'85 300CD - Heater only blows warm air when still

[frnorke]

My heater will only blow warm air when the car is running and not moving. Once the car gets moving, over a short amount of time (perhaps 1-3 minutes) the warm air becomes cool and then cold. Of course the temperatures outside in Virginia are cold right now also.

Is there somekind of "cold air" door that is normally supposed to be closed and controlled by the temperature "wheel" that is opening and letting in air from outside?

Anyone have any other ideas?

Thanks.

[rocky raccoon]

Bad pun, sorry.

All that stuff, the doors and flaps that port heat/cooling air around are vacuum controlled. I think you have a pretty bad vacuum leak somewhere.

Check underhood and inspect your vac lines soft rubber connectors. If they have never been replaced, the connectors are due. Look particularly closely at the small bundle of vac lines that go through the firewall. If you don't see anything wrong under the hood, you will have to pull out your glove compartment to inspect vac connections there. If nothing there, you may need to pull your HVAC control unit and possibly the instrument cluster to find the rest of the vac actuators.

Go slow and methodical and you will be successful. A MityVac tester would be very helpful.

Any other vac operated stuff not working? Door locks, trunk or fuel door, seat backs, engine stop. That may offer a clue.

[my123ca]

Originally Posted by frnorke

Is there somekind of "cold air" door that is normally supposed to be closed and controlled by the temperature "wheel" that is opening and letting in air from outside?

Anyone have any other ideas?

Thanks.

Yes there is the fresh air flap but that is normally closed when you have the temp wheel in heat mode. Check your aux. pump if its working. Pull the plug on the mono heater valve. That should give you full heat.

[fdanielson]

The heat is totally controlled by the flow of hot water and there is no blend door like you may see in other vehicles.

You most likely have a bad monovalve diaphragm. When those split the water pressure gets behind it and makes them do funny things. The classic test of removing the monovalve electrical connector can help but I've had instances where the malfunctioning valve still did not stay open. The aftermarket monovalves sold today are not always great either. My heat had similar problems to yours and didn't work correctly until a pulled the heater hoses and backflushed the heater core with a garden hose and then trolled ebay until I got a genuine Bosch monovalve insert.

My auxiliary pump was also not running due to dirty contacts on the k1 relay in the pushbutton control unit. Cleaning those contacts restored the operation of the pump and the footwell flaps. Even though people say the pump is not needed if it is not running it acts as a restriction to flow. So it should either be working correctly or bypassed.