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Why does my heater blow cold air?

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Dear Tom and Ray:


My 1980 Chevy Luv pick-up truck runs and behaves just fine. But last week I noticed a strange thing that happened twice. Once while discharging a passenger and again while waiting for a longer-than-average red light, the heater started blowing cold air. When I resumed movement the heat came back. Any ideas on what could cause this?
Carroll

RAY: Your coolant level is low. The heat that warms the passenger compartment is taken from the engine coolant as it passes through the heater core. But the heater core, which is behind the dashboard, is at the "end of the line" in the cooling system. That means it's the last component to get coolant, and if the level were low, it would be the first thing to be deprived.

TOM: I had the same problem when I lived in a third floor apartment that had steam heat. I was the last one to get the heat. So while the people on the first floor were lounging around in their Bermuda shorts, I had to wear three pairs of Bronko Nagurski Long Underwear just to keep my teeth from chattering.

RAY: Moreover, Carroll, the fact that this happens when the car is stopped adds credence to our theory. The coolant is circulated by the water pump--which runs off the engine. So when the engine is running slowly--like when you're discharging a passenger or waiting at a red light--the pump is pumping slowly. That's when you notice the heat loss. If you continue to lose coolant, you'll eventually have no heat no matter at what speed the pump is turning.

TOM: Top up the coolant and you'll get your heat back. But more importantly, have the system checked for leaks. If left unattended, coolant leaks can do more than just make your teeth chatter. They can lead to serious engine problems--problems that even Bronko Nagurski Long Underwear can't solve.

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