Is it bad that Doug's heat is connected to the gas pedal?

Dear Car Talk

Dear Car Talk | Feb 01, 1991

Dear Tom and Ray:

My 1981 Lincoln Continental turns the cabin heat on full blast whenever I step on the gas hard. This happens summer or winter, whether the heat or AC is on. Climbing a hill requires turning off the automatic climate control or maybe opening a window or two. The local dealer has spent about 15 hours on the car and cannot find the problem. He's added Freon, and claims to have checked for vacuum leaks or problems with the vacuum operated mechanisms that could cause the heat to come on. Do you have any other suggestions? I don't particularly like 120 degree air on a hot summer day--or a cold winter day either.

RAY: Well, Doug, we don't know what's wrong with your car, but we do think your mechanic is on the right track.

TOM: It probably is a vacuum related problem. The engine naturally produces vacuum (ie. suction) whenever it runs. The vacuum is highest when the engine is at idle, and lowest when the engine is working hard (when you open the throttle to climb a hill, for example).

RAY: And that engine vacuum is used to control a number of different things. One of them is the ventilation system. On cars like yours with automatic climate control, the heat and air conditioning are on all the time. The vacuum controls a series of "blend doors" that regulate the amount of hot or cold air coming into the cabin. If the vacuum gets too weak, the doors can malfunction and the heat can come in when it's not supposed to.

TOM: If you're not losing vacuum due to a leak, your engine may just not produce enough vacuum. This can happen when an engine is old and worn out, and it may mean that an engine rebuild is in your tarot cards.

RAY: But before you do anything crazy like that, take the car to another dealer for the proverbial second opinion. He may find a vacuum leak that your mechanic may simply have missed.

Get the Car Talk Newsletter

Got a question about your car?

Ask Someone Who Owns One