Common rail diesel fuel pressure problems

I have a 2008 Jeep Grand Cherokee 3.0L CRD. I had to replace the fuel tank a few weeks ago because the old one was leaking pretty bad. Now off and on since then the engine light comes on and it goes into limp mode. I have seen two codes on the ECU. P0087 which is common rail fuel pressure too low and P0088 which is common rail fuel pressure too high. I changed the fuel filter a week after the tank was replaced, thinking maybe it was clogged. The engine light stayed off for a week after that but then it came on again. I then sent it in to the shop and they had a look at it to make sure they didn't mess up on the tank replacement but everything was fine there. They cleared the code again and it stayed off for about a week again. Then it came on again so I cleared it myself. Now a few days later it is coming on again and won't stay off. It seems to mostly go in limp mode when it's cold. It runs fine and has good power when not in limp mode. I put an additive in this tank of fuel that is supposed to clean dirt out of the system. Anyone know what's going on here?

A couple of things come to mind here...

The first would be air in the fuel system. Did you bleed the air out of the fuel system?

Air in this fuel system will cause oscillations on the fuel pressure sensor and cause both HI/LO pressure codes to be set.

Second, If the air was bled, voltage level checks on the fuel pressure sensor should be performed, it might very well be pooched.

If you have access to a scanner with live data, the fuel pressure PID should be available in Global OBDII.



Let's do some voltage checks on the fuel rail pressure sensor.

Make sure the battery is fully charged.

There are three wires, 5 volt ref, Signal and Ground. They're colors are in the above pic.

Since you're getting both codes, I'm expecting you will see erratic voltage readings on the signal wire.

KOEO, Sensor plugged in, and backprobing the harness side...

What is the reference voltage? should be near 5 volts
What is the Signal Voltage? should be 0.45-0.60 volts

KOEO, Sensor Unplugged, and backprobing the harness side...

What is the reference voltage? Still should be near 5 volts
What is the signal voltage? Should be near 5 volts

On this system low volts on the signal is low fuel pressure, high volts on the signal is high pressure.

Start the motor and observe the signal voltage while backprobing and sensor plugged in. It should not jump up/down to much, the voltage changes should be relatively smooth on a good sensor.

If the voltages are all over the place for no rhyme or reason even with some rpm changes, the sensor is more than likely bad, or you have air in the system.

Air in the system was the first thing that came to mind for me as well. I've seen this on several 24v trucks over the last few years. I don't know the system of your vehicle off the top of my head like some people do but on the trucks I worked on I saw an oscilloscope pattern of the SCV bouncing up and down - someone suggested it must be sticking but I learned it had already been replaced by mechanics the day before. Then I scoped SCV vs fuel rail pressure and it was almost like the pattern of an O2 sensor vs STFT - zig-zagging back and forth across each other. I then put a clear hose on the return line and you could see bubbles coming out the return every now and then. The fuel pump was bouncing against the air bubbles and the SCV was fighting to get its pressures and they would dip too low then too high in the process - like your fault codes.
On the one truck it was a cracked filter housing - we moved the clear hose between components, first suspecting a crack in the fuel pick-up pipe at the tank and narrowed it down to the fuel filter housing eventually. On the next one the mechanics had changed everything in the system to fix the problem - except for the two (worn) rubber/metal washers right at the intake of the high pressure pump - it was sucking air right at the pump.
So I would say either a scope pattern of fuel rail pressure or a clear hose on the return will let you know where to look next.

Need to also check the signal to the fuel metering unit. If it has a short it would spike highs and lows in the fuel pressure