I have a 2003 Mitsubishi Eclipse with 48,000 miles on it. During the winter when tooling along on the highway the heater blows comfortable warm air. However, when I stop at a stop light the heater blows cold air until I get underway again. And, NO, the AC is not on!
Low coolant can cause this. When driving the water pump sends coolant faster and I guess at higher pressure than at idle. The others on here are more knowledgeable and I may be off target a little.
I’d be inclined to suspect that the thermostat is stuck open and the engine is cooling down. It’s transferring a lot less heat to the coolant when it’s idling.
If your coolant is full there could be an air pocket somewhere that is causing this. Also check to see if Mitsu. has a TSB( technical service bulletin)
I have this same problem, except the heat isn’t that great when driving either…then goes almost cold at idle. Could this still be air in the system? Coolant level is good. One hose going into the heater core is very hot, the other just warm. Any ideas?
If checking the coolant proves to be full - check heater hoses going into the heater core - if one if hot and one is warm or cold you have a blockage in the heater core - you can try to backflush the core with clean water but that may not work depending on how clogged it is but worth a try before replacing the heater core
I have never felt heater hoses while working but seems to me the heater core might have a block. Don’t take my word for it though.
Let me add one more thing. Does it also do it when you accelerate? If so it may be a vacuum problem.
That sounds like what my problem is…on a 99 STS, had the notorious head gaskets go, so I think someone might have used some sort of stop leak to fix it before they were replaced properly. Could it still be air in the system though?