Q. I recently had some major repairs done on my engine. The damage was caused by “pinging” and was avoidable, according to my mechanic. I was told the noise the engine made was a warning sign, and had I known its significance, I might have brought the car in sooner, heading off the damage. I’m not one to cry over spilled milk but thought it might be a benefit to your readers if you could spread the word about this.

Anne North, San Francisco

A. Ouch! Your unpleasant experience is certainly worth sharing with others and might spare some from a similar fate. While we’re at it, let’s look at some other common engine noises and what can cause them.

Engine pinging (preignition) and detonation are both forms of improper combustion. Pinging is a light, repetitive sound much like shaking small marbles in a tin can. The sound might range from barely audible to fairly loud. Detonation is a deeper knocking sound that is hard to miss. Either sound may occur during acceleration or increased-load steady state driving such as climbing a hill, and are worsened by higher than normal engine or ambient temperature.

How do these sounds occur? The air/fuel mixture drawn into each cylinder is supposed to be ignited at the correct instant and burn smoothly, pushing the piston downward. Instead, the air/fuel charge might preignite or explode, causing dramatically increased heat and pressure. Causes include lower-than-specified gasoline octane, advanced ignition timing, elevated engine temperature, carbon buildup within the combustion chamber, a lean air/fuel ratio or a fault in the EGR (exhaust gas recirculation) system.

How big a deal is pinging/detonation? A very brief clatter during tip-in acceleration might be considered acceptable, but frequent or continual pinging/detonation will take a toll on pistons, valves and head gaskets.

Pinging/detonation rarely, if ever, occurs during cold engine operation, helping to distinguish them from other engine sounds.

Here’s a few more engine sounds and their likely causes (each will vary with changes in engine speed).

n A continual clatter, light in tone, might be caused by improper valve adjustment or valve-train wear.

n A deep knocking sound, often worse when first starting the engine or at hot-engine idle, might be caused by worn crankshaft bearings.

n A squealing sound, usually worse during engine start-up, is likely the result a loose alternator belt.

n A continual chirping/squeaking sound is probably caused by glazed drive belts.

n And, finally, a growling/whirring sound can be the result of a faulty water pump.

Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif.

E-mail him at under-the-hoodjuno.com or write to him in care of Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. He cannot make personal replies.)

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