What Engine Knock Sounds Like
Engine knock — sometimes called "big-end knock" — is a repetitive, hollow, metallic banging or knocking sound from deep inside the engine. It becomes noticeably louder when you accelerate hard or put the engine under load (going uphill, overtaking). At idle it may be a dull thud; under full throttle it intensifies to a rapid knocking that's hard to ignore.
It's a fundamentally different sound from the ticking of a valve train or the rattle of a heat shield. Engine knock comes from the bottom end of the engine and resonates through the block.
What Causes It
The most common cause is worn or failed connecting rod (big-end) bearings. These plain bearings sit between the connecting rods and the crankshaft journals. As they wear, clearance between the rod and the crankshaft increases, and the metal-to-metal impact at each combustion event produces the characteristic knock.
Main bearing wear produces a similar sound but is typically lower in pitch and less affected by RPM variation — more of a rumble.
Detonation (spark knock/pinging) is a different phenomenon — the fuel-air mixture ignites prematurely due to heat or incorrect octane, producing a sharp metallic pinging sound under load. This is an ignition/fuel issue rather than mechanical wear.
Why It Happens
- Oil starvation — insufficient oil level or a failed oil pump starves the bearings of lubrication, causing accelerated wear
- Extended oil change intervals on a high-mileage engine allow oil degradation and sludge build-up
- Running low oil for an extended period (ignoring the oil pressure warning light is the most common cause of catastrophic bearing failure)
- Oil pressure warning light ignored — if this light comes on and the engine is not shut down immediately, bearing failure can occur within minutes
Severity: Do Not Ignore This
Engine knock is not a "monitor and see" situation. Left unaddressed, worn rod bearings will fail completely, allowing a connecting rod to break free of the crankshaft — a "thrown rod" event. At best this destroys the engine internally. At worst it puts a hole in the engine block. Either way the engine is lost.
If you hear a new knocking sound, pull over safely, check the oil level, and if the sound is confirmed, do not drive the car. Have it towed.
Repair Options
The repair depends on the extent of damage:
- Early-stage bearing wear (no scoring): Rebuild involving new rod and main bearings, plastigauge measurement, and reassembly. $1,500–$3,500 NZD depending on the engine and access.
- Scored crankshaft journals: Crankshaft needs to be reground (if repairable) or replaced. Adds $500–$1,500 NZD to the above.
- Extensive damage: Engine replacement from a quality second-hand unit (common with Japanese import engines). $2,500–$5,500 NZD fitted.
Oil: The Most Important Factor
The single best way to prevent engine knock is regular oil and filter changes with the correct specification oil. Most NZ mechanics recommend 10,000 km intervals for conventional oil and 15,000 km for full synthetic — or the manufacturer's recommendation, whichever is shorter.