QuestionQUESTION: At start up, the heater/defrost/front vents will only blow cold/ambient air, while at idle, even after the car is completely warm (thermostat shows mid way). It will begin to blow hot air only after the car starts moving. I can stop and idle and it will continue to blow hot air. I can't find anything that tells how the system is supposed to operate, but believe there should be a vent that will open after the car is warm and may be sticking. Am I on the right track? Can you point me in the right direction?
ANSWER: Steve,
I would replace the thermostat with a new one first. Next, I would be looking at the heater hoses and see if there is a restriction in the incoming water hose to the heater core. Those are the two most common causes. Now, I cannot see that your heater controls would work while running and not at idle. That is why I think it is a flow problem and not a blend door problem.
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QUESTION: Okay thanks, I'll check flow to the heater core. Just to give a little more detail, after stopped(engine off) for a while (~5-10 minutes), I hear the vents changing position. Is this change based on time or temperature?
AnswerSteve,
This is the vacuum leaking off since there is a reservoir that holds vacuum for a time. It is used for backup vacuum like when you accelerate and the engine vacuum drops. Also, when the vehicle is off. But, it will not hold forever. If the vacuum reservoir has a leak or the check valve is defective you could be loosing vacuum and that could cause some problems with the vacuum actuated doors. But, for your heating issue this would not cause it. That is, unless the heater blend door is moving to the cold setting uncommanded.
