Knock-knock. Who's there? Your engine.

Dear Car Talk

Dear Car Talk | Feb 01, 1996

Dear Tom and Ray:

My 1982 Toyota Corolla is equipped with a manual gearbox and has 110,000 miles. At about 80,000, the engine developed a tendency to knock under certain conditions. Curiously, it knocks most severely under moderate throttle, with minimal or no knock at wide-open throttle. I had the car tuned up and had the timing checked. 89 octane gas seems to stop it. But I wonder what's causing the knocking?

RAY: If it knocks three times, it could be Tony Orlando and Dawn. But more likely, it's a lazy Exhaust Gas Recovery (EGR) valve.

TOM: Knocking is also known as "pre-ignition." When the temperature in the cylinders is too high, or the octane (i.e. ignition point) of the gasoline is too low, some of the gasoline begins to burn before it's supposed to. And when those flame fronts of burning gasoline crash together in the cylinders, you hear knocking.

RAY: The EGR valve is supposed to open gradually as you step down on the gas. And believe it or not, it actually introduces some exhaust back into the combustion chambers. And since that exhaust burns less efficiently in the cylinders than the oxygen it's crowding out, it lowers the temperature and eliminates knocking. Your EGR is sticky or lazy, and is only opening when you apply full throttle, and not during normal driving.

TOM: So I'd have the EGR circuit checked. Make sure the valve is working and the ports are not plugged up. You might even check with your Toyota dealer, because some Toyota models were retrofitted with a vacuum amplifier and other such devices to try to beef up a weak EGR design.

RAY: But if it's not relatively simple to fix (i.e inexpensive), this is one of those cases with an older car where you SHOULD just use higher octane and forget about it.

TOM: By the way, Alexander, the EGR's role in the elimination of knock was one of those great cases of dumb luck. The EGR was actually designed to reduce nitrous oxide (NOx) emissions, which are created most readily at higher temperatures.

RAY: And when the engineers discovered that the EGR eliminated knock too, they told their bosses they were in seclusion working on the "knock problem," and took two weeks off.

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