Dear Tom and Ray:

I have a 1989 Honda Accord Lxi which knocks and pings when it is under load unless I use super premium gas. What's actually going on when it does this? What is pinging? -- Kenny

Ray: I asked my 12-year-old son what pinging is, and he described something he does after he picks his nose. But I assume you're looking for more of an automotive explanation.

Tom: Pinging, knocking and detonation are all terms for a condition known as "pre-ignition," Kenny. That means some of the gas and air mixture is igniting in the cylinder before it's supposed to. And when that extra flame front crashes into the normal flame front (created by the spark plug), you hear a noise, and that noise is pinging.

Ray: The CDC (Centers for Diesel Control) in Atlanta tell us there are four major causes of pinging. The first three are related to excessive heat.

Tom: The first is severe engine overheating. If the engine is running too hot for any reason, the temperature in the cylinders can simply be too high. In that case, some of the mixture can ignite before the spark plug fires just from the intense heat.

Ray: The second major cause of pinging is carbon buildup inside the cylinders and on the pistons. When too much carbon collects, it can reduce the size of the cylinders (increasing the compression and temperature of the cylinder contents) and retain excessive heat itself.

Tom: The third common cause of pinging is a malfunctioning exhaust gas recirculation (EGR) system. The EGR is supposed to send non-combustible exhaust gas into the cylinders to lower the temperature of the mixture.

Ray: How does 800-degree exhaust gas LOWER the temperature in the cylinders? Because it doesn't burn, and it crowds out some of the oxygen that would have burned and made an even hotter flame. And if that EGR is not working properly, the cylinder temperature can be too hot and the engine can ping.

Tom: And finally, bad timing can cause pinging. The ignition timing determines when the spark plugs fire. And if they're set to fire too early, the stuff will begin to burn too early.

Ray: The use of higher-octane gas often makes the pinging stop because it has a higher ignition point. By requiring a higher temperature to make the gasoline burn, you reduce the likelihood of it "pre-igniting" somewhere else in the cylinder.

Tom: But premium gas doesn't address the underlying problem. So start by checking the EGR system, the cooling system and the ignition timing. And if none of those things fix it, then you can consider increasing your donation to your local oil company by permanently using a higher-octane fuel.

How can you tell if a used car is in good condition -- or even OK, for that matter? Find out by ordering Tom and Ray's pamphlet "How to Buy a Great Used Car: Things That Detroit and Tokyo Don't Want You to Know." Send $3 and a stamped (55 cents), self-addressed, No. 10 envelope to Used Car, PO Box 536475, Orlando, FL 32853-6475.

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

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