Hello peeps! Been lurking for a while & decided to share this:
DON'T THROW THE BABY OUT WITH THE BATHWATER JUST YET
OK, I seem to have had most of the symptoms of failing SCV: loss of power, hesitating around 2000rpm, error codes: rail leak or rail pressure control valve malfunction, injection system malfunction, P0093... all sounding scary.
First time, changing the fuel filter helped a lot - it was black all the way up: figure blocked filter - not enough fuel under load. Makes sense. Then the problem came back, so out with the filter - this time it was only black 30% up the filter element. Hmmm. Problem did seem to go away, though.
Problem back the other day: now quite bad. I recorded some data with OPCOM - plotted desired rail pressure and actual rail pressure VS time. It showed sudden and large deviations: pressure rising, wanting 80MPa, suddenly falling to 15MPa.
Anyway, I took the top off the fuel filter assy and noticed that the filter housing body was not brimming full of fuel, but about an inch down. So I topped the filter up, put the pipes back on and finally topped it up through the 'bolt hole' at the top, put the bolt back in and.... THE CAR WAS BACK ON FORM
OP COMMed the desired vs actual rail pressures and the graphs were an exact match. Cool!
So, as these cars do not have a separate lift pump, but rely on the fuel being sucked up right from the tank, it seems like I have a small leak that's allowing air in. This air would collect at the highest point: the fuel filter housing. Once sufficient, this would mean the fuel pump starts to recieve air and fuel, causing pressure drops, especially when a load is placed on the engine - only enough fuel is pumped by the high pressure pump to supply the engine: that's what the SCVs do. It makes sense that the ECU interprets the pressure drops as a leak in the high pressure side and throws the codes.
Reckon this is definately worth a try before binning a big chunk of change on fuel pump bits... certainly buys a few at the bar.