Troubleshoot Engine

Identifying and Fixing Engine Knocking at Idle

engine knocking when idling

Modern engines are complicated pieces of machinery that have hundreds of moving parts. The sounds coming from an engine can quickly let you know if an engine is running smoothly or if there is something wrong.

Engine knocking noises that come from an engine that is idling can be caused by an incorrect fuel-air mixture, bad spark plugs, engine timing issues, a faulty idle air control valve, or faulty engine sensors such as the engine knock sensor or mass air flow sensor.

Engine knocking is a common sound that can often only appear under specific conditions such as when the engine is cold or when idling. Sometimes it will disappear when driving and then reappear when the vehicle is stopped.

This can often be caused by a problem with the fuel supply or with a specific engine sensor that may be faulty.

What Causes Engine Knocking When Idling? (5 Common Causes Explained)

Engine knocking that only occurs when the engine is idling is usually caused by a problem with the fuel delivery to the engine, inferior fuel quality, incorrect engine timing or a problem with mass air flow sensor or engine knock sensor.

If an engine is not running smoothly when it is idling the cause is usually different to engine knocking when driving. When you are driving, the engine is operating at higher speeds and is under more pressure.

This usually exposes problems with worn internal components such as piston rods, piston bearings, and engine valves. When idling, the engine is operating under different conditions, is using much less fuel, and is often cold.

This means that problems with fuel delivery to the cylinders and management of the intake of air is more likely to cause engine issues that may lead to knocking noises.

Here are a few of the main causes of engine knocking when the vehicle is idling.

Cause 1: Inferior fuel quality Fuel (and lower octane fuel)

The quality and composition of the fuel-air mixture is very important when an engine is starting and is still cold. When an engine is cold and idling, more fuel is usually added to the mixture to offset the coldness that affects the vaporization of the fuel in the cylinder.

The engine will usually idle faster when colder to compensate for the unburnt fuel so that it is forced out of the engine quicker. Sometimes there may be a problem with engine timing, bad spark plugs or fuel that is of bad quality or too low an octane. This will affect the timing of the ignition of the fuel and as a result, it may not burn completely.

This can lead to specific engine knocking noises known as fuel knock, as some of the unburnt fuel can auto-ignite after the expected fuel ignition has occurred.

Cause 2: Problems with Engine Timing

Another cause of engine knocking when idling is incorrect engine timing. The correct timing of the internal parts of the engine is obviously critical when it comes to keeping the engine running smoothly.

When it comes to engine knocking, the timing can have a massive effect on how precisely the spark fires, and this can have a knock-on effect on how well the fuel ignites and burns.

When idling, there is less fuel being burnt to keep the engine ticking over. Spark plugs generally need to fire hundreds of times per minute in an exact sequence to keep the engine running smoothly. Problems with the timing can mess with the spark plug sequencing which can leave unburnt fuel in the cylinder that can self-ignite causing knocking.

Cause 3: Faulty Engine Sensor

Bad engine sensors can also cause engine knocking. Engines have dozens of sensors that relay information to the ECU to help keep it running correctly.

When the engine is idling certain sensors such as the mass air flow sensor, throttle position sensor and fuel pressure sensor are important for keeping the engine running while using very little fuel and air.

A bad mass air flow sensor will not be able to accurately monitor the amount of air entering the engine and this can adversely affect the fuel-air mixture. The throttle position sensor keeps track of how open the throttle is and works with the idle air control valve to make sure adequate amounts of air continue to enter the engine when idling.

Cause 4: Faulty Idle Air Control Valve

A bad idle air control valve is a very common reason for engine knocking when idling. The idle air control valve is responsible for diverting air into the intake of the engine when the throttle body is completely closed.

If the correct amount of air is not getting into the engine to mix with the fuel then this can cause the engine to run rich (too much fuel). This can create a problem with unburnt fuel remaining in the engine cylinders and escaping into the exhaust fumes that can self-ignite, resulting in engine knocking.

A bad idle air control valve will also cause an irregular idle speed that leads to a rough-running engine that can stall easily. It can also create exhaust popping and backfiring if unburnt fuel is making its way to the exhaust manifold.

Cause 5: Worn Valve Lifters

Another common source of engine knocking and tapping is worn valve lifters. Hydraulic valve lifters are fitted to every engine to aid in the opening and closing of the intake and exhaust valves. These lifters ensure the quiet operation of the valves and help to reduce wear and tear on the valve stems and seals

When the lifters become worn or are operated with insufficient engine oil present then they can start to make a tapping noise when they are going up and down. If left in this state then they can become excessively worn and this can lead to damage to the rocker arm, valves, and the lifters themselves. This can lead to engine knocking which can be especially loud when the engine is idling.

What Next – How To Fix Engine Knock When Idling

As explained above, most engine-knocking noises that occur when idling are because of a problem with the fuel supply and combustion within the engine cylinder. So it’s important to make sure that the engine is getting a steady supply of good quality, contaminant-free fuel.

  1. Run an engine diagnostic using an OBD-II reader. You should start by running an engine diagnostic using an OBD-II scanner to see if there are any engine error codes that may point to a faulty engine sensor or faulty part of the fuel delivery system. Look out for codes related to the fuel pressure such as P0087 or codes related to bad fuel injectors or fuel pump.
  2. Check for a faulty engine knock sensor. A bad engine knock sensor can contribute to engine knocking by relaying bad information back to the ECU related to the current running of the engine. A bad engine knock sensor should register engine error code P0325.
  3. Check the mass air flow sensor and idle air control valve. Remove the mass air flow sensor and check for dirt or cracks. Check the wiring that feeds power to the mass air flow sensor. Remove and check the idle air control valve for damage. You can also check the idle air control valve using a multimeter to test for adequate resistance. You should get a reading of between 7 and 25 ohms if it is working correctly.
  4. Check the timing belt and serpentine belt condition. Problems with the various drive belts fitted to the engine can cause problems with the engine timing that can lead to engine knocking. If the timing belt has stretched or is damaged it may affect the timing of the engine. Auxiliary belts can also restrict the movement of the crankshaft pulley if they are damaged. Worn or stiff pulleys or tensioners can also affect the engine timing and should be replaced if they are not operating smoothly.
  5. Try high-octane fuel or a fuel additive. The type of fuel you are using may be causing the knocking, especially when the engine is cold. Double-check that you are using the correct octane fuel for the engine and try changing to a higher octane fuel if possible. An octane booster additive may also help, just make sure to check the owner’s handbook before adding one as some engines can suffer damage if extra additives are used in the fuel tank.

FAQ – Engine Knock When Idling

1. Can bad spark plugs cause knocking when idling?

Yes, worn-out or carbon-fouled spark plugs can cause engine knocking when idling. This is because the spark plugs will no longer be capable of producing a good quality spark that ignites the entire fuel mix in a controlled manner until it has burnt up completely. This can lead to unburnt fuel self-igniting at a slightly different time and this causes cylinder misfires, engine knocking, and a rough engine.

2. Can the wrong engine oil cause engine knock when idling?

Yes, the wrong engine oil can cause engine knock. Modern engines operate under very tight tolerances and it’s very important that the correct engine oil specification is adhered to when changing the engine oil.
The wrong oil may be too thick or too thin when the engine is cold and when it warms up. It may also be lacking the correct protective additives that are necessary to prevent premature oil degradation and the protection of the various engine components.
If the engine oil is not doing its job it will cause premature engine wear that will lead to engine knock. Engine components such as piston rods and bearings, and hydraulic valve lifters can be damaged by using the wrong oil and this will lead to engine knock, especially when the engine is idling.

3. What happens if you don’t fix engine knock?

If you don’t fix an engine that knocks, it will usually get worse and lead to permanent damage over time.
In some cases, and with some engines, a slight engine knock can be considered normal. All engines make their own distinctive noises but most of the time engine knock is not normal and is caused by a fault that will only get worse over time.
Engine knock that is caused by bad fuel combustion or a faulty sensor will inevitably lead to damage within the engine. Damage to the piston face, valve seals and face, piston rod and bearings, valve lifters, and the rocker arm can all be attributed to a rough engine that knocks.

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About the author

The Motor Guy

The Motor Guy is a passionate car enthusiast with a love for troubleshooting and diagnosing all sorts of vehicle problems.

With years of experience in OBD diagnostics, he has become an expert in identifying and solving complex automotive issues.

Through, he shares his knowledge and expertise with others, providing valuable insights and tips on how to keep your vehicle running smoothly.

- 12 years experience in the automotive industry
- ASE Master Automobile Technician
- A Series: Automobile and Light Truck Certification, A9 Light Vehicle Diesel Engine Certification
- Bachelor's Degree in Information Systems

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  • I noticed my car has developed a knocking sound when it idles, and it seems to go away when I’m driving but comes back when I stop. What could be causing this issue, and how should I go about diagnosing it?