Engine Knocking Pinging Rattling

engine knocking
engine knocking

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 spark knock. (engine knocking)

It’s not necessarily a problem with your fuel delivery system.  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.

EGR Valve
EGR Valve

The EGR valve should 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.

engine knocking
engine knocking

Testing the knock sensor,

The knock sensor can be tested by tapping on the engine near the sensor. You should do this while watching 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 (spark knock). 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 engines. This includes 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.

spark knock
spark knock

(4) Compression ratio too high. (engine knocking)

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.

(5) Cheap gas. (engine knocking)

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. Or, if you always buy gas at the same gas station, try a different gas station. Also don’t buy the cheapest gas you can find.

(6) Engine overheating.

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