engine.
Never ending air bubbles in the cooling system is likely from a bad head gasket. Compression gasses are pushing past the head gasket into the water jacket. I’m guessing that when they replaced the head gaskets a year ago they just replaced the gaskets and didn’t have the heads resurfaced at a machine shop.
That would contradict all my test results though. Compression is solid and the bubbles don’t contain combustion gasses.. Are there different conditions I can try with my testing? I’ve already tried at both temperature extremes, double and triple checked too because I was convinced it had to be a leaking head gasket or crack.
I wouldn’t expect a compression test to show anything because it’s a very small type of leak. If it was a large leak it would blow the hoses off. What you could do to confirm a head gasket leak is apply compressed air to each cylinder individually using a compression tester fitting and then watch for bubbles in the radiator. It might take a few minutes for bubbles to appear. Test with engine off, of course, and test cold engine vs warm engine. Make sure that the transmission is in neutral (if it’s a standard transmission) before you put the air pressure to it because the engine will turn over suddenly as the pressurized cylinder moves to the bottom of it’s travel.
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