What Causes Engine Knock in Cars?
2 Answers
Engine knock can be caused by excessively advanced ignition timing, excessive carbon buildup in the engine, excessively high engine temperature, or incorrect air-fuel ratio. Excessively advanced ignition timing: If ignition occurs too early, most of the air-fuel mixture will have already burned while the piston is still in the compression stroke. The remaining unburned mixture will then be subjected to extreme pressure and auto-ignite, causing knock. Excessive carbon buildup in the engine: Excessive carbon deposits in the combustion chamber not only increase the compression ratio but also create hot spots on the carbon deposits, leading to engine knock. Excessively high engine temperature: If the engine operates in too hot an environment, causing high intake air temperature, or if the engine coolant circulation is poor, it can result in high-temperature engine knock. Incorrect air-fuel ratio: An overly lean fuel-air mixture will increase combustion temperature, which in turn raises the engine temperature and makes knock more likely. Solutions: Reduce the ignition advance angle; use gasoline with the specified octane rating; clean carbon deposits from the combustion chamber; if knock occurs when driving uphill, promptly shift to a lower gear; when starting the car, do not shift into high gear too early; if knock occurs under heavy engine load, close the throttle slightly.
After all these years of repairing cars, I've seen that knocking issues are mostly caused by poor fuel quality. Using low-octane gasoline can easily cause the air-fuel mixture to detonate prematurely in the engine, which is common with cheap fuel from small gas stations. Carbon deposits are another culprit—too much carbon buildup on the piston tops can become ignition points under high temperatures, especially in older models or cars driven under heavy loads for long periods. Ignition timing set too early, worn-out spark plugs, or a clogged air filter leading to a lean air-fuel mixture can also trigger knocking. I've even seen it happen when the engine overheats—a failing cooling system can cause cylinder temperatures to skyrocket. For prevention, I recommend using high-quality gasoline, regularly cleaning carbon deposits, checking spark plugs and the ignition system, and avoiding prolonged high-speed driving in summer. If you hear knocking sounds, don't ignore them—getting it checked ASAP can save you a lot of money.
