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Tuesday, January 25, 2011

Causes of Spark Knock

The things that usually cause spark knock (detonation) are:

EGR valve that is not working. The EGR valve is supposed to open when the engine is accelerating or lugging under a load. 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 (spark knock) when accelerating, driving up a hill, or when the engine is lugging under a heavy load.

The knock sensor can be tested by tapping on the engine near the sensor (not the sensor itself) with a wrench while watching spark timing and/or knock sensor input on a scan tool to see if it sends a timing retard signal.

NOTE: Over advanted ignition timing can also cause the same thing (spark knock). But on most late model engines, ignition timing is not adjustable and is controlled by the engine computer. 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. This is usually more of an issue with older, high mileage engines or vehicles that are only driven for short trips and never fully warm up. 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, it will increase the engine's static compression ratio. Or, 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 because the forced air induction system increases compression. This usually requires using premium fuel.

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. Be warned, though, that some 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.

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.

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