Quote:
Originally Posted by Essentials
If with the car running and the radiator cap on, there is still bubbles coming up through the fluid in the expansion tank. Does that mean the radiator cap is not working correctly?
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That is the exact symptom of a leaking radiator cap
IF there is no air in the system. Bubbles come out continuously with a hot engine, and coolant is sucked in to replace the vapor gap when it cools. When a system slowly loses coolant, a pinhole leak in the cap is by far the most likely culprit.
Bubbles are to be expected from a recently-filled system that hasn't been completely bled. Less careful folks just add coolant to the reservoir until it stabilizes.
Normally, the reservoir is actually an expansion tank. Hot liquid water squeezes out of the system under pressure, and then is sucked back in as it cools.