This fault remains fresh in my mind. That is because it is one of the seven faults I discuss in our webinars. Covid-19 border closures have restricted my training travel so in late 2020 we released a series of webinars being ran once a week with coverage throughout the world. The fault I am referring is a surging 2009 VW Golf. The customers complaint was the vehicle surges under load and was more noticeable above 100km/h. Our member found code P0088- Rail pressure too high present in the system.
Understanding that a fuel rail pressure code could be triggered by several components or causes such as a fuel restriction or air in the diesel fuel, you must first assess what type of fuel system the vehicle is fitted with. This VW model uses Bosch. Most VW/Audi’s are fitted with Bosch fuel systems and on the odd occasion have been known to use Continental. This design has a Fuel Metering Valve (FMV) located on the pump and a fuel pressure regulator screwed into the end of the rail (opposite end of the fuel rail pressure sensor). This Metering Valve is referred to as ‘Normally open type’. It does not require voltage to operate, without power and the piston is moved by a spring to a preset position. This position enables an increase in fuel flow, thus raising the rail pressure. On this model when disconnected from the power, the engine will start, idle and drive. This is only possible when a pressure regulator is fitted to the rail to compensate for the change in fuel flow, altering the pressure.
One of my popular diagnostic tools is the Eliminator (a separate fuel supply). In this case I didn’t believe it was required for this diagnosis as the vehicle was not displaying symptoms needing me to test the low-pressure fuel supply. Symptoms for this type of fault could be stalling, difficult to start or not starting at all. In this case, I have asked for the fuel rail pressure readings on the scan tool. Some scan tool descriptions of fuel pressure can be misrepresented. To assist with this, I have provided some common descriptions:
CRD Pressure, FRP, Fuel Pressure, rail pressure, reference fuel pressure.
I prefer a scan tool to display two pressures. Desired and actual. If you only have actual, it is difficult to reference what the vehicles computer is requesting. Look for descriptions similar to-
Desired, demand, required, requested or fuel pressure.
You can determine which is desired rail pressure by looking what the pressure reading is with the key on engine off. This should be display 200 bar or more.
The actual pressure should be less than 10 bar after switching the engine off.
I’ll be looking for a large fluctuation in pressure at idle. + or – 20 bars at idle is excessive and a sign there is an issue. When this vehicle fault was occurring, the rail pressure fluctuates high in revs. Whilst driving, the fuel pressure demand was 630 bars to 640 bars. The actual pressure was fluctuating between 400 to 800 bar.
To assist with confirmation of your diagnostics, disconnect the metering control valve on the pump. With care, test drive the vehicle. The engine check light will illuminate and may or may not be in limp mode due to the disconnection of the valve. If the vehicle will drive smooth and the noticeable surge is no longer present, you have confirmed a failed Fuel Metering Valve.
Solution/repair:
Remove and replace the metering valve, as necessary. Ensure you thoroughly clean the area and use dust caps before removing any pipes.
Before carrying out any such replacement of fuel system components, always remove fuel filter to inspect the quality of the fuel. With any contaminants present especially water or petrol, YOU MUST REPLACE ALL components in the High pressure CRD system. Contamination does not bypass anything! Once you have determined the system is clean, proceed to replace with new genuine metering valve and new fuel filter. Carry out a relearn of the metering valve and other related components in the system.
