Fluctuating heater temperature

i have a 2001 chevy blazer. it might be the worst car i’ve ever owned. i live in alaska, so temperatures right now are around zero. i warm up my car for about ten minutes before driving it, and when i get into the car, the fans are blowing out moderately warm air. when i start pressing down on the gas pedal, the heat comes out very hot. however, if i come to a stop for more than 20 to 30 seconds, the air coming out turns ice cold. but then gets very hot again as i start to accelerate. what gives and how much is this going to cost me?

The problem might be with air in the cooling system.

Get the vehicle up to operating temperature. While the engine is idling, loosen the upper radiator hose clamp slightly. Take a flat tipped screwdriver and slip it between the radiator hose and the neck of the radiator and see if any air purges from the cooling system. If you’re sure there’s no air left in the cooling system, then the problem might be with worn/erroded waterpump impeller blades.


If the air temperature door in the HVAC system is vacuum controlled, and there is a vacuum leak, it could do exactly what you describe. When you are idling there is high vacuum, and the temp door moves one way. When you accelerate there is lower vacuum, and the door moves the other way. Look for a leaking or loose vacuum line, or a leaking vacuum actuator on the temp door.

This is expected. When the engine is working harder it produces more heat.

Mmm… that’s not exactly right. Even when the engine’s on the cooler end of it’s operating range, the hot air should still be coming out uncomfortably hot-- somewhat less than 195 or so degrees that the thermostat keeps the cooling system at. You shoudn’t really be able to tell the difference between the temperature of the air when it’s on the cool end of the operating range and the hot end. If the thermostat is stuck open, however, you may be able to tell the difference between the cool air and lukewarm air, which is what it sounds like might be happening.

I’d say the air in the cooling system is a possibility as is a vacuum leak, but the first thing I’d do is change the thermostat-- it’s good to do every so often anyways.

I’ve NEVER owned a vehicle that would pump out cold air when when idling…Even in temps at -20.

When the engine is COLD, make sure the radiator is full and the overflow bottle is about 1/2 full.