maverick54

Noticed the coolant level was quite low in my car. So carried out a number of checks over the past week. No external leaks anywhere, did a pressure test.. Did a sniff test and no exhaust fumes in the coolant. Car not overheating. The only strange thing I have found is when my car as been standing overnight outside, sometimes for upto 12hrs and I remove the coolant bottle cap, the coolant is still under pressure and the coolant comes rushing back up into the bottle with some force. Now I know when its hot and under pressure the hoses can expand and fluid can fill these hoses and cause the coolant level to drop, but surely as the engine is switched off and the coolant begins to cool should it not push its way back up into the expansion tank giving me a correct level? Not sure how the pressure expels itself from the system as it cools. Could a faulty expansion cap couse this problem to happen?