Poor quality fuel can come from the ingress of contaminants due to a dirty filter, the low or high-pressure pump breaking up, or a build-up of corrosion in the fuel system. Introducing water or petrol will also rob the diesel of its lubricity, thus causing the valve seat to wear prematurely.

If fuel quality is inferior, the fuel will erode the seat of the Pilot valve, preventing the ball from shutting off the valve and, therefore, not allowing the pump to build up rail pressure.

The industry-recognised “back leakage” or “injector return test” can check whether any injectors are suffering from high return.

Symptoms of high return in a common rail injector include:

• Difficult or non-starting of the engine
• Un-even tick over
• Engine cutting out under acceleration
• Once started engine is unable to accelerate
• Lack of power
• Low fuel pressure faults