Key summary of what causes car engine knocking:
- Engine knocking is often caused by issues like low oil pressure, worn spark plugs, faulty fuel pumps, or damaged engine components.
- Using the correct engine oil and keeping up with maintenance helps prevent knocking and reduces engine wear.
Ignoring knocking noises can lead to severe engine damage and expensive repairs, so problems should be checked quickly.
The average cost for replacing a cam belt is £469
Low oil levels is one of the main causes of engine knocking
Spark plugs need replacing anywhere from 30,000 to over 100,000 miles
Engine knocking is an abnormal sound resulting from improper combustion or mechanical wear within a vehicle's internal combustion engine. This guide identifies the six primary causes of engine knocking, including worn cambelts, low oil pressure, and faulty spark plugs and provides actionable steps for diagnosis and prevention to avoid catastrophic engine failure.
1. Worn cambelt
Also known as a timing belt, the cambelt synchronises the rotation of various other components to ensure the engine valves open and close at the exact right moment. Eventually, a cambelt can become worn or frayed, causing a knocking sound inside your car.
Cambelt replacement requirements differ based on the car make and model. Take a look at the owner’s manual, and if you’re in or approaching the recommended range, your cambelt could be the culprit.
2. Low oil pressure
Engine oil is a vital lubricant that keeps your motor running smoothly. Without it, the engine will seize, and the internal components can ‘knock’ against each other, which can lead to serious damage in the long term.
If the engine oil level is too low, the oil pump picks up air instead of oil, and air does not provide the lubrication needed to protect the moving parts in the engine. Knocking happens as a result of the bearings getting starved of oil, and they begin to melt from the heat of friction.
If your oil pressure is low, you may also have a leak. Try checking your oil and topping up if necessary. If you find it running low again quickly, this indicates a leak in the system, and you should contact a mechanic to organise a repair.
3. Valve lifter failure
A valve lifter is another essential engine component that transfers motion to open and close the valves. If your oil hasn’t been changed in a while, dirt can accumulate in the lifters, leading to a persistent knocking noise every time you start the engine. Lifters can also become clogged when you use the incorrect grade of oil for your engine.
Changing your oil and running a cleaning agent through the engine could be the simple solution to your engine knocking sound. Your vehicle’s handbook will guide you on how often the oil needs to be changed for your particular vehicle and the type of oil to use.
4. Old spark plugs
Spark plugs generate the electricity needed to ignite combustion in your engine. Without them, your car wouldn’t start. While spark plugs typically last thousands of miles, they do eventually need replacing. Your vehicle manual will specify how often you need to replace yours.
If your engine knocking sound is accompanied by other issues such as misfiring, stalling, struggling to start, and a ‘check engine’ light, it’s time to book in with a mechanic to have your spark plugs replaced.
5. Bad fuel pump
A malfunctioning fuel pump can disrupt the precise air-fuel ratio required for combustion. When the pump fails to deliver enough fuel, the engine runs 'lean' (too much air, too little fuel). This lean condition increases combustion chamber temperatures, often leading to pre-ignition and a noticeable knocking sound.
A trained mechanic must perform a fuel pressure test to confirm if a replacement is necessary.
6. Worn engine bearings
Engine bearings reduce friction while supporting moving machinery. They facilitate the smooth movement of the pistons as they travel up and down in the engine, turning the crankshaft, which transfers power to the wheels
Over time, they can become worn out or come out of position, creating a rattling noise as the pistons clunk against the crankshaft.
If this is the cause of your engine knocking sound, you’ll need to book in for a repair with a qualified mechanic.
What to do if your engine is making a knocking sound
As soon as you notice a knocking sound, you should stop driving and park in a safe location.
Investigate the issue yourself by checking your engine oil, any warning lights, and reading your owner’s manual for instructions. For the most accurate diagnosis and repairs, you should always consult a mechanic.
Frequently asked quetions
While some of the reasons behind an engine knocking noise are relatively simple to remedy, you shouldn’t drive your car until the issue has been diagnosed and resolved.
Continuing to drive can cause further damage to your car. If your engine fails on the road, you could even end up in an accident.
At the very least, you may find yourself with a more expensive repair bill as the fault continues to wreak havoc beneath your bonnet.
How to prevent engine knocking
To help prevent knocking or pinging noises from your engine, it's crucial to follow the preventative maintenance schedule recommended by your manufacturer. This includes:
- Changing the engine oil and using the correct viscosity/grade per the owner's manual intervals
- Replacing air filters, spark plugs, timing belts, fuel filters, etc. on schedule
- Addressing any fluid leaks promptly to maintain proper lubrication levels
- Using high-quality gasoline and considering periodic fuel system cleaners
- Keeping up with all other scheduled maintenance for emissions
Sticking to the manufacturer's guidance for maintenance using approved fluids and parts allows components to operate properly, reducing the strain that leads to knocking noises. Consistent care is key to preventing costly engine damage down the road.
While some cases of engine knocking can’t be avoided, it’s best to stay on top of a preventative maintenance schedule to decrease your chances.
For more vehicle maintenance tips and expert advice, explore our other helpful blog posts.
Explore our car fluids
Sources for further reading
Guide to timing belt replacements, RAC
Signs of Low Oil in a Car: What to Look For, Oil Changers
How often should you replace spark plugs?, Carwow