Knock, knock. No, we’ll spare you the bad joke, because we are discussing those annoying engine knocks.
Some people call it knock, some call it ping, others call it clatter. No matter what you call it, it is the sharp metallic sound that comes from inside your engine because of detonation or pre-ignition.
Though the sounds are similar, the causes of detonation and pre-ignition are not the same. Both, however, are a result of abnormal engine combustion–when two (or more) flame fronts crash into one another.
It is important to note that combustion in the engine’s cylinders is not an explosion. It is a very rapidly burning fire. Normally, the flame front burns smoothly across the combustion chamber, building pressure, as it consumes the air/fuel mixture.
When flame fronts collide, pressure in the combustion chamber rises rapidly and at the wrong time. We hear the knocking noise as the pistons, their connecting rods and bearings resonate.
Knock typically occurs in hard acceleration, such as when you are merging into highway traffic or under heavy load conditions, such as pulling a trailer up a steep hill. Brief knocking–less than a couple seconds–is nothing to worry about.
Uncontrolled knock, however, can cause some nasty engine damage. We remember reading about a Ford Bronco that was towed into the shop with a hole through the top of one of its pistons caused by engine knock.
Combustion usually occurs when the spark plug sparks. But sometimes, combustion begins before the spark. This is called pre-ignition. Detonation occurs when a second flame front is set off after the spark plug has started the first fire.
Pre-ignition usually is caused by hot spots in the combustion chamber. A sharp edge, such as a nick on a piston or a bit of head gasket protruding into the combustion chamber, can get hot enough to glow and start the fire before the spark. Spark plugs that are the wrong heat range also can cause pre-ignition, as can misrouted spark-plug wires.
In the case of the Bronco, the owner replaced the spark plugs and ignition wires himself. Being tidy, he routed and bundled the wires neatly parallel to one another, instead of the original random arrangement. Problems occurred when one ignition wire induced a voltage in its neighbor, which triggered a spark on the wrong plug at the wrong time. Ford even issued a technical service bulletin to this effect for 1987-94 5-liter V-8 engines.
Detonation is closely related to the octane rating of gasoline. The octane number we see posted on the pumps is really the anti-knock index, an averaging of the research octane number and the motor octane number: (R + M) / 2.
Engines are designed to run on gasoline with a certain octane rating. High-compression engines need higher octane. So do turbocharged and supercharged engines.
The research octane number measures the gasoline’s ability to resist knock at low speed and gentle operation, such as city driving. The motor octane number measures the fuel’s capacity to resist knock at high speeds or heavy engine loads.
Many modern engines have knock sensors tuned to the sound frequency wavelength of engine knock and report the event to the engine control module (computer). Generally, you will hear only a few knocks before the ECM begins retarding the ignition timing until the knock stops. In cases of severe or persistent knock, fuel economy and performance may suffer.
Other things can lead to knock caused by detonation including ignition timing that is too far advanced, a lean air/fuel mixture (because the cylinder temperature gets hot enough to combust spontaneously), a faulty EGR (exhaust-gas recirculation) valve, increased compression due to carbon buildup in the combustion chamber, hot intake air temperature or a hot engine.
As you can see, a knocking engine is nothing to laugh at. To prevent costly damage, you could switch to a higher-octane fuel, but that is treating the symptoms, not the disease. Fixing the problem will save you money in the short run through better fuel economy and in the long run by preventing costly engine damange.
Replace the spark plugs with ones of the proper heat range. Service the cooling system that may have buildup in the coolant passages, a bad thermostat, poorly functioning radiator or malfunctioning fan. Have the EGR system checked and cleaned. Remove the combustion chamber carbon buildup. Sometimes adding combustion chamber cleaner to the gas tank can do the trick; in severe cases a professional technician would be the best bet.
Once everything is back to normal, your engine should remain quiet–knock on wood.
———-
Bob Weber is an ASE-certified Master Automobile Technician, having recertified every five years since 1978. Contact him at MMTribune@netscape.net.
