Heater blows cold at idle...

[93 x_SPORT]

This might be the most ridiculous thing you ever heard, but my heater blows cold at idle. It could be cooking you out of the truck but when you stop at a light or go thru the drive-thru the thing blows cold. Then when you get going again, it blows warm. The temperature gauge stays steady the whole time. Any ideas why this would happen?

This is a '97 sport with Automatic and 4 wheeldrive.

 

[BrooklynBay]

The pump turns faster when the engine RPMs go up. If you are low on coolant, it's probably not circulating in a continuous flow. If you have an electric heater control valve, the voltage going to it could also fluctuate if the valve is not opening all of the way, due to a partial clog. Your water pump could have a partial clog on the area going to the heater core. First check your coolant level, and troubleshoot from there.

 

[93 x_SPORT]

Thanks for the reply! I will check the coolant level before I leave work tonight. Hopefully it's something simple like that.

 

[93 x_SPORT]

It appears that the coolant is low. Will add some tonight. Hopefully that will clear up this issue. Hopefully I didn't burn up or shorten the life of my water pump.