No Heat at Idle/Heat Only at Acceleration.

Davidson Mott

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I'm trying to hunt down an issue I'm having with no heat at idle or yet alone just cruising on my 2000 GMC Yukon Denali. About the only time I get heat is when I'm either accelerating or when the engine is under additional load. It was doing it before I changed the heater core and still doing it after changing the heater core. The heater control valve is in good shape and working. I have bled the cooling system multiple times and yielded no result change. Also the inlet and outlet are pretty warm as well.
 

Schurkey

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no heat at idle or yet alone just cruising on my 2000 GMC Yukon Denali. About the only time I get heat is when I'm either accelerating or when the engine is under additional load. It was doing it before I changed the heater core and still doing it after changing the heater core.
Typical symptom of low coolant level. Takes water pump RPM to toss some hot water into the heater core.

I wouldn't rule out a failed thermostat.

Seems unlikely that it's an air-door problem as it does get hot...sometimes.

The heater control valve is in good shape and working. I have bled the cooling system multiple times and yielded no result change.
Consider raising the front of the vehicle as high as you can jack it up, and using one of those Lisle radiator funnels. Fill the funnel, rev the engine, see if you can move any air out.

Verify operating temperature of the engine using a scan tool, or a thermometer in the radiator fill neck.

Also the inlet and outlet are pretty warm as well.
Pretty warm? They should be so friggin' hot you can't hold them. 180+ degrees for the inlet, mabye 170-ish for the outlet hose.
 
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