I drive a two hour drive twice a week. The heat works in my vehicle quite fine but two or three times per trip I get this blast of cold air through my vents. I don't understand why this is happening. I just wonder if anyone else has this problem.

Thanks.

 

I just started having this problem where when i stop so does the heat but when i start going again its warm again.

 

does having diffrent vent positions or heat selection make any diffrence. you might have a air bubble in your cooling system, does your engine temp ever jump slightly?

Heffty i think your heater core might be going out, it could also be your water pump but if the engin heat is good then it is probably the core.

 

I'm not convinced this is a heater core problem. Ever since we bought our '99 Blazer with 39K miles, it has occasionally done this - but ONLY when going up hill or otherwise in a situation where vacuum would be minimal. I think that in our case it's caused by a drop in vacuum to the air control flaps, allowing them to default to vent position until we let up on the throttle and vacuum is restored.

When does yours do this? Is it tied to acceleration or pulling hills, which would be a low-vacuum position?

(See, I'm old enough to remember vacuum-driven windshield wipers, and they NEVER worked when going uphill because that meant wide open throttle, which reduces vacuum available. For those of you who know John F. Kennedy's quotes: "Ich bin eine old fart.")