Actros Fuel Rail Pressure Fault – Loss of Power Diagnosis

This 2017 Mercedes Benz Actros MP4 (963) had lost all power, was rattling loudly and blowing excessive black smoke. The customer said it would occur soon after going around a corner.

But was it a left or right turn? It’s not such a stupid question when diagnosing. I have come across quite a few similar faults in my time. One that sticks in my mind is the old Perkins in the ACCO. It would stall only when turning left. At first, I thought my boss was taking the piss and it was another apprentice prank, but he wasn’t kidding! As it turned out, the lift-pump valve would lift off its seat, causing a drainback of fuel, particularly when it was low on fuel in the tank. The vehicle would then stall. One of the diagnostic signs was its eventual, though hesitant, restart after pumping the hand primer at the base of the lift pump. Turning right was just fine; it was left turns that were the issue.

This was not the case with this vehicle. During earlier discussions with our Diesel Help member, they said they had been able to get the fault occurring at any speed and motion.

Before picking up the scan tool, scopes and other expensive test equipment, discuss all symptoms thoroughly with the operator. There will be key points that will assist without stripping the vehicle bare searching for a fault.

One important detail was determined during the diagnosis; the Mercedes was only faulting when the engine was warm, after driving for 10 minutes. After further testing we pinpointed that it was not until it reached operating temperature that something changed.

We know fuel quantity changes depending on the change in the temperature read by fuel-temperature sensors. Look at your scan tool for desired and actual. This can provide you with an idea of what the electronic control unit (ECU) or module is demanding.

On some occasions, at idle this vehicle would almost stall. The fuel-rail pressure (FRP) dropped to as low as 170 bar – the minimum common-rail pressure is 200 to 300 bar. Fuel pressure faults like this are often misinterpreted or diagnosed incorrectly — see fuel pressure fault SID78FMI5 — and shortly after it would come back up to around 300 bar and idle. The vehicle was running very rough, blowing smoke and had a noticeable injector rattle but would idle OK.

Fault Codes

During this diagnostic case there were no fault codes. In some cases, you may experience the following:
• 35040E – The leakage in the highpressure control circuit is too high.
• 9D0010 – The specified rail pressure is not reached.
• 9D0012 – The specified rail pressure is exceeded.

For a broader understanding of how faults like this fit into modern diesel systems and how to approach them correctly, see our guide on Know the diesel fuel system you are diagnosing.

Our member provided an incredibly detailed explanation of the Actros diagnostics implemented so far. The fuel-rail pressure sensor readings were checked with the engine not running (KOEO) at both cold and warm temperatures. The pressure fluctuated heavily and – on some occasions, more so when warm – remained high between 300 and 3000 bar.

From here we were able to confirm an open or short circuit. To confirm this, disconnect the FRP sensor with KOEO and read the rail pressure on the scan tool; only with the ignition switched off, and disconnect electrical plugs. According to our workshop data-provider HaynesPro, the Actros’ maximum rail pressure is 2400 bar.

Before jumping to the conclusion that the rail sensor is the issue, make sure you test the wiring circuit between FRP sensor and ECU/ECM. It is possible another sensor on the same circuit could be interfering. On this vehicle application, the circuit has a low pressure fuel sensor and boost sensor with built-in temperature sensor.

After all circuit wiring was tested and the other sensors were confirmed to be operating as required, the final confirmation of the fault was the technician pouring hot water onto the sensor when the engine was cold. Cold, it ran fine. As soon as hot water was applied, the problem began to occur.

This confirmed the rail sensor would require replacement. On some applications it is not safe to replace only the sensor. If there is no sealing gasket or torque specification available, replace the rail assembly – including the rail sensor – as a complete assembly to avoid costly contamination or an internal leak.

Make sure you replace the correct fuel-pressure sensor. Some of these Actros applications also run a low fuel pressure sensor. HaynesPro has a broad coverage of Mercedes and European truck applications. Including many specifications, including parts locations, to help with diagnosis and replacement.

However, when diagnostics stall or test results don’t align with expected system behaviour, this is where real-world diagnostic support becomes critical.

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

The common-rail pressure sensor is located on the rail. This sensor detects the fuel pressure in the common rail, converts the pressure into a voltage signal and sends the signal to the ECU.

The ECU supplies 5V to the fuel-rail pressure sensor on the 5V reference circuit. The ECU also provides a ground on the low circuit. Higher common-rail pressure provides higher fuel-pressure sensor voltage while lower pressure provides lower fuel-pressure sensor voltage. We believe this rail sensor failed due to corrosion, possibly caused by the washdown process and/or natural wet weather conditions.