Thrust Sensor vs HPFP

[Dvent]

So, you guys have managed to help me get my 2007 A4 running again by swapping out the lpfp.
I'm still having a little trouble so I'm asking for some help again.
I still have a check engine light for low fuel rail pressure.

"00135 - Fuel Rail/ System Pressure - Too Low - sporadic 008 - Implausible Signal"
"00135 - Fuel Rail/ System Pressure - Too Low - sporadic 002 - Lower Limit Exceeded"

Right now I'm leaning more towards replacing the pump itself. I attached a graph from the last live data I got.
It seems to fluctuate in proportion to what is required from the engine.
But the pressure is nowhere near what it needs to be.
If it was bouncing up and down more, I would lean towards changing the thrust sensor.

What do you guys think?

Engine Speed RPM Rail Pressure (Actual) BAR Rail Pressure (Specified) BAR
720 11.60 50.00
720 11.60 50.00
720 12.98 50.00
720 12.98 50.00
720 12.98 50.00
720 12.54 50.00
720 12.54 50.00
720 12.54 50.00
720 11.94 50.00
720 11.94 50.00
1320 11.94 50.00
1320 25.03 50.00
1320 25.03 55.25
1440 25.03 55.25
1440 26.32 55.25
1440 26.32 60.42
1600 26.32 60.42
1600 28.30 60.42
1600 28.30 62.63
1640 28.30 62.63
1640 28.81 62.63
1640 28.81 63.65
1680 28.81 63.65
1680 29.39 63.65
1680 29.39 63.93
1680 29.39 63.93
1680 29.18 63.93
1680 29.18 64.09
1680 29.18 64.09
1680 29.30 64.09
1680 29.30 63.94
1680 29.30 63.94
1680 29.27 63.94
1680 29.27 64.07
1680 29.27 64.07
1680 29.51 64.07
1680 29.51 64.22
1880 29.51 64.22
1880 29.51 64.22

[Audibot]

Did you change the fuel filter as well? A clogged filter could be why the lpfp died in the first place.

[Dvent]

Did you change the fuel filter as well? A clogged filter could be why the lpfp died in the first place.

Yes, filter was changed when figuring out the original problem. Fuel lines were blown out as well. LP sensor was also replaced.

[Audibot]

Then that sensor on the hpfp would be my next target. They should be fairly inexpensive, IIRC.

[Dvent]

Then that sensor on the hpfp would be my next target. They should be fairly inexpensive, IIRC.

This sensor?


Or this one?

[Audibot]

The top one. At least that's the one I've had to swap out a few times when I got codes.

[Dvent]

Yeah, that's the lp sensor. I've swapped it out and even gotten the audi dealership to give me a replacement for free.

Sent from my SM-N920P using Audizine mobile app

[Brillo]

The fuel rail pressure plotted above is the high pressure part of the fuel delivery system. If the pressure is low there will be noticeable effects especially during high demand (high gas pedal inputs and at higher RPM). It could be low for a variety of reasons: 1) The pressure relief valve (prv) could be malfunctioning by relieving rail pressure (lifting) at too low a pressure. I believe that it is purely a pressure activated mechanical valve normally set to lift at 126 bar. 2) The pressure sensor (front side of the HPFP, top photo in post #5) could be malfunctioning by sending bogus information to the ECU, causing it to modulate rail pressure to too low a value. I believe this is the sensor responsible for your logs in the plot above. 3) The solenoid valve ((not a sensor) right upper side of the HPFP, bottom photo in post #5) could be malfunctioning by not allowing the fuel pressure in the fuel rail to meet ECU requested demand level. I believe this is the valve between the low pressure and high pressure side of the fuel delivery system that controls the ingress of low pressure fuel into the HPFP. Find out which one is malfunctioning before you spend a lot replacing these components.

I suspect your pressure sensor may be malfunctioning because the pressure seems to be systematically low at the rpm range you provided in the plot above. The prv is likely not the problem because it would manifest at higher pressure end of the spectrum. It controls pressure during high demand and also protects the fuel rail from over pressurization by lifting allowing pressure to bleed of back to the low pressure side of the system or the fuel return, I don't remember which.

The solenoid could be malfunctioning but I'm not sure how to assess this - not sure its input signal can be logged. Also, I don't know if it can be replaced independent of the HPFP. Others please chime in here.
Brillo

[Dvent]

Pressure sensor has been replaced a couple of times now, once for free from audi. Haven't checked into the prv yet, but that'll be what i figure out how to test. The solenoid, like you said, I have no idea how to test. I'll have to look more into that one as well.

Sent from my SM-N920P using Audizine mobile app

[Brillo]

Have you checked your cam follower? Does your engine oil smell even remotely like gas? A leak in the pump piston seal would allow fuel to contaminate the engine oil and possibly not allow the pump to achieve very high pressure. Can you plot the fuel rail pressure out to a higher RPM? Any other possible leaks in the fuel system. If so would you probably would have already noticed a strong gasoline smell. Are you running an overly rich mixture - black spark plugs and sooty exhaust. Just throwing stuff out there to consider.