Heat only while truck sits idle

Hi flushed heater core, flushed and burped system and changed thermostat. Heater fan works great. But only getting good hot heat while sitting idle
Any ideas?

 

Fan still works when you are in motion? Might be blend door problems. If you look under the heater controls (got to get on the passenger side floor) move your fresh air circulation handle and watch all that stuff move down there. Temp Control should move the cables as well. I have a 98 and the HVAC controls are cable driven. Not sure if your 99 would be as well. Need some more troubleshooting before you can really tell.

 

Yes fan works while in motion, yes cables move when checked blend control. Also check cables under hood

 

Seems like something in the blend door to me. When you move the air from outside pushes on it enough to open it up.

 

Sounds like there might still be air in the hoses and coolant isn't circulating properly.
Does the new thermostat have a bypass hole in it?
Squeeze both radiator hoses. You should hear/feel the "Giggle valve" in the thermostat "tinking"

Start the engine cold with the radiator cap OFF.
As the engine warms up coolant will spill out a little, then the level might drop when the thermostat opens. It won't hurt anything to run at full operating temperature without a radiator cap as long as you keep it topped of.
This way you'll get all the air out.

Replace the cap. (check the seal for cracks too)
Note coolant level on overflow tank.
At idle, where does the engine temp gauge usually sit?
After driving for 20 Minutes. Where is the engine temp gauge?

If results don't change.
Next, after the engine cools. Disconnect both heater hoses and hook up a garden hose to one heater hose. (you can just hold the hoses together by hand) You may be lucky and just need a cleaning.
Disconnect the hose and flush it the other way.

 

These steps are all completed, temp gets up to normal run temp when idle and heart works good while sitting idling. System has been flushed. No cracks in rad cap

 

Heat works great blows hot while sitting in driveway running

 

At idle with the heat on, accelerate to cruising speed. If your heat then stops/slows you have a vacuum door that's most likely acting up.

 

Did you have good heat when moving before you replaced the T stat and flushed everything? If so, I'd hazard a guess that the T stat is stuck in the open position.

 

Where is vacuum door? Is it something I can fix or check on my own?

 

No heat before while driving prior to changing t stat

 

I once had an old truck that had a worn water pump impeller, lots of miles on it. it wouldn't circulate water well. when parked it seemed fine but when moving down the road the water spent too much time in the radiator and heater core and would get cold. It was like at faster rpms it just swirled the water around in the pump instead of moving it. I dont know... just something I ran into once. maybe try looking in the radiator to see if the coolant moves when the thermostat opens? GL

 

What temperature is your new thermostat? If it opens at too low of a temp, you'll only get any heat when you're parked.

 

New T stat is rated for 180

 

99 is all cable driven climate controls so disregard vacuum comment.

 

I had a buddy with this issue. Burp the system. I bet you there is an air pocket. His car wasn't overheating and at idle he had heat but when he started driving no heat.

Also check the heater valve. make sure it is opening all the way.

 

Probably not that, then.

I know you said that you burped the system, but I wouldn't be surprised if it still had an air bubble or two in there that was the culprit. If your T-stat is working right (it should be; it's brand new) then a bubble is the most likely cause.

 

If there is air in the system you can elevate the nose of the truck to get the radiator cap above the heater core hoses and burp it just to make sure - other than that taking some dash panels off to inspect the HVAC would be my next move...

 

I have to ask... are the heater hoses installed correctly?