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Spark Knock (Detonation): Engine Knock Under Acceleration

Spark Knock is a pinging noise, rattling or knocking which might be heard while the engine is working hard under load (passing on the highway, towing a trailer, driving up a mountain, etc.) or is accelerating. When the fuel is detonating it called spark knock.

Detonation is when the fuel explodes rather than burning. It happens when there is compression and to much heat in the combustion chamber. It’s comparable to preignition, but preignition is when the fuel ignites before the spark happens due to a hot spot within the combustion chamber. Preignition could burn up a hole all over the surface of a piston.

Detonation is not good for the engine, it may could break piston lands and the rings, can damage the rod bearing, also can causing the head gasket to fail if it happen over long period of time.

Piston Damage
Piston Damage

Causes Of Spark Knock

This common reason why spark knock (detonation) occurs:

Faulty of the EGR valve

When the engine is lugging under a load or accelerating, the EGR valve is supposed to open. In this case to dilute the air / fuel mixture there will be a few exhaust suction by the vacuum intake through the EGR valve. This will prevent knock and lowering the combustion temperatures. The EGR valve operation should be checked, there a maybe in valve port or the valve pintle has a carbon deposits buildup that could block the exhaust flow back to the engine. If you found a carbon deposits, clean of it using carburetor cleaner and wire brush, and if EGR valve is defective replace it.

Knock Sensor Is Bad

Knock sensor is part of the engine and will tell the computer systems to retard the ignition timing when it detect the detonation. Premium grade fuel is required by your car engine, however you just fill the tank with mid-grade fuel, in some case the PCM will retard the timing due engine working hard under load that causing the detonation where the knock sensor will detect it. This will protect the engine against the detonation and consequently reduce the power a bit. When the spark timing should retard, but it’s not, then the knock sensor is not working. Therefore when under a heavy load the engine is lugging, accelerating or driving up to the hill, spark knock (rattling sound) or pinging may be heard.

How to test knock sensor is tapping the engine using a wrench that nearby the sensor (do not tap on sensor), and look close on scan tool or knock sensor and/or spark timing to make sure if timing retard signal is sent.

NOTE! Spark knock also can caused by overrated ignition timing. However, on most modern engine (late model), engine computer control the ignition timing and it is not adjustable. Reprogramming the PCM is the only way to change the timing advance.

Excessive Carbon Buildup On The Pistons And Combustion Chambers

This is more of a problem with higher mileage engines or vehicles which are driven for short trips and never warm up. Clean up the combustion chamber using additive of fuel system or treating an engine using a dose of top cleaner usually works. There a machine called MotorVac that used by some repair shops to cleaning up the engine carbon. A concentrated detergent used by the MotorVac machine to flush out the combustion chambers and the fuel injection system.

Compression Ratio Too High

The static compression ratio of engine will increased when cylinders has been bored to oversize and an engine have been rebuilt. Or to restore flatness the resurfaced have been done in the cylinder head, this way will also increase the static compression ratio of engine and reducing the combustion chamber volume. The power will increasing by this change, however on regular 87 octane fuel there also have a risk of detonation. If you do that kind of modifications, retarding the spark timing and/or using fuel with higher/premium octane is recommended (89 or 93). Higher risk detonation also happen on engine that are turbocharged or supercharged because compression increased by forced air induction system. Premium fuel is needed for this kind of engines.

Low Octane Gas

Octane rating 87 is usually for regular grade gasoline. Spark knock may caused the fuel octane rating is not 87 due cutting corners by the station of gas and the refiner. Try a a tank premium gasoline or mid-range to fix this. And beware, not all gas station say the truth about the octane rating, some of them cheat on this and what is given to you is not always to have the octane rating as claimed. Knocking can be reduced by fill the tank with a premium gas, it’s costs more but your engine may need it. Try another gas station and don’t buy a gas on same gas station regularly. Cheapest gas may have a poor gas quality. Find a good brands such as Mobil, BP and Shell.

Engine Overheating

Fuel detonation may caused by some problem such as sticking thermostat, bad water pump, a plugged radiator, cooling fan fail, or low coolant and will make the engine will running too hot.

Excessive Turbo Boost

The engine may damage and causing the detonation if the engine is turbocharged and there a excessive boost pressure. MAP sensor (Manifold Absolute Pressure), waste-gate and the engine computer is control the boost pressure. When the turbo produce a pressure and it exceed the preset level, then the waste-gate should opened for a vent. Over-boost and detonation could be happen or even possible the engine is damage if MAP sensor doesn’t send a report correctly about boot pressure to the computer, or sensor inputs are not correctly processing by the computer, or the waste-gate is closed and isn’t opening.

Lean Fuel Mixture

Not enough fuel and to much air (lean fuel mixture) may caused the detonation rather than rich fuel mixture or normal fuel mixture. There are many things that cause the lean fuel mixture, it’s possibly due faulty of MAF sensor (Mass Airflow), engine vacuum/air leaks, low fuel pressure or a dirty fuel injectors. A weak fuel pump or fuel pressure regulator leaks are possibly cause of the low fuel pressure.

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