Common symptoms of engine knocking
Detecting engine knocking early can save you from major repair bills. Here are the most common signs to watch out for:
Tapping or knocking noise from the engine
The most obvious symptom is a metallic knocking, pinging, or rattling sound coming from under the bonnet. This noise often appears when the car is accelerating, climbing a slope, or carrying heavy loads. It may disappear at low speeds and return when the engine is under stress.
Noise gets louder during acceleration
If the knocking sound increases as you press the accelerator or drive uphill, it usually points to a combustion problem. Normal engine noise stays consistent, but knocking that gets louder with speed is a warning sign.
A knocking engine often feels weaker. You may notice slower acceleration, hesitation while speeding up, or overall poor performance. This happens because the engine cannot produce power efficiently when combustion is uneven.
Drop in fuel efficiency
When fuel does not burn properly, the engine uses more fuel to produce the same power. If your car’s mileage suddenly drops without any obvious reason, engine knocking could be one of the causes.
Check engine light
Modern cars are equipped with sensors that detect unusual engine behaviour. If the engine knocking becomes frequent, the check engine light may turn on. This is a signal that the engine control system has detected a problem that needs attention.
Ignoring these signs can turn a small issue into a major mechanical failure
Main causes of engine knocking noise
Engine knocking might be caused by anything from very minor maintenance mistakes to more complicated mechanical problems. It is important to identify the reason before avoiding extensive engine wear and repair.
The following are the most common reasons why an engine would begin knocking:
1. Wrong or low-quality fuel
The quality and nature of the fuel determine the combustion process. Each engine is designed to run with a certain octane rating. Using fuel with an octane rating below the engine requirement can lead to pre-ignition and cause knocking. Poor-quality fuel can contain several impurities that can interfere with optimum ignition, leading to dysfunctional explosions in the cylinder.
High-performance cars or high-compression engines are particularly prone to octane fluctuation. Regular use of the incorrect fuel can lead to not only knocking but also to long-term piston, valve, and cylinder wall damage.
2. Bad spark plugs
Bad spark plugs can also interfere with the engine's efficiency and should be maintained properly to avoid this. A routine inspection of the spark plugs is required to avert massive ignition problems and knocking sounds. Following the service schedule provided by the manufacturer is ideal in these cases.
3. Timing issues
Inefficient combustion and knocking result when the spark fires too early or too late in the compression stroke.
Mistimed engine performance can be caused by a broken timing chain or belt, a malfunctioning sensor, or a broken electronic control unit (ECU). Engine timing issues are especially hazardous because they upset the synchronisation of engine pistons and valves. If not repaired, such issues might lead to rough engine idling, a loss of power, or engine failure.
4. Carbon buildup in the engine
Carbon deposits can form on the piston heads, intake valves, and combustion chambers over time due to incomplete fuel combustion. Carbon deposits occupy available space in the air-fuel mixture, effectively increasing the compression ratio within the cylinder.
A higher compression ratio produces premature fuel detonation, which results in the knocking noise. In severe instances, the buildup may also create hot spots that preburn the fuel mixture before it is ignited by the spark plug. Cleaning the engine and adding good-quality fuel additives will reduce the carbon buildup and keep the engine in a smooth-running state.
5. Faulty knock sensor
Current vehicles have knock sensors that sense unusual vibrations or sounds made by unusual combustion. During normal functioning, the knock sensor tells the ECU to correct ignition timing and fuel injection to avoid damage.
But an improper knock sensor will not pick up such deviations, and knocking continues unabated. In certain other instances, the ECU might confuse sensor readings, causing improper adjustments and accelerating the problem. Replacing a malfunctioning knock sensor on time guarantees proper engine management and avoids possible mechanical failure.
6. Worn engine parts
Moving components such as bearings, pistons, and connecting rods deteriorate over time. Proper clearance between such moving components produces metallic tapping or knocking noises, especially under acceleration or load.
A loosely fitted connecting rod bearing, for example, makes a heavy, thudding sound with rising engine speed. Mechanical thudding is an indication of internal wear and must be inspected immediately by a professional mechanic. Failure to do so can cause the engine to fail disastrously and require expensive repairs.
How to fix engine knocking
If your car shows signs of engine knocking, it’s important to address the issue as soon as possible. Fixing the root cause early can prevent costly repairs later.
Diagnostic scan
A mechanic will first run a diagnostic scan to identify fault codes related to timing, sensors, or combustion issues. This helps pinpoint the exact cause of knocking.
Use the correct fuel
If knocking is caused by low-octane or poor-quality fuel, switching to the recommended fuel grade often helps immediately. Draining contaminated fuel may also be necessary in some cases.
Replace spark plugs
Worn or dirty spark plugs are replaced with the correct type recommended by the manufacturer. This restores proper ignition and smooth combustion.
Clean carbon deposits
Fuel additives or professional engine cleaning methods are used to remove carbon buildup from the combustion chamber and valves. This helps restore normal compression and prevent premature ignition.
Check and adjust timing
Mechanics inspect ignition and valve timing using specialised tools. Faulty components like timing belts, sensors, or ECU settings are corrected or replaced as needed.
Replace faulty sensors
If the knock sensor or related sensors are defective, replacing them allows the engine management system to function correctly and prevent knocking.
Most of these repairs require professional equipment and expertise, so it’s best to get your car checked at a trusted service centre.
With the ACKO Drive Service Centre, taking care of your engine is simple. Book a service slot online, and we’ll pick up your car from your doorstep for free. Our skilled mechanics will run a complete diagnostic, fix issues like worn spark plugs, carbon buildup, faulty sensors, or timing problems, and return your car in top condition.
Track the progress easily via the ACKO app or website, and enjoy transparent pricing so you know exactly what you’re paying for. Let us handle the repairs, so you can prevent bigger, more expensive engine problems in the future.
How to prevent engine knocking
Preventing engine knocking is far easier and cheaper than fixing engine damage. A few good habits and regular maintenance can keep your engine healthy.
Use the recommended fuel
Always fill your car with the fuel grade specified in your owner’s manual or on the fuel cap. Stick to this grade consistently and avoid mixing fuel types. Refuel at trusted stations and keep your tank above a quarter full to prevent sediment from reaching injectors, which can cause uneven combustion and engine knocking.
Replace spark plugs on schedule
Spark plugs control the timing of fuel ignition. Check your service records and replace them according to the manufacturer’s recommended interval. Use the exact type specified for your engine and ask the technician to inspect the spark plug gap during replacement. A proper spark ensures smooth combustion and reduces the risk of knocking.
Keep the engine clean
Carbon deposits in the cylinders or on valves can increase compression, leading to premature ignition. Prevent buildup by using quality fuel additives every 5,000–7,000 km and periodically driving at steady highway speeds to burn off deposits. During servicing, request intake or throttle body cleaning if recommended for your vehicle.
Service your car regularly
Follow the service schedule in your owner’s manual - usually every 10,000 km or annually. Ensure timely oil and filter changes, and request a full engine diagnostic scan even if there are no warning lights. Regular maintenance keeps components like timing belts, sensors, and ignition systems functioning optimally, reducing the chance of knocking.
Bottom line
Engine knocking must never be overlooked. It might be due to something as trivial as low-octane fuel or dirty spark plugs, but it could also be due to something more nefarious, such as engine problems. Brought in early and fixed, it prevents damage to costly components, keeps fuel mileage high, and prevents costly repairs.
A softly purring engine is as much an indication of decent service as it is of a good, efficient, and long-lasting car. Regular servicing, careful fuel selection, and a sensitive ear for even a sound of impending doom all help keep the engine free from engine knocking.
Frequently asked questions
