Causes and Troubleshooting of Diesel Engine Knocking

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Causes and Troubleshooting of Diesel Engine Knocking

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Symptoms of knocking during diesel engine operation include:

Heavy knocking noise from medium speed to low speed, black smoke from the exhaust pipe, occasional backfires or flame ejection, and overheating of the engine body.

The main causes of knocking are as follows:

1. Overly Early Fuel Supply Timing

If the injector starts injecting fuel before the piston reaches the specified injection position during its upward stroke, combustion begins prematurely.

High-pressure gas in the cylinder impacts the piston, producing a rhythmic, crisp “tap-tap” knocking sound, which becomes especially noticeable when the throttle is reduced.

Remedy:

Adjust the fuel supply advance angle by adding gaskets between the fuel injection pump and the gear housing to delay the fuel supply timing to the specified value.

Each 0.1 mm increase in gasket thickness delays the advance angle by approximately 1.3°–1.7° of crankshaft rotation.

2. Poor Fuel Combustion

Insufficient injection pressure, injector dribbling, poor fuel atomization, and excessively late fuel supply timing can all lead to incomplete combustion and subsequent knocking.

Knocking caused by such faults is usually dull and husky, accompanied by engine overheating, black smoke, exhaust backfires, or flame ejection.

Remedy:

Remove and inspect the injectors, readjust injection pressure and timing, and replace components if necessary to meet specifications.

3. Wear of Delivery Valves

Worn delivery valves result in two main issues:

Increased clearance between the pressure-relief ring and the valve seat, reducing sealing and pressure-relief efficiency;
Excessively high residual pressure in the high-pressure fuel line, leading to over‑fueling and violent combustion that causes knocking.
Knocking becomes particularly obvious when the throttle is lowered.

Remedy:

Replace the delivery valves and valve seats.

Note

It should be emphasized that a technically sound diesel engine may sometimes produce rhythmic knocking during low‑speed operation.

This is caused by vibration from a rapid rise in gas pressure during normal combustion, not the abnormal knocking described above.

Published：Mar.24.2026 Viewed：103

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Related Information

1. Overly Early Fuel Supply Timing

2. Poor Fuel Combustion

3. Wear of Delivery Valves

Note

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