Engine spark knock what is it? Is it bad and what can you do about it?

If your engine makes an annoying knocking, pinging or rattling sound when you accelerate or work your engine hard with a load – like driving uphill, passing a slowpoke or towing a trailer – then you’re probably experiencing engine spark knock.
Engine Spark Knock-EGR Valve Blockage
Engine Spark Knock-EGR Valve Blockage

It’s not necessarily a problem with your fuel delivery system, but it does mean that your fuel is detonating, or exploding erratically rather than burning smoothly as it should. Eventually, this could lead to a failed head gasket, broken rings, cracked piston lands and/or flatten your rod bearings.

(1) The EGR valve that is not working.

When the engine is accelerating or lugging under load the EGR valve should open. This allows intake vacuum to suck some exhaust in through the EGR valve to dilute the air/fuel mixture slightly. This lowers combustion temperatures and prevents knock. Inspect the operation of the EGR valve, and check for a buildup of carbon deposits on the valve pintle or valve port that may be blocking the flow of exhaust back into the engine. Clean off the carbon deposits with a wire brush and carburetor cleaner, or replace the EGR valve if it is defective.

(2) A bad knock sensor.

Your engine has a knock sensor that should detect detonation and tell the computer to retard the ignition timing. If your engine requires premium grade fuel, but you are using regular or mid-grade fuel, the knock sensor should detect any detonation that may occur when the engine is working hard under a load and cause the PCM to retard timing. This reduces power a bit but protects your engine against detonation. However, if the knock sensor is not working, spark timing will not retard when it should. Consequently, you may hear a pinging or rattling sound (engine spark knock) when accelerating, driving up a hill, or when the engine is lugging under a heavy load.

By tapping on the engine near the sensor is one way to test it. Next watch spark timing and/or knock sensor input on a scan tool to see if it sends a timing retard signal.

NOTE: Over advanced ignition timing can also cause the same thing (engine spark knock). The engine computer controls ignition timing is not adjustable on today’s engines. The only way to change the timing advance would be to flash reprogram the PCM.

(3) Excessive carbon buildup in the combustion chambers and on the tops of the pistons.

Carbon build up
Carbon build up

Older high mileage engines. Vehicles that never fully warm. Only driven short distances. Treating the engine with a dose of top cleaner or a fuel system additive that also removes carbon from the combustion chamber can usually clears this up. Some repair shops use a machine called a MotorVac to perform an engine carbon cleaning procedure. The machine uses a concentrated detergent to flush out the fuel injection system and combustion chambers.

(4) Compression ratio too high.

If an engine has been rebuilt and the cylinders have been bored to oversize. This can increase the engine’s static compression ratio. If the cylinder head has been resurfaced to restore flatness. This will reduce the volume of the combustion chamber and also increase the engine’s static compression ratio. These changes will increase engine power, but also the risk of detonation on regular 87 octane fuel. Such modifications may require using higher octane 89 or 93 octane premium fuel and/or retarding spark timing. Engines that are supercharged or turbocharged are also at much higher risk of detonation. This is because the forced air induction system increases compression. This usually requires using premium fuel.

Engine Spark Knock
Engine Spark Knock

(5) Cheap gas.

Regular grade gasoline is supposed to have an octane rating of 87. If the gas station or their refiner is cutting corners and the fuel is not 87, it may knock. The fix for this is to try a tank of mid-range or premium gasoline. Some gas stations cheat on this too, and don’t always give you the octane rating claimed on the pump. Premium costs more, but may be required to reduce the knocking. Or, if you always buy gas at the same gas station, try a different gas station. Don’t buy the cheapest gas you can find. BP, Shell and Mobil are all good brands.

(6) Engine overheating.

If the engine is running too hot because of low coolant, a cooling fan that isn’t working, a plugged radiator, bad water pump, sticking thermostat, etc., it may cause the fuel to detonate. This could cause engine spark knock.

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