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THE GOODS : Computer's Confusion Can Cause Hesitation

October 21, 1994|RALPH VARTABEDIAN | TIMES STAFF WRITER

Question: I have a 1986 Chevrolet Celebrity with a four-cylinder engine. When I suddenly accelerate from either a stop or to pass another car, the engine hesitates a few seconds. I have changed the fuel filter and the transmission fluid and run a few cans of fuel injection cleaner through the gas tank, but nothing has corrected the problem.

Please advise.

--J.W.S.

Answer: The remedies you have tried are inexpensive and simple, but miss the mark. The problem is not likely to be caused by a dirty filter or clogged fuel injectors, but rather by the complex computer system that operates the engine.

The 2.5-liter "Iron Duke" engine in your Celebrity has two sensors--the throttle position sensor and the manifold absolute pressure (MAP) sensor--that help the car's computer determine the correct amount of fuel to feed the engine.

In a fuel-injected engine, when you push down on the accelerator pedal, more air flows into the engine's intake manifold. A cable from the pedal opens up a valve that controls the amount.

The engine needs extra gas for additional air. So, the pedal is also connected to the throttle position sensor, which allows the computer to order the fuel injectors to squirt more gasoline into the manifold.

The throttle sensor, a rheostat, has copper wire wound around a core and a copper brush that moves along the wire. The wires can become corroded and the rheostat fails to operate reliably.

The second possible problem is the MAP sensor, which measures the vacuum inside the intake manifold where fuel and air are mixed.

When you depress the accelerator and the extra air rushes into the manifold, the vacuum level drops. The MAP sensor detects this and tells the computer the engine needs more fuel.

Each sensor provides a separate reading of how much power the driver wants. If one fails, the computer will need a few seconds to determine something is amiss and get information from the backup sensor to make a correction.

The effort by the computer to correct the discrepancy between sensors is probably the hesitation you are experiencing.

Here's one way you may be able to narrow down which sensor is failing. The MAP sensor also serves to tell the computer the outside barometric pressure, which is affected by weather and altitude. As the pressure drops, the computer supplies less gasoline to the engine. So, if you also have the problem on low pressure days or on trips in the high mountains, it is likely that the MAP sensor is the culprit.

Also, any mechanic with a diagnostic computer can quickly determine which sensor is failing.

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