What are the causes of engine knocking?

[DeJude]

The causes of engine knocking are: 1. Carbon deposits in the combustion chamber; 2. Using gasoline with an octane rating that is too low; 3. Poor quality gasoline; 4. Incorrect ignition timing. Symptoms of knocking: 1. Knocking creates sudden shock waves inside the cylinder that impact in all directions, causing strong vibrations in the engine's pistons, cylinder walls, connecting rods, crankshaft, etc., resulting in irregular metal knocking sounds; 2. Overheating of the cooling system; 3. Incomplete fuel combustion, with black smoke in the exhaust. Solutions: 1. Reduce the ignition timing advance; 2. Use gasoline with the specified octane rating; 3. Remove carbon deposits from the combustion chamber; 4. If knocking occurs when the car is going uphill, promptly shift to a lower gear; 5. Do not shift into a forward gear too early when starting the car.

[StCarlos]

I ran into this issue last time, there was always a metallic knocking sound during acceleration. Detonation, to put it simply, is abnormal combustion inside the engine, and the most frustrating part is using low-octane gasoline. Like my turbocharged car, it has a very high compression ratio and must run on premium 95 octane or higher. But once I tried to save money and filled it with 92 octane, and that's when the trouble started. Carbon buildup is also a major culprit. When carbon deposits accumulate on the piston top, they act like heating elements, causing the gasoline to self-ignite before the spark plug fires. The mechanic at the repair shop showed me with an endoscope—the carbon layer was as thick as a coin. Abnormal water temperature is also to blame; issues with the cooling system can turn the combustion chamber into an oven. Detonation is really deadly—over time, it can even crack the piston. Now, I always check the octane rating with a magnifying glass when refueling.

[DelAnthony]

After a decade of car repair, five main causes of engine knocking have been identified: Fuel issues top the list, where low octane rating or water contamination in gasoline can cause premature ignition; Second is severe carbon buildup in the combustion chamber - those black deposits act like hot irons, igniting the air-fuel mixture too early; Third is incorrect ignition timing, caused by faulty control units or aging distributors advancing the spark too soon; Fourth is excessive intake air temperature, particularly common in turbocharged vehicles or those with intake system leaks; Fifth is overzealous forced induction modifications - some young enthusiasts recklessly flash tuning programs, pushing cylinder pressures beyond the engine's design limits. The most straightforward way to diagnose these issues is by reading knock sensor data streams with a diagnostic tool. For older vehicles, an annual carbon cleaning is recommended.

[AylaAnn]

It's like sudden oil splatters in a frying pan. Normally, the engine should combust only after the spark plug ignites, but if the air-fuel mixture is accidentally ignited at the end of the compression stroke, two flames collide and cause a detonation. Low-octane gasoline is particularly prone to auto-ignition in high-compression engines, and carbon deposits in the combustion chamber of older cars can form hot spots. Last year, my cousin's car experienced knocking during a high-altitude trip—the low-oxygen environment slowed combustion, and the ECU over-advanced the ignition timing, making things worse. Occasional mild knocking isn't a big issue, but if the dashboard warning light comes on or there's persistent metallic knocking, it must be checked. Modified cars especially need to pay attention to fuel quality and the cooling system.

[OLondon]

Delaying the spark plug's operation can be much better. When the knock sensor detects abnormal vibrations, the control module will retard the ignition timing to alleviate the issue. But if the sensor is damaged, it becomes troublesome—my old BMW once had corroded wiring, causing the ECU to receive incorrect signals and continuously delay ignition, pushing fuel consumption up to 15 L/100km. The oxygen sensor malfunction also plays a part, as a lean fuel mixture can cause combustion temperatures to skyrocket. Some think knocking can be fixed by just driving at high speeds, but enduring it long-term might even fracture the piston rings. Last time I saw an engine repair bill that could buy a motorcycle, so it's best to avoid fuel additives during the break-in period of a new car.

[PatrickAnn]

Preventing engine knock actually has its tricks. Fuel quality is the most critical factor. For direct injection vehicles, it's best to take a high-speed run monthly to clear carbon deposits. I make it a habit to clean the throttle body every 20,000 kilometers. During my last maintenance, a leaking intake manifold was discovered, where hot gases flowing back directly caused the air-fuel mixture to boil. Spark plug condition should also be prioritized - old spark plugs with excessive gap can cause delayed ignition leading to abnormal combustion. Replace inaccurate coolant temperature sensors immediately as they directly affect air-fuel ratio adjustment. Engine knock most easily occurs when climbing hills with AC on in summer - shifting to manual mode and downshifting to increase RPM not only protects the engine but also saves fuel compared to flooring the accelerator. For modified cars, it's recommended to strengthen the cylinder block before increasing compression ratio.