Otto Explains — Engine Noises
Why Is My Engine Knocking?
Short Answer
An engine knock can come from the way fuel burns in the cylinders or from worn or poorly lubricated moving parts. Don't try to work out which from the sound. If a new, heavy or persistent knock starts, pull over safely, switch off and arrange a professional assessment, even if no warning light is on.
What is engine knocking?
Drivers use the word for several different problems, and the urgency is different for each. Combustion knock comes from the way fuel burns in the cylinders. A mechanical knock comes from moving parts inside the engine and can be serious. A knock over speed bumps or potholes is often not the engine at all: clunks and rattles there can come from the exhaust or the suspension. The cause of a knock from the engine should be confirmed by testing, not judged by ear, so treat any new knock from the engine as a reason to pull over and have it checked.
What should you do if the engine starts knocking?
Don't keep driving to see whether it goes away, and don't rely on the sound to decide how serious it is. Pull over when it is safe, switch the engine off and, once the engine is off, check the oil level. If the knock is new, heavy, persistent or getting worse, or the oil is low, don't drive on, even if no warning light is on: worn bearings or a failing oil pump are a job for a mechanic, not something to drive on, and carrying on with a strange noise can turn a small problem into a breakdown. Arrange for the car to be assessed where it is or transported to the workshop.
What if the red oil warning light comes on as well?
Stop safely as soon as you can, switch the engine off and don't drive on: bearing damage can begin within minutes of running on low oil pressure. Low oil also increases friction and can wear connecting rods, pistons and bearings. Once stopped, you can check the oil level, but if the light stays on or the knock continues, the car should be assessed where it is or transported, not driven to a workshop.
What is combustion knock, and what causes it?
In combustion knock, also called detonation or pinging, part of the fuel-air mixture away from the spark plug ignites on its own under the pressure of the advancing flame, and the colliding flame fronts make the knocking sound. It happens more often with low-octane fuel and is related to ignition timing; using fuel with a lower octane rating than the manufacturer recommends is a common cause. A related problem, pre-ignition, is when a hot spot in the combustion chamber ignites the mixture before the spark. It is most often caused by a spark plug of the wrong heat range or over-advanced ignition timing.
Can combustion knock damage the engine?
Yes. Heavy knock can break or erode parts of the combustion chamber and often leads to pre-ignition, which sharply raises cylinder temperature and pressure and can damage pistons and valves. Don't ignore a light knock either, and don't keep driving with it: check that you are using the fuel grade your manufacturer recommends, and have the engine diagnosed with a scan and testing before relying on the car again.
Could the knock be coming from the suspension instead?
Clunks and rattles over speed bumps, potholes or rough roads more often come from the suspension, steering or exhaust than from the engine. That is a separate set of problems, explained in Otto's guide to suspension knocking. If you are not sure where a noise comes from, have the car checked rather than guessing.
Who should diagnose the knock?
A qualified technician, using tests rather than the sound alone. AutoTronix Garage handles mechanical engine diagnosis and repair, and AutoTronix Lab carries out diagnostic scans and electronic testing. The cause should be confirmed by testing before any part is replaced.
Common Questions
- Can I keep driving with a knocking engine? Don't drive on if the knock is heavy or getting worse, or if the red oil warning light is on: pull over safely, switch off and have the car assessed or transported. Even a light knock should be diagnosed before you rely on the car.
- Will higher-octane fuel stop the knocking? Combustion knock happens more often with low-octane fuel, so using the grade your manufacturer recommends matters. But knock is also related to ignition timing and other faults, and a mechanical knock has nothing to do with fuel, so a change of fuel is not a fix: have the engine checked.
- Can I tell from the sound whether it is serious? Don't rely on the sound. A new or persistent knock from the engine is a reason to stop, switch off and have it tested, with or without a warning light.