Hi all,
I am trying to troubleshoot a P0191 DTC and low fuel pressure at the rail. I have about 24 PSI at the rail during cold idle, measured from both OBDII and at the schrader valve. I recently purchased the car, so I thought a new fuel filter was in order anyway. The old one had some resistance. Replaced it and the problem persisted. I next suspected a lazy fuel pump so I replaced that too. No luck. Still only getting 24 PSI at the rail. It idles a little rough and has low power and does not like to climb hills. This problem began suddenly the other day. I wonder about the fuel rail pressure transducer, but it should be working properly if it is reading the same pressure as an analog gauge, but maybe this thinking wrong? Thanks!

 

Does anyone know what the circled part is? If I didn't know any better I would have thought it was a fuel pressure regulator, but I am not aware of this car having one. Thanks

 

that’s the pulse damper


fuel pressure is regulated by the pressure sensor on the intake close to bank 1. same part as the 4.6 2V mustang

 

Thank you, xalty. I knew about the FRP transducer, but I didn't know what the pulse damper was. I didn't think that this car had a regulator in addition to the FRP sensor, but I couldn't figure out what the damper was.
Do you have any thoughts about my low fuel rail pressure? I am going to check for any clogs in the system. If that doesn't prove to be helpful I'll swap out the pressure transducer tomorrow afternoon. I am going to get a spare pressure relief valve too, from another fuel pump housing, just in case.

 

How many miles has he vehicle done?

 

149k.

 

Okay, so this is resolved.

Here are all the checks that I did and what finally fixed the problem.

1) Checked that the fuel rail pressures sensor (FRP) matched an analog fuel pressure gauge. Both measured 24 PSI and eliminated the FRP as a culprit in my mind
2) Next step was to replace the fuel filter, as a clogged filter might lead to low pressure at the rail. Problem persisted
3) Replaced the fuel pump motor, but retained the housing. This seemed like the most likely culprit, but no luck. Still 24 PSI
4) I decided to bite the bullet and go to a nearby salvage yard. Unfortunately they did not have any X350s, but they did have a 2002 S-type. I removed the entire fuel pump housing and the FRP
5) Replaced the FRP as a sanity check. Still 24 PSI
6) I next blew out all of the fuel lines from the tank to the rail. Reassembled and still 24 PSI
7) At this point I concluded that something in the fuel pump housing must be bleeding off pressure, so I swapped the whole fuel pump unit from the S-type into the XJ8. 55 PSI!

I examined the housing for cracks in the shell and in the hoses. I noticed something suspicious with check valve. There seemed to be a slight gap where the two halves are bonded to the middle piece. I gently squeezed them together, but the gap remained. So then I gently pulled them apart and ...


Check valve

And the check valve just fell apart. So it seems that the check valve was bleeding pressure in the tank, not allowing the fuel system to achieve the 55 PSI the ECM was requesting. It fell apart with so little force that I can't believe it was able to withstand 24 PSI. This fuel pump was purchased by the previous owner for around $800 in 2019. It has less than 6k miles on it. This is astonishing to me. It should be noted that it appears to not be a genuine part, but for that price I would have expected genuine.

Well, anyway, the Jag is back on the road and I now have a spare FRP and three functioning fuel pump motors and two complete fuel pumps.

If you find that you have low pressure at the rail using both the OBD II reading from the FRP sensor and an analog gauge while having no external leaks outside the tank, I would pressure test entire fuel pump assembly to rule out any hidden leaks that would be difficult to detect and might be a very cheap fix. Even with the brand new pump motor and the junkyard unit, I am only into this repair $60 and will have two fully functioning fuel pumps once I replace the check valve on my bad one. Very soon I will replace the S-type pump motor with my new one, since I do not know the history of that car.

Thanks for your help!