1999 Pontiac Sunbird Overflow cap vacuum

Hi there,

I would say that the cap has deteriorated with age and the seal has become dislodged, the fact that it has sealed it self with a vacuum onto the filler is a sign that the cooling system is working (at leased sealed) as required, what has happened is that as the engine coolant heats up it expands and pressure is built up, this pressure is required to raise the boiling point of the coolant, when the coolant cools the pressure drops and the valving in the cap allows air back into the reservoir, the seal has failed and the vacuum created by the cooling coolant has dislodged the seal and stuck it on the filler, replace the cap and all should be ok.

Mark (mhpautos)