To me, it sounds like you have a water pump that is not circulating
very well at idle (maybe the vanes are eroded) in combination with a
stuck-open thermostat.  How long since you changed the coolant?

Allan

Matthew Snook <matt@SNOOKSBAND.COM> writes:

> Another heater question.  Curiosity, I guess.
>
> My heaters seem to work.  I have some leak in the back that I can smell
> briefly whenever I turn on the rear fan.  Other than that, all is well.
>
> But!  When I pull up to an intersection with a light or a stop sign, the
> temperature of the heated air starts dropping immediately.  If I sit for
> more than a minute or so, it seems to be blowing room temperature air.  Not
> as cold as what's outside, but not necessarily heated, either.
>
> Meanwhile, when driving the temperature gauge stays down near the bottom
> mark.  When I stop and idle, that needle will eventually start to climb,
> 'til it nearly touches the warning light.
>
> Then, when I take off again, the temperature gauge goes back down, and the
> temperature of the heater air goes back up to nice and toasty.
>
> What's up with all this temperature change?
>
> Matt
>