Strange Cooling Issue

[Kabong]

Greetings gurus,
I am having a strange issue with my baby, and I figured this was the best place to start. While driving the engine stays at a normal temperature. However, occasionally when at idle the engine heats up rapidly. I can compensate for this by putting the car in neutral and revving the engine until the temperature needle drops. Once moving again it is fine. I say only occasionally, as sometimes it is not an issue at idle, although the frequency of this has been increasing since I first noticed the issue.

I suspect it is either the thermostat, or water pump, but could it just be the fan sensor? I would rather not spend extra cash on the parts if not needed, as far as I can tell the fan comes on normally.

Thanks

[courtenay]

The most important part of the cooling system is the air that passes through the radiator when the car is moving. When the car is stopped, and idling, the only air passing through the rad is the air being sucked through by the fan. As a result, your temp will increase until you start driving again. Unless you're getting into the red range on your temp gauge, this is quite normal.

[Kabong]

It does get up to the red range if left unchecked. When I see it rise I rev the engine to about 3500-4500 rpm and it will start to cool off.

[So Cal Mark]

sounds like the thermostat is installed incorrectly, but does the fan come on when idling? If so, the temp still climbs with the fan on?

[NSDave]

Does anybody know if the water pump impeller blades are plastic or metal? If plastic they might be worn, and the water pump not circulating enough water at idle. Maybe the pump's belt is slipping? How's the tension?

[Kabong]

It looks like the fan is not coming on all the time, I'm going to change out the switch. Hopefully this does the trick...