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Recently, I noticed some strange behavior in my car. Normally, when you check coolant level when cold then you drive your car and re-check your level after complete cool-down, it is at the same level you have checked it before driving. This is also the case in my car (and most other cars too). However, the strange thing that happened to me is that I checked my coolant level when cold. I drove my car and after 40 mins of driving, I checked coolant level with the engine running at operating temperature (Yes, obviously it was higher than cold). Up to this point, everything seems normal but from now on, the strange thing happens! I checked my coolant level after 48 hours and to my surprise, coolant level was higher than the last time I had checked when cold!! as if the car has produced some extra coolant by itself!!
After some inspection, I found that this abnormal behavior happens ONLY when I open and close the radiator cap with the engine running hot and then recheck the coolant when the engine is cold. In all other cases, the cold level is the same before and after driving.
You might say that AIR is entering the system somehow and causes coolant level to rise but it doesn't make sense because it should happen all the time not just when I open and close the cap when the engine is hot and running.
I have experienced this behavior two times but it didn't use to happen in the past even when I opened the cap with the engine running.
I have no justifiable explanation for this strange behavior.