Mechanics

WHY IS MY CAR OVERHEATING?

2007 Pontiac G6 • 91,000 miles

2007 pontiac G6, 3.5 litre engine. My car overheats as I drive it, and the heater does not get warm. It is winter here about 36 degrees and it over heated after driving in less than 5 minutes. Sometimes if your on the hiway for a long stretch the heater will get warm but if you slow down it cools. Also noticed the fan seems to stay on a long time after you stop the engine. We replaced the thermostat, added coolant, and there doesnt seem to be any leaks, although I'm not sure the water is circulating. Any help on what might be wrong would be greatly appreciated
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Dajewoo
December 26, 2011.



You're right about lack of circulation. The clue is you'll find the upper radiator hose near the engine is hot but there is no hot coolant circulating into the heater in the dash. If the system was low, including after replacing the thermostat, there is likely an air pocket that has to be bled out. Look for a bleeder screw near the thermostat housing. If there isn't one, look for a sensor near the thermostat that can be removed to burp the air out. Thermostats only open in response to hot liquid, not hot air, so an air pocket will prevent it from opening and cause overheating.

If you bleed out a lot of air but the problem comes back in a few days, suspect a leaking head gasket as the most likely cause. It will continue to allow air into the coolant and coolant will go out the exhaust. There are two different tests your mechanic can perform to identify a leaking head gasket.

Caradiodoc
Dec 26, 2011.
Couldnt find a bleeder valve or sensor to take off, its pretty crowded, but took off each radiator hose and filled with coolant. Seemed to remove the air and now is working great again. Thanks for the advice, it helped put me on the right track

Tiny
Dajewoo
Dec 26, 2011.
Dandy news. Can't argue with success. Keep an eye on the coolant level in the reservoir. If it starts going down, you can add a bottle of dye to the coolant, drive the car until the level goes down some more, then search with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. If you find it inside the tail pipe, suspect a leaking head gasket.

Your mechanic can also perform a "sniffer" test. That involves drawing air from the radiator through a glass cylinder with two chambers partially filled with a special dark blue liquid. If combustion gases are present, that liquid will turn bright yellow. Combustion gases get forced through a leaking head gasket, or less commonly through a crack in the head, and cause that air pocket by the thermostat, and when the engine is off and cools down, coolant is drawn into that cylinder and forced out the tail pipe later. Coolant can also be sucked into the cylinder while the engine is running.

Caradiodoc
Dec 26, 2011.