Engine knocking is a rhythmic knocking or pinging sound from the engine, and the cause decides how serious it is. A light metallic ping under load is usually detonation, where fuel ignites too early, and is often fixed with the right fuel or by curing overheating or carbon buildup. A deep, heavy knock that rises with engine speed is often rod knock from worn bearings, which is serious: stop driving and check the oil.
Types of engine knock
| Sound | When it happens | Likely cause |
|---|---|---|
| Light pinging or rattling | Accelerating or climbing a hill | Detonation (spark knock) |
| Deep, heavy knock | Rises with RPM, often louder off the throttle | Rod bearing wear (rod knock) |
| Hollow knock when cold | First minutes after a cold start, fades as it warms | Piston slap |
| Light, fast ticking | At idle, at half engine speed | Valvetrain: lifters, rockers or valve lash |
| Ticking that fades as the engine warms | Cold start | Exhaust manifold leak |
Detonation (spark knock)
Detonation happens when part of the air-fuel mixture ignites on its own after the spark, colliding with the normal flame front. Common causes are fuel with a lower octane rating than the engine needs, carbon deposits that raise compression and create hot spots, overheating, a lean mixture, and ignition timing that is too far advanced. Modern engines have knock sensors that pull timing when they hear it, so a faulty knock sensor can show up as a check engine light with a knock sensor circuit code. Use the octane your owner’s manual specifies, and fix cooling or fueling problems before they damage pistons.
Rod knock
Rod knock comes from wear between the connecting rod bearings and the crankshaft, usually after oil starvation, low oil pressure or very high mileage. The knock is deep and gets faster with RPM. Check the oil level right away. If the level is fine and the knock remains, the bearings are worn, and continued driving can break a rod and destroy the engine. Rod knock usually means a rebuild or replacement engine; see how to tell if an engine is blown.
Ticks and other noises that sound like knock
Valvetrain ticking from a lifter or rocker is lighter and faster than rod knock. Some engines are known for it; our 5.7 Hemi tick guide covers one common case. Exhaust leaks tick loudly when cold and quiet down as the metal expands. Loose heat shields, accessory pulleys and a failing harmonic balancer can also knock or rattle, so locate the noise before assuming the worst.
What to do if your engine is knocking
- Check the oil level and pressure first. Low oil is a common cause of serious knocks.
- Note when it happens: cold or warm, under load or at idle, and whether it follows engine speed.
- Use the correct fuel and look for overheating if the sound is a ping under load.
- Pinpoint the noise with a mechanic’s stethoscope, or have a shop listen to it.
- Stop driving if you hear a deep knock that rises with RPM.
If the engine needs replacing
A knocking engine with worn bearings is often better replaced than repaired. We carry short blocks, long blocks and complete engines; find yours on our engines by engine code page, compare short block vs long block, and follow an engine break-in procedure after installation.
More guides: knock sensor · oil pressure light on · engine ticking vs knocking · seized engine · hydrolocked engine · diesel knocking · bad oil pump symptoms · bent pushrod symptoms.
Trouble codes: P0325 Knock Sensor 1 Circuit · P0327 Knock Sensor 1 Circuit Low · P0332 Knock Sensor 2 Circuit Low · P0324 Knock Control System Error · P0330 Knock Sensor 2 Circuit Bank 2 · all OBD2 codes.
Shipping is a flat $80.00 per order to US addresses, with 15-day free returns. Questions? Ask a parts specialist.