The Vehicle Started the Day Normally

A diesel vehicle in New Zealand performed well when first started.

After a long period of driving, however, engine power gradually decreased.

If the vehicle was parked for a while, the problem became less noticeable again.

Because the symptom appeared during continued operation, fuel delivery was investigated.

The Fuel Tank Was Not Venting Correctly

As fuel left the tank, air needed to enter to replace the volume.

The tank ventilation system was restricted.

Over time, this created increasing vacuum inside the tank.

The low-pressure fuel system then had more difficulty drawing fuel.

Why the Problem Looked Like Injector Weakness

As fuel supply decreased, rail-pressure performance under load also became harder to maintain.

The driver experienced:

• reduced power;
• hesitation;
• weaker acceleration.

These symptoms could easily be interpreted as insufficient injector delivery.

A Simple Observation Changed the Diagnosis

The workshop compared fuel-supply behavior early in the drive with the condition after extended operation.

The change developed gradually rather than appearing on one cylinder.

Inspection of the tank ventilation system identified the restriction.

Repair Result

The ventilation issue was corrected.

The vehicle was then driven for a similar operating period.

Fuel delivery remained stable and the gradual power loss did not return.

Case Conclusion

This New Zealand case demonstrates why time and operating duration can be important diagnostic clues.

A fuel tank ventilation problem can reduce supply gradually and create symptoms that resemble injector weakness.

When the complete engine loses power only after extended driving, the workshop should examine shared fuel-supply conditions before individual injectors.