My car has been losing coolant slowly for months without visible leak anywhere, overheating, oil in coolant, milky oil, sweet smell and bubble in recovery tank. The coolant level in tank droped about 1cm every 30-40 miles driving in average. Coolant level kept lowering even after it was below MIN marking.
There is minor condensation on under-side of oil fill cap, it always reappears few days after I clean it, does this show a very minor water or coolant keeps going into oil? But oil from dipstick looks clear and clean.
3 minutes after cold start, much damp white smoke and a few drips keep coming out from tailpipe for about 15 minutes then completely disappear after engine completely warms up.
Every time when I remove the overflow tank cap with completely cold engine, I can hear a popping sound that seems like some air in coolant system escaping out. Does this show a small pressure from engine is leaking to coolant system?
For pressure test, the coolant system can hold 18 psi pressure very well for more than 5 hours with cold engine. When I kept connecting pressure tester to overflow tank and started engine without pumping pressure, the pressure on gauge didn’t increase quickly, stayed at 5 psi after engine warmed up, then I pumped it to 15 psi, it slowly went up to 19 at most after 1 hour idling, and finally when engine got cold I saw coolant level was 0.5cm lower (or about 5oz less) than it was before this test. But I still did not see any external leak.
Moreover, the recovery tank cap has been changed, the dealer also checked transmission pan and spark plug, and did TK test (combustion gas detection) , they couldn’t find the leak too.
I’m at a loss. Any suggestion? Could it be a very small and early stage blown head gasket or head crack without obvious symptoms?
There comes a new clue. As I mentioned, after 1 hour high pressure test with engine idling yesterday, coolant level droped 0.5cm. Today I started my car and just leave it idling for half hours without pumping additional pressure, after engine was cold, I saw the coolant level still droped about 0.5cm. So no matter the engine was running for 1 hour or half hours, and no matter there was high pressure or normal pressure in coolant system, my car lost the same amount of coolant. It seems the car loses coolant only in the first half hour after engine cold start. What can be the cause?