While on a trip today (around 300mi) I pulled into a parking lot to move some cd's around and left the engine idling while I was doing this. When I pulled into the lot, the temp gauge was at the normal 1/2 way up position, but when I pulled back onto the road I noticed the temp needle rapidly going towards the red zone. I turned the heater on high to try and cool things down as much as possible and pulled back over. Checked under the hood and didn't find any leaks. After letting it sit for a while, the temp came back down so I took off again and haven't had the temp come up that high since (stayed steady back at the 1/2 up position). I'll probably try bleeding the coolant tomorrow, but other than a small air pocket or something what would cause something like that?

 

Faulty thermostat. It is not opening when its gets to its boiling point to allow recirculation of the coolant flow.

 

Ditto, Thermostat is sticking. Next time it happens just rev the motor a few times. Usually the extra pressure will get it to move.

 

Don't really need a third, but I'll do it anyways.
Agreed, thermostat. Had that happen once or twice before.

 

except i'll add- just replace the frickin thing... lets not dick around here. it's cheap and easy to do, so just put in a new one so you won't have to worry about it.

 

its true that it could be the thermostat but u might also have a blockage in the rad. so if u change the t-stat take the rad off and flush it out.