At idle, your pump is just pushing less hot water through the heater. They all do that to some extent. Even with an S50 engine in my car I can feel the heater less effective at idle than when driving. But the heater core is just a small radiator, and can get plugged just like the main radiator -- that would feel like the heater valve was not all the way open.
If it was the thermostat, the water temp gage would read low. |