OK, my 2001 XK8 would set P0172 &n P0175 from time to time. You could clear the codes and they would stay away for a while, but always came back, so I started doing diagnostics.

Turns out the fuel pressure was around 40 psi, key on, engine off, but would shot up to 100+psi when you start the engine. Tried a second gauge just to verify that it wasn't a gauge error and got the same results. Next step was to pull the return line off, put on a piece of hose and ran it into a gas can, started the engine and got normal pressures, so I'm thinking I have a blockage in the return line.

Next step, I applied compressed air to the return line and it initially took quite a bit to get bubbling noises in the tank, but after a minute of so I could dial the pressure back to 25 psi and could still get bubbling. At this point I was thinking that maybe it was fixed so I put everything back together and started it up. Now I get 40 psi, key on,engine off, and it flickers rapidly around 100psi, down to maybe 80 up to ??.

Next try was a different fuel pressure regulator. Unfortunatly, same results, so I am a bit stumped and thinking/hoping/praying I'm missing something or someone else has fought this demon before.

I was further confused by the parts breakdowns I was finding, which showed a different configuration than my car. Spent hours downloading and installing the Jaguar EPC and now see that my car is just as it should be. I see there is a connection, pn NJA 6522CB, between the two halves of the return line, does anyone know if it is a check valve or regulator?

I'm at a lose at this point, my next option is to start inspecting the return lines for a damaged or crimped spot. Does anyone know where they run and where the above mentioned connection is? Has anyone run into this before? Car is pretty new to me, so I don't have any history to go on.

Sorry to be so wordy, just wanted to post my steps to see if I was missing something.

Any and all help is MUCH appreciated!

 

Well, pulled off the return line again and attached a hose going to a gas can and pressure is normal, so new fuel pressure regulator is working correctly.

Pulled return line on my XJ6 and it flows freely. I know the X300/XJ6 is an earlier design, but that leads me to believe the XK8 should flow without restriction. Can anyone confirm this?

I won't get a chance to work on it anymore until Friday, but will keep an eye on this thread and bump it from time to time.

Any input or experience is greatly appreciated!

 

It appears that you have diagnosed the problem pretty well. Return line into a can, pressure correct, Return line back to the tank, pressure too high.

Blockage in the return line.

Sorry, haven't personally seen that but short of running over a large curb and crushing the return line it all seems strange as the fuel going back is fully filtered.

 

Yeah, I'm sort of baffled. The unknown is the connector between the front and rear action of the lines. It lists for about $50, but at dealer prices it could be just a connector and nothing special. I guess my next step will be to get under the car and trace the line from front to rear. I really dread the idea of having to replace a line; I replaced a front to rear brake line once and it was an all day hassle.

Thanks!