Fuel pressure too high (P0088) and high fuel pump duty cycle

kjhiggins · Feb 10, 2023

This is a continuation of a previous thread but since two of the three issues mentioned in its subject line were resolved (P0171 and high LTFT) with a PCV diaphragm replacement I thought I'd post a new thread specific to the fuel pressure issue I'm facing.

I'm getting a P0088 code (fuel pressure too high) in my 2010 XC70 T6. When I look at other diagnostics the fuel pressure is in the 650 kPa range and the fuel pump duty cycle is constant at 80%. I installed a new spare FPS but no change in the P0088 code or measured fuel pressure. I hooked up DICE/VIDA to read the fuel pump duty cycle from start and it jumps immediately to 80% and stays there no matter what the driving condition or fuel pressure. So, what determines the fuel pump duty cycle? From my understanding the ECM tells the PEM what it should be based on measured fuel pressure and perhaps driving situation. Why would mine be stuck at 80% when the pressure is obviously too high already? Is the duty cycle read by VIDA the commanded or actual duty cycle? I've read the PEM controls the pump duty cycle by controlling the ground for the pump while supplying a constant 12v power. Could the ground be shorting so the pump is on too much? Or could 80% be a default when some other control signal is missing?

volvogod · Feb 10, 2023

In VIDA go to product specs, Specs other to show various specs.
The fuel pressure spec is 480 kPa +/- 5, Duty cycle 50% =/- 10%.
The ECM wants to see 480 kPa at the fuel pressure sensor on the fuel rail. This is the only place that the ECM knows what the fuel pressure is. The ECM tells the PEM to vary the ground side of the fuel pump to maintain fuel pressure.
I could not find a fuel filter for this car. The fuel pump motor could be wearing out, so the PEM commands more duty cycle to maintain fuel pressure. Tough call if it is a fuel pump. I do not feel that it is a failing PEM.
Your last line about 80% is it default for something? It could be. The PEM is mounted on top of the fuel tank.
Try to look up in that area with a borescope camera for mouse nest or wire chewings. There is not any access plate from under the rear seat area. The fuel tank has to be dropped for fuel pump or PEM repairs.
#2 on tank pic is the PEM location.

Below is from VIDA about Fuel pressure Control. See line in Red.
-------------------------
Fuel pressure control, diagnostics
General
The fuel pump control module has no functions for diagnostics. Fuel pressure control components and functions are diagnosed by the engine control module (ECM).
The following components and functions are diagnosed by the engine control module (ECM). Diagnostic trouble codes (DTCs) can be stored for each component and function:
Fuel pump control module, a diagnostic checks the power supply. A diagnostic trouble code (DTC) will be stored and the fuel pump will not work if there is no power supply to the fuel pump control module
Fuel pressure sensor with temperature sensor, a diagnostic checks the pressure and temperature signals to see if they are outside the normal operating range of the sensor. If a fault in the fuel pressure sensor is detected, the fuel pump (FP) will run at a predefined power and the pressure is governed by the by-pass valve in the fuel tank
Fuel pressure, the engine control module (ECM) compares the requested pressure with the measured pressure(signal from the fuel pressure sensor). If the measured pressure deviates excessively from the requested pressure, or if the pressure pulses, a diagnostic trouble code (DTC) will be stored for incorrect fuel pressure. The diagnostic is not active if a diagnostic trouble code (DTC) for the fuel pressure sensor is stored. A diagnostic trouble code (DTC) for fuel pressure will be stored if there is a fault in the fuel pump.
Communication cable. The engine cannot be started if there is a fault in the communication cable between the engine control module (ECM) and the fuel pump control module. A diagnostic trouble code (DTC) will be stored for the fault.
For further information about diagnostic trouble codes (DTCs), see Diagnostic trouble code (DTC) information for the diagnostic trouble codes (DTCs).
Components and functions which cannot be diagnosed:
If there is a fault in the by-pass valve in the tank unit, the starting process of the engine will take longer if the valve is leaking fuel when the fuel pump is off. If the valve does not open when there are pressure peaks in the fuel system, engine operation will be affected negatively, especially after engine braking. The fault can also cause rough idling.
--------------------------------------

kjhiggins · Apr 17, 2023

To follow up on this issue, I finally had the chance to pull the CEM and clean all the connector contacts. I didn't see any obvious oxidation or corrosion but the problem now seems to be gone. I'm not sure if was the cleaning or the effective system reset because the battery was disconnected for many hours. I had already done an adaptations reset using VIDA with no difference so I didn't think a battery disconnect reset would make a difference. I've driven it 100 miles and the fuel pressure stays in the 365-375 kPa range and the fuel pump duty cycle sits around 60%, which may be a bit high but I'll live with it for now.

pbierre · Apr 18, 2023

The 60% duty cycle (nominal 50% on a brand new car) merely indicates a pump that has 13 years on it. The servo control system easily adapts to aging of the fuel pump. You've been promoted to senior DIY repair mechanic!

Fuel pressure too high (P0088) and high fuel pump duty cycle

Our Vendors

Top Contributors this Month

Recommended Communities