Why does my AC change to heat when I step on the gas pedal?

Dear Car Talk

Dear Car Talk | Feb 01, 1997

Dear Tom and Ray:

We have a 1985 Oldsmobile Delta 88 with 64,000 miles on it. With all of the ventilation controls off, the heat will suddenly come on. With the air conditioner on, it suddenly changes to heat when I step on the gas hard, and then returns to AC when I release the gas pedal. With the controls set for outside fresh air, it will change to heat on its own. Do you know why it's doing this? -- Euclid

RAY: Did you buy it used, Euclid? Maybe this car was originally from Anchorage, and it's having withdrawal symptoms.

TOM: It's a vacuum problem. All the ventilation controls and the heater control valve on this car are operated by vacuum. And the default position (i.e., when there's no vacuum or very low vacuum) is "heat."

RAY: So when there's not enough vacuum, the blend doors and the heater control valve will begin to open and allow heat in.

TOM: When you turn on the AC or the fresh-air vent, or turn the whole ventilation system off, engine vacuum is supposed to keep the valves and doors closed. But for some reason, the vacuum you're getting is insufficient.

RAY: You may have a broken hose somewhere, or the control unit on the dashboard may be faulty and may be creating a vacuum leak.

TOM: And the reason it gets worse when you step down hard on the gas is because that's when the engine naturally produces the least vacuum. So if you suddenly need to climb a hill or pass somebody, you step on the gas, the engine vacuum decreases, the doors open, and, voila! You get heat.

RAY: Have your mechanic look for a vacuum leak, Euclid. Finding it and fixing it should take the heat off this situation.

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