Did you ever find a fix for your problem? I seem to have the same issue. 99.2% load and 19,850 psi when I get the P0087 code and CEL comes on.
Mine also tops at around 24,000 psi, so you’re down on pressure. The system regulates pressure by restricting the return flow out the Fuel Pressure Relief Solenoid Valve on the back end of the fuel rail, but also restricts fuel volume by cycling the Fuel Quantity Control Valve on the high pressure pump. HP fuel volume produced by the HP fuel pump is “consumed” by the:
- injector fueling events (combustion)
- injector pilot valve return flow (leak-off)
- fuel rail pressure relief solenoid valve
The pressure relief valve is controlled using a PWM signal from the ecm. The duty cycle of this signal can be viewed in Live Data on many T1N-aware scan tools… mine ranges broadly from less than 20% at idle to around 70% highway cruising. A higher value indicates “more closed” - a more restrictive relief valve setting.
If the combined volume of the fueling and leak-off is greater than the pump is producing then the system will loose pressure even with the relief valve running “maximum closed” with a 100% duty cycle signal. If that happens the ECM will set a fault code…
This can happen due to:
- excessive leak-off of injectors (also causes hard starting)
- leaking fuel rail relief solenoid valve
- fuel quantity control valve sticking closed
- worn HP fuel pump (relatively rare)
There are two leak-off test procedures described in the service manual: one for the injectors, the other for the fuel rail solenoid. Replacement of the leaky components is then required. (note that MB does not recommend servicing the OM647’s individual fuel rail components… you must swap the whole rail)
Unplugging the FQCV might diagnose a sticking valve, assuming it can open back up, though this will likely throw a separate code, which may then prevent building full power and invalidate the test.
Good luck!
-dave