August 1989

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Dear Tom and Ray:
I have a practically new fuel injected 1978 Volvo with 173,000 miles on it. When I step on the brake while idling, the rpms drop, almost to the point of killing the engine. The brake fluid level has been fine. The mechanics are baffled. Do you have an answer?
Allen

RAY: You probably have a vacuum leak in the power brake booster. An internal combustion engine creates natural suction when the intake stroke sucks in gas and air. Modern engines use this vacuum to operate things like ventilation controls, cruise controls, and power brakes.

TOM: In your case, there's probably a leak in the vacuum system which occurs when you activate the power brake booster. This confuses the fuel injection, which constantly monitors the amount of air being sucked into the combustion chambers. The leak makes the fuel injection system think that less air is coming in, so it cuts the amount of gasoline to match. This is probably what causes your rpm to drop.

RAY: Have someone step on the brake while you put your ear near the brake booster. Vacuum leaks are usually accompanied by a hissing noises. If you hear hissing, start looking for a repair shop with clean carpets. This is a sure sign that they understand vacuums.

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© 2006 by Tom and Ray Magliozzi and Doug Berman. Distributed by King Features Syndicate, Inc.

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Auto repairs can be costly! Save money by ordering Tom and Ray's pamphlet: "Ten Ways You May Be Ruining Your Car Without Even Knowing It!" To order, send (check or money order) to Ruin, P.O. Box 536475, Orlando, FL 32853-6475. You can also order online.