i have a 97 cavalier the coolant is starting to bubble and overflow the water pump and radiator have been replaced recently the fan comes on when i start the vehicle (sometimes not always)but will not turn on when the a/c or defrosters are on nor when the temp gauge is high (not overheating but high) my question is why would the fan turn on when the car starts but not when the ac/defrosters are on or when the gauge is reading hot?

 

Does it have electric fans?

 

yes it does i just tryed swaping out relays with no luck im leaning towards overflow cap (not pressurizing the system) or thermostat?

 

could be, check the cap first. if you need you can usually rent a tester from the big name auto parts store. as far as the thermostat... i don't think it is the culprit. if all else fails and you still do not have a cause, your comment about bubbles makes me think of a busted head gasket causing combustion gases to leak into the coolant passage

 

if it was a head gasket wouldnt the car overheat?wouldnt there be a sign of a busted head gasket like anitfreeze in the oil.or white thick smoke or hesitation in the vehicle?its not taching out like its overheating i watched the gauge when it got hot the fan still didnt come on