Why does my heat only work at the "90 degree" setting?

Asked by Feb 09, 2016 at 09:20 AM about the 2007 Chevrolet Silverado 1500 LT1 Ext. Cab SB 4WD

Question type: Maintenance & Repair

Any setting below 90 degrees and the heat will turn to cold air. Blower motor
works across all settings (low to high) and the air conditioning seems to be
working fine. I recently flushed my coolant and  made sure to burp the heater
core. Never encountered a problem like this before and not sure where to begin
diagnosing the issue. Any help is much appreciated.

2 Answers

37,975

Could be the blend door actuator that controls the temp. blend door.

65

I believe you still have air in your coolant system. This may work for you. While the engine is cool, open you radiator cap then start your engine. Turn your heat on full blast at the hottest temperature setting. With the car idling, pour more coolant directly into the radiator until no more can be added. You should see small bubbles coming up. Next, slowly rev the engine a few times and then repeat the process of adding more coolant directly into the radiator again. Revving the engine slowly will draw more coolant into the system and push air out by spinning your water pump. Do this entire process as many times as you can.

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