Slovenian Light Commercial Diesel Vehicle Shows Persistent Idle and Low-Speed Torsional Vibration Despite Balanced Injector Corrections as Dual-Mass Flywheel Damping Is Investigated

The Vibration Was Initially Treated as a Combustion Problem

A light commercial diesel vehicle in Slovenia developed noticeable vibration at idle and during low-speed gear changes.

Because diesel engines can show similar symptoms when one cylinder produces less torque than the others, injector correction values and cylinder contribution were checked first.

The fuel-system data did not reveal a clear imbalance.

The investigation then moved to the component positioned between engine torque pulses and the transmission: the dual-mass flywheel.

Diesel Combustion Naturally Produces Torsional Pulses

A diesel crankshaft does not deliver perfectly continuous torque.

Each firing event accelerates the crankshaft, while compression strokes and drivetrain resistance create alternating torque changes.

A dual-mass flywheel is designed to reduce how much of this torsional movement reaches the transmission.

The simplified path is:

Cylinder torque pulses → Primary flywheel mass → Internal springs/damping → Secondary mass → Transmission

If the internal damping deteriorates, normal engine torsion can become much more noticeable.

Balanced Injector Data Changed the Fault Boundary

The workshop compared two possible sources:

Combustion-Origin Vibration

One or more cylinders generate significantly different torque.

Driveline-Origin Vibration

Combustion is relatively balanced, but the system that isolates crankshaft pulsation is no longer functioning correctly.

Balanced injector corrections did not prove that every engine component was perfect, but they reduced evidence for a major fueling imbalance.

Low-Speed Operation Made the DMF More Noticeable

Dual-mass flywheel problems can become more obvious during:

• idle;
• engine shutdown;
• low-RPM acceleration;
• clutch engagement;
• and gear changes.

These are operating states where torsional speed variation is relatively large compared with average crankshaft speed.

Technicians therefore paid attention not only to vibration amplitude, but to when the vibration appeared.

Mechanical Areas Reviewed

Inspection focused on:

• excessive rotational free play;
• abnormal spring movement;
• internal damping condition;
• axial or radial movement;
• heat discoloration;
• clutch condition;
• and abnormal noise during engine shutdown.

The exact allowable movement depends on the flywheel design and manufacturer procedure.

Why Injector Replacement Would Not Address the Cause

If the cylinders are producing torque normally but the dual-mass flywheel cannot absorb the torsional pulses, new injectors will transmit their combustion torque through the same damaged damping system.

The driver can therefore continue to feel roughness even after unnecessary fuel-system work.

Technical Lesson

This Slovenian case shows why diesel vibration diagnosis should separate torque generation from torque isolation.

The important question is not only:

“Are the cylinders producing equal torque?”

It is also:

“Is the driveline damping system correctly absorbing normal diesel torque pulsation?”

FAQ

Can a dual-mass flywheel make a diesel engine feel like it has an injector fault?

Yes. Loss of torsional damping can create vibration that resembles uneven combustion.

Are balanced injector corrections enough to confirm a bad flywheel?

No. The flywheel and other mechanical vibration sources still require direct inspection.