Dear Tom and Ray:

If anyone ever told this female that I'd be looking forward to an automotive column every week, I'd never have believed it. But it's true! I'm writing because our 1989 Mercury Marquis has an "ailment" that our mechanic cannot solve. My husband says I should "forget about it." But I happen to be the family member who worries about everything. When our engine is cold, the Check Engine light flashes on and off whenever we accelerate after a Stop sign. After the engine warms up, the problem disappears. Our mechanic told us that as long as the light doesn't stay on, it isn't a problem. He recommended changing brands of gas for a while. That didn't work. Please give us your expertise. -- Iola

Tom: We don't have any expertise to spare, Iola, so we can't give you much. But we can give you more than that mechanic who told you to change brands of gasoline. That's the lamest advice I've heard since my brother suggested we write a weekly newspaper column together.

Ray: It has nothing to do with the brand of gas you use, Iola. The Check Engine light is turned on by the car's computer when some piece of the engine management system is malfunctioning. When one of the engine sensors sends a signal to the computer that is out of normal range, the Check Engine light comes on.

Tom: So it could be any number of sensors. Fortunately, you don't have to count on us to guess which one. All you need is a mechanic who knows what a Check Engine light means. Most of the time, the computer stores a "code" when it activates the Check Engine light. And all your mechanic has to do to diagnose the problem is plug his scanner into the car's computer.

Ray: And usually, the scanner will then spell out what's wrong. For instance, the scanner might say "coolant temperature sensor," or "oxygen sensor."

Tom: Or, if the scanner is broken, it might say "Try different brands of gasoline, Iola."

Ray: If the computer hasn't stored a code, then your mechanic has a couple of choices. He can dig deeper, in which case he'll probably want to keep the car overnight and scan it when it's cold. If he's scanning it when the fault actually occurs, the scanner should give him the answer.

Tom: Or, alternatively, he can start taking educated guesses by replacing sensors one at a time. I'd look first at the sensors that kick in when the engine is cold, since that's when your light is flashing. And I'd start with the coolant temperature sensor and the MAP sensor (that sensor reads the Manifold Absolute Pressure, not the maps in the glove compartment). Good luck, Iola.

Tom and Ray share secrets mechanics don't want you to know in their 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.

© 1996 by Tom and Ray Magliozzi and Doug Berman Distributed by King Features Syndicate, Inc.

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