The Mystery of the Check-Engine Light

Question: I have a 1982 Chevrolet Impala. The check-engine light comes on while I’m driving on the freeway. Sometimes, it goes off within minutes; at other times it stays on for long periods of time. It did this when the car was new and then it stopped for a long time. Now it is occurring again. What would you check in the engine if the light comes on?--J.L.

Answer: The check-engine light is part of a sophisticated diagnosis system that is supposed to help mechanics pinpoint problems in the engine without a lot of trial and error. In many cases it works just the way it is intended, but in other cases mechanics aren’t well enough trained to know how to use the system.

When the check-engine light comes on, it means an on-board computer has detected and identified a problem somewhere inside the engine. The classification of the problem is stored inside an electronic memory and can be recalled by a mechanic. The information is stored for 50 cycles of the ignition switch, which sounds like a lot but really gives you only a week or less to get the car into the garage. If the problem reoccurs, it is stored for another 50 cycles of the ignition switch, though.

The diagnosis system can detect such problems as low voltage in the charging system, a malfunction in the carburetor that results in an improper fuel-air ratio, an ignition problem and even such concerns as a cruise control that is on the blink.

The newer General Motors cars have computers that can diagnose and store up to 50 or 60 problems. The older ones (such as on your car) are capable of identifying and storing only 15 problem codes.

To extract the codes from the computer, a mechanic uses a special instrument or scanner that plugs into a terminal under the dashboard and prints out a code that indicates where the problem has occurred. Then, the mechanic can use more traditional means to verify if there is a problem and how to fix it.

All this is great when it works. But as many motorists know all too well, the computerized diagnosis system doesn’t ensure a dependable car or an easily repairable car. That’s because cars are more complex today, and even if the computer has correctly identified a problem, it isn’t always easy to find a solution.

You should have a qualified mechanic examine the car. Most good shops have the instruments or other means to extract the codes out of the computer. Then you can know for sure whether you have an engine problem or a computer that is half-witted.

Q: I own a 1987 Honda Accord LX with an automatic transmission. Since the car was new, I have experienced an acceleration sensation when cruising at highway speeds. The engine speed will increase by about 200 revolutions per minute for a fraction of a second, just long enough to shift the transmission into a lower gear. This happens with the cruise control on or off. Honda tells me this is normal. Is it?--J.G.

A: It doesn’t seem like that sort of irritation should be normal, but it is. The Honda Accord automatic transmission has a so-called lock-up feature that physically causes the transmission to lock up its fluid clutch.

The lock-up will come undone with only moderate variations in accelerator pedal pressure. It’s worse in driving conditions when the car is going uphill, downhill, upwind or downwind, because that tends to trick the transmission into an improper shift.

The system is controlled by a cable that runs from the engine to the transmission, telling the transmission how much load is being put on the engine. This cable is adjustable, and if the lock-up feature is too sensitive, you should have the cable adjusted. But chances are that it is adjusted correctly, and you just aren’t used to the transmission unlocking and shifting with only small changes in accelerator pressure.

Vartabedian cannot answer mail personally but will respond in this column to automotive questions of general interest. Do not telephone. Write to Your Wheels, You section, The Times, Times Mirror Square, Los Angeles 90053.