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Some Engine Knocks May Be Normal

January 23, 1986|RALPH VARTABEDIAN | Times Staff Writer

Question: I have a 1984 Mazda truck with engine-knocking noises when I climb hills. The problem started after a routine valve-and-timing adjustment by the dealer. They said the timing was off and they fixed it, but that's when the knocking started. Should I have the timing reset to where it was before the tuneup?--T.M.C.

Answer: The decision to "detune" your engine depends on the severity of the knock, which is sometimes called a ping. Many newer cars operate with a mild ping, and owners' manuals often advise that such a ping is normal.

Engine knock is caused by the uncontrolled and uneven combustion of fuel in your engine. Modern engines run hotter and leaner to maximize fuel economy, but hot and lean conditions also contribute to the knock. At the very least, knocking is an irritation, despite manufacturers assurances that it is not harmful to the engine.

If your engine knocks consistently, even on level roads and at constant speeds, you have a definite problem that needs attention. In certain cases, an engine can experience "pre-ignition," which occurs when the fuel mixture ignites inside the cylinder before the spark plug fires. The force of the explosion can blow a hole through the top of a piston.

You can eliminate mild knocking by switching to a higher-octane gas with more so-called anti-knock ingredients. You can also eliminate knock by retarding the spark timing from its normal setting, which essentially detunes the engine. Either solution is bound to increase your operating costs.

Deliberately detuning the engine will reduce your fuel economy. In addition, it will increase engine emissions and impair acceleration. It may cause spark plug fouling.

I suggest you opt for buying a higher-octane gasoline. Burning premium gas will cost somewhat more, but it may be no more than the additional gasoline you would have to purchase by detuning the engine. Also, it does not have the other negative side effects.

You should also experiment with different brands of gasoline. Even though their octane rating is the same, one brand may perform better in your engine. You may also get by with mixing half regular and half premium. You may eventually decide to live with the mild knock you are experiencing.

If you are determined to eliminate the knock, another possible solution is to install a knock limiter, which is a device that senses knocking and retards the spark advance momentarily. The devices are costly and unnecessary much of the time.

Q: I would like you to comment on the rubber timing belts on the 2.2-liter Plymouth Reliant engines. How long are the belts supposed to last? If the belt breaks, can it severely damage the engine?--R.A.M.

A: Rubber timing belts have become commonplace on modern engines, especially four-cylinder engines. The belts transmit power to overhead camshafts, which operate the intake and exhaust valves on most four-cylinder engines.

The grooved rubber belts run on pulleys on the exterior of the engine and perform the function that was once done by internal chains and gears. Many experts have been skeptical of rubber belts replacing metal gears.

So far, however, experience with the belts has been fairly good. Chrysler belts have accumulated well beyond 100,000 miles wear without failure. They can also be easily checked for proper timing.

On 2.2-liter Chrysler engines, a broken belt cannot damage the engine. The rubber belts will maintain correct timing as long as the belt doesn't jump grooves.

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