I have a '99 Pontiac Montana with an overheating problem.?

[teresa.sass]

The only time it acts up is when the air contdt.is on or when stuck in traffic. I bought a thermostat, but am not sure if the problem is a stuck thermo, or if it could be something else. Any suggestions?
Ok, so how do I go about checking the fans to make sure they are working properly?

 

[regi @aamco.com]

check the fan

 

[infoman6617]

only a few thing cause over heating thermostat will be one either stuck open or close bad hose could or fan but id say system flush its not flowing enough or the fan is not blowing at its fullest if it was the thermo it will either take a long time to heat up or it will heat up fast .... ok try the belts first check for tightest or old and dry cracks.... if thats good then have the system flushed... and replace the thermostat also after it runs hot any time does mess with them

 

[baldie]

if it was the thermostat, it would heat up all the time, wheather the ac is on or off. try flushing out the cooling system. also make sure the fan is kicking on. some cars has 2 fans, 1 for ac and the other for cooling the engine.but i would first flush the cooling system, then check both fans.

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[donwhy60]

If the thermostat is stuck closed it very well could be the problem. While you have the thermostat out, flush the cooling system. This may help with the overheating problem.