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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.

1 Answer 1


A possible explanation, depending on how you are doing your checks on coolant level -

If you open the expansion tank while the coolant is hot, you will be releasing hot air. As the system cools down, due to contraction of the cooling air you will now have a low volume of air in the expansion tank and hence a low pressure that will pull coolant into the expansion tank making the volume appear to increase.

If you open the expansion tank when the coolant is cool, you will be allowing air into the expansion tank which will be at atmospheric pressure which will allow the coolant to drop.

  • What I get from your answer (in simpler terms) is that when I open the radiator cap (my car doesn't have an expansion tank) when the engine is hot, the hot air above the coolant is released and there will be less pressure to push the coolant level down when it cools down.
    – LFY MP7.3
    Commented Jul 1, 2022 at 20:45

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