Diesel Help works with all sectors of the automotive trade, from heavy-commercial to light passenger-vehicle markets, so we see a range of very different diagnostic approaches.

With passenger vehicles, customers are generally more accepting of a cost-effective diagnostic approach. This often means the vehicle remains off the road until a correct and confirmed repair outcome is achieved.

In the commercial sector, it is often the opposite. A truck sitting idle during diagnostics is seen as costing the operator as much as replacing parts without a confirmed diagnosis.

In the current supply market, future trends may force the heavy sector to adopt a diagnose-to-repair approach. This is Diesel Help Australia’s preferred diagnostic pathway. The following case study ultimately reached a successful outcome, but not without significant investment in parts and labour.

The vehicle

This case involved a 2005 Volvo FH16 cab-over prime mover, fitted with the D16C610 engine.

The truck had accumulated 1.6 million kilometres and was still running well — until it developed intermittent loss of performance and stalling, accompanied by the fault code:

SID78FMI5 – Supply pump actuator / fuel pressure too high

Before Diesel Help members became involved, another workshop had already attempted repairs, replacing the fuel lift pump due to it being a known common failure.

Understanding the system before diagnosing it is critical

Before going deeper into the diagnostics, it’s important to understand the system fitted to this engine.

The D16C uses a Delphi Electronic Unit Injector (EUI) system.

Anything fitted with wiring, a fuel supply pump, and electronically controlled injectors is often mistaken for common rail. However, there is a significant difference between EUI and common-rail diesel systems. This distinction is critical when diagnosing faults — start here: Know the diesel fuel system you are diagnosing.

Delphi EUI system operation

Unlike common rail, the EUI system does not use a separate fuel rail.

The electronically activated fuel pump operates a low pressure feed, min. 1 bar to a max. 3 bar. Fuel is drawn from the tank, passes through two fuel filters, and is then delivered through internal ports in the cylinder head.

Once fuel reaches the injectors:

• A portion is injected into each cylinder at the correct timing

• Excess fuel returns from the cylinder head

Injection pressure itself is not generated by the pump.

Instead, the engine camshaft creates the high injection pressure. During rotation, a separate rocker assembly driven by the camshaft pushes down on each injector plunger, increasing internal injector pressure. This is why injector rocker adjustments are critical and must be performed during servicing.

Sensor inputs influencing fuel delivery

Multiple input signals influence this system via the ECU, including:

• Camshaft position sensor
• Crankshaft speed and timing sensor
• Intake air temperature and pressure (MAP)
• Coolant temperature
• Fuel temperature
• Pedal position sensor
• Atmospheric pressure sensor
• Airflow meter

On some D16C engines, an additional sensor is fitted — the crankcase ventilation pressure (CCP) sensor.

Crankcase pressure sensor function

The CCP sensor is rarely seen in vehicle applications and depends on:

• Blow-by flow

• Under-pressure generated by the oil separator

Blow-by gases originate mainly from combustion gases passing the piston rings, though valve guide wear or turbocharger faults can also contribute. These gases contain exhaust mixed with oil vapour.

A high-speed separator removes oil from the gases. Under suitable conditions, the ECU stores the minimum difference between crankcase pressure and barometric pressure (BARO) over a given time period.

The diagnosis begins

Using an inline pressure gauge, Diesel Help members confirmed the SID78FMI5 fault. The scan tool indicated elevated pressure, and physical testing showed fuel pressure exceeding 3.8 bar.

According to Jaltest specifications, this was outside acceptable parameters.

The decision was made to replace the fuel pump with a genuine unit. While pressure initially dropped, the fault returned — and became more frequent.

During further testing, a clear hose fitted to the injector return line revealed air ingestion returning from the injectors.

Experience matters

During this diagnostic process, I was attending the 2022 Australian Automotive Aftermarket Expo in Melbourne. A technician who regularly reads Australian Diesel Mechanic Magazine and had attended one of my training courses stopped to chat.

His workshop primarily dealt with heavy-vehicle applications. When I explained the symptoms, he mentioned a known issue where injector sleeves fail, allowing combustion gases to enter the fuel system.

This insight proved critical.

Injector investigation

Based on the air return issue and further inspection, the rocker cover was removed. Scope testing of the injectors revealed drop-outs on cylinders 3 and 4.

At this point, the decision was made to replace the injectors.

The vehicle had been serviced by the same workshop for over 400,000 km, yet there were:

• No records of injector replacement

• No evidence of cam adjustment for injectors

Ideally, the injectors would have been removed and tested before replacement. However, when ordering the new injectors, the original set was sent away for testing. One injector was confirmed faulty.

A remanufactured injector set was installed, and adjustments were carried out. This resulted in an immediate improvement — the air return issue was resolved — but loss of power and stalling remained.

The real fault revealed

As the crankcase pressure sensor heated up, its internal resistance increased. This caused a false pressure reading, triggering engine shutdown.

During further investigation, another fault was uncovered.

The root cause of the SID78FMI5 supply pump actuator fault turned out to be an incorrect earth connection.

At some point, another workshop had found a spare wire when replacing the fuel pump. Believing it to be an earth, they fitted a ring terminal and grounded it directly to the pump body.

In reality:

• The wire had broken from the main harness

• It needed to be rejoined so it could earth and switch through the ECU

• While the pump still operated, the ECU could no longer recognise it correctly

This explained why the fault code first appeared immediately after the pump replacement.

This was causing the engine shutdown issue. As the crankcase-pressure sensor heated up, electrical resistance increased, resulting in a false reading and shutting down the engine.

Fuel pressure faults can also contribute to incorrect regeneration behaviour and ongoing DPF issues — particularly when underlying system faults are not identified — see repeated forced DPF regeneration problems.

If you're dealing with a fuel pressure fault that doesn’t make sense, access Diesel Help technical support and get guidance from technicians working on these systems every day.

Final outcome

Although the diagnostic process was lengthy, it resulted in a successful repair. I was extremely impressed with our members’ professionalism, persistence, and respect for:

• Their customer’s business

• Their own staff

• The wider industry

This case is a perfect example of why understanding how a diesel fuel system actually operates is essential before replacing parts — particularly on non-common-rail systems.

For more real-world diesel fault case studies, explore our diesel diagnostic resources.