1999 626 2.5L Manual transmission. The car overheats at idle, but cools with higher engine RPM. What causes this?

Hmmmmm. Restricted coolant flow - worn out water pump impeller - Radiator cap - bad temp sender, bad fan switch. Is the engine actually overheating? I don't mean does the gauge read hot but is it actually hot?

The water pump was just replaced less than a year ago, so I doubt its the impeller.the caps don't appear to have any leaks, good seals throughout, not low on coolant at all. But how would I know if its actually getting hot or just misreading? Every time it starts getting to the red line on hot I rev it up for a few second and cool it of again. And both fans are functioning properly.

You have to actually test a pressure cap. If it releases pressure at too low of a pressure, the coolant will boil and won't cool anything. Man.....it sounds like a flow problem for sure - air flow or coolant flow.

Oops, I didn't answer your question - got stuck on the problem. The easiest way to check for overheating is with a scan tool to actually read what the (ECT) engine coolant temperature sensor is reading. One indicator is a really full overflow tank (if equipped) and the sound of boiling water when you shut it off hot. Darn, I really can't explain to you how I would know - I can tell when an engine is overheating by the heat coming off of it, sounds it makes, odors, etc. Monitoring the Engine Temp PID is your best bet.

Well the car overheated fully today on the way to a mechanic. Overflow coming out of the overflow tank.steaming. So it is definitely getting hot. But it goes from normal to hot in maybe 15 seconds, and then cooled off nearly as fast when revved. Until on the way to the mechanic, in which it would not cool off fast enough.