New problem developing with the 1990 300ce. Once I let the car warm up the heater blows nice and hot as long as I'm moving. If I stop at a stop light though, it starts blowing whatever the outside temperature is. So I have to turn the heater off at all of the stop lights and turn it back on when the light turns green. Not a huge hassle, but something that I'd definitely like to fix if its not too complicated. Ideas?

Thanks

 

My guess would be the thermostat. It's not closing when the coolant temp drops.
I've had that with other cars.

 

Is there a way to repair a broken thermostat or would I need to replace it? Also, I don't have any other problems with the air conditioner or the heater except when it's at idle. Would that rule out the thermostat or is it still consistent with the problem I'm having?

 

It's consistent. And as far is I know, you can't fix a thermostat. But they're simple and cheap (assuming Benz uses them, and not some computer controlled diversion flapper, or some other obscenely expensive widget).
What the thermostat (more accurately called a vernatherm) does is:
When the coolant is below a certain temperature, it prevents it from flowing through the radiator. When the coolant reaches temp, the thermostat opens and allows the coolant to go to the radiator.
The thermostat uses a spring that has metals that contract and expand at different rates, and it slowly opens and closes based on temp.

So it constantly adjusts the amount of coolant going to the radiator based on coolant temperature, and works to keep the coolant at a relatively constant temp (outside of extreme outside temps).
One reason that it prevents coolant from going to the radiator when the coolant is cold, is to heat up the coolant faster so you get heat in the car faster (the engine also works best when engine is within a certain temp range that it's designed for).

So, when it's cold and your car is idling, the engine is not producing nearly as much heat as when it's producing power. When I was young and poor, the thermostat on my Jeep stuck closed. I had to pull it out, and couldn't afford to replace it, so coolant was always going through the radiator. I could watch the coolant temp go up and down when driving in cold weather. Up when applying throttle, down when idling.

Now, this is only one possibility, but it's the easiest and cheapest to check and fix. Another possibility is a restriction in the heater core, and when coolant pressure from the water pump goes down (at idle), enough coolant can't flow through to keep it warm.
Also check for kinked coolant hoses, as that could have a similar effect.

That's the cheap and easy stuff. If those aren't the solution, you're over my head.

PS: Here's some good troubleshooting info to see if your thermostat is working properly:
http://autos.yahoo.com/maintain/repa...ques018_2.html