Dear friends, I need your help and experience: my 2003 XK8 was throwing me some P0193 codes, which means "fuel rail pressure sensor circuit high input".
As first tought, I swapped the sensor for an OE genuine, but the problem remained and the code came back after a while.
I also changed the sensor connector to a new one - no help - so I decided to go deeper:
I measured the voltage on the sensor output with an oscilloscope and I figured out that the values were perfect, around 3.8V, the fuel pump is regulating nicely (duty cycle from the ECM: 30%-40%), until the car is going downhill and the throttle is closed and the engine is turning above 2000 rpm.
At this point the fuel pressure starts to ramp up slowly and after 10-15 sec, the fuel pressure sensor output reaches 5V which is the maximum limit.
I also checked the fuel pump paralelly and the pump is off (no current, turn off signal from ECM) when using engine brake.
My question for the pros: my system is a non-return fuel injection system - is there a pressure relief valve built in the pump unit (C2N1146) or somewhere else that can be faulty?




Picoscope Fuel Pressure Sensor Out while going downhill with engine brake.

 

In addition to my previous post, today morning I checked the pressure relief valve, built in the fuel pump, by adding pressurized air into the fuel pressure test valve (schreder):

- Opens (pop-off) at 5.5 Bar

The problem is that at this pressure, the sensor has already reached his limit, not to mention that with vacuum in the intake manifold, it will reach his limit even before.

My problem is now narrowed down to two thing:

1.) Pressure sensor wrong type.
2.) Pressure relief valve opens at wrong pressure.

I suspect more the pressure sensor type problem, because the labels are not matching with the old one:

The old was labeled: Ford XW4F 9F972 BB
The new is labeled: Ford 3R3E 9F972 AC

I also tried with 2 aftermarket, unlabeled sensor with the same bad result.
Can someone tell me the difference between the sensors?
Maybe USA modell have different sensor for the same MY?

I ordered a used sensor with the old label from ebay, and when it arrives, I will have to measure the two different pressure sensors characteristic.

 

Please help me by telling the name and function of the device shown on the picture. I can't find in the partlists and manuals.

 

Have you checked the vacuum line to the sensor (make sure it is not loose or leaking)? Also, fuel pressure is available over OBDII (a cheap ELM327 can read it). Should be a solid 55 psi in all circumstances (idle or acceleration).

Downhill closed throttle is also high vacuum, so check your fuel trims (ELM327 again). Ideally, long term trims should be, say, 5% or less (I can see the patchwork on the intake tube, so maybe you have leaks of metered air).

I have had P0193 before on a replacement sensor (the original one developed a leak). I returned it for a replacement. This (third) one seems to be holding up.

Last, probably would not hurt to replace the fuel filter if the car is new to you (driver side rear wheel well). Make sure you double check the fitting. Here in the US, the part is incorrectly referenced by the parts store (they list a filter with barb fittings vs. need to be quick-connect). Double check with a S-type 4.2L.

Best of luck, keep us posted.

 

Thanks for the advices, my vacuum line to the sensor is new and I have a good OBD reader/logger (Foxwell NT624), SDD, JTIS, oscilloscopes and lot of patience as well. I used to monitor the fuel pressure trough the OBD, but in that special case, I prefer the oscilloscope measurement, since it can reveal electrical issues (for example: noise or spikes) and the measurement limit (5V) of the sensor, these are not revealed by the OBD.
Naturally, both method has their advantage/disadvantage, best is to know which one (or both) to use in case.

What really would help me, if somebody could measure the fuel pump's regulator (pop-off valve) opening pressure on a XK8.
This can be easily done by connecting a regular tyre air filler to the fuel rail schrader valve and slowly increasing the pressure until this pop-off valve opens and one can hear the bubles from the fuel tank.
(It is a good idea to bleed the fuel system with the same air filler device after this test. The remaining small amount of air will leave the system trough the injectors after few minutes)

My valve opens at 5.5 Bar (80 psi) which make me think that somebody fitted an XKR pump in my car, because I read that in the supercharged engine, the fuel pressure is set to 5 bar (72 psi).

Another theoretical question: the same sensor is used in both XK8 and XKR (part number AJ87977), according to the documentation. How is it possible to maintain a fuel pressure of 5Bar (72 psi) in the XKR engine, with the same sensor, if my sensors (3 pieces) reaches 5V (maximum) at the pressure of 4.5 Bar (65 psi)?

 

The fuel pressure reading is always taken in reference to manifold pressure, not general atmospheric pressure. That is why the sensor has that vacuum (or pressure for the XKR) line. So even if you have forced induction, all you are measuring is the difference.

I say replace the fuel filter (cheap) and check the condition of what comes out. It it is anything other than completely clean, focus on cleaning the system (injector cleaner, etc.). How is gas quality in Hungary anyway (never been there, but Europe's is generally very good). People have reported nasty dark fuel coming out when that filter is neglected.

Altogether, I vaguely remember folks posting about a bad batch of these expensive XK8 fuel pumps, specifically when it comes to that return feature. I believe the issue was the inability of the car to maintain fuel pressure during stops, and causing hard starts (vapor lock?). So maybe the opposite of what you are describing, actually.

So, as per your description, if you log the fuel pressure over OBDII, it remains at 55psi all the time, except when you are at high vacuum (downhill, off the gas, above 2000 rpm). And at high vacuum, the fuel pressure reading goes up above 56 psi, and eventually maxes out with a P0193. Correct?

 

I am totally aware of the differential pressure measurement, but the measurement range of my tested sensors doesn't fit, even with vacuum, even with induction.

Your understanding is correct, except that the pressure is rising slowly going downhill, because of the radiated heat of the engine. (See the measurement graph above)

 

Today I found something interesting in the S type service training manual:

• The fuel rail pressure relief valve opens at approximately 4.14 – 4.48 Bar (60 – 65 psi) to relieve excessive fuel rail pressure.

If the same pressure values should be present in my car, than this can be the root of my problems, since my fuel pump integrated pressure relief valve opens at 5.5 Bar (80 psi) which is too much.

 

What did you ever figure out on this fuel pressure issue? I have the same problem with same fault and same fuel pressure rising on decel going down hill . My fuel pressure gauge is showing 60 psi while the scanner is showing 73.2 psi .

 

Sorry for getting into the old thread. I am new to forum, didn't find any link how to creat a new thread.
My 04 xk8 has same code p0193 with p1582. Car starts, revs and drives good but don't have the thrust feel of a v8 as my previous v8 jag had. Have to push hard throttle some time to touch the floor, the revs up but slowly. In first gears it's quick but then it doesn't give the feel of a v8. Have a little crack in air pipe. The fuel pressure on the scanner is 55 psi with slight fluctutaion between 54.3 - 55.8. When the engine is off the pressure is 70 psi. I checked the pressure while driving it and it's the same 55 psi with minor fluctuation. I am not an expert, just got a basic scanner and basic knowledge. The fuel gauge was also playing up, it's steady now for last few days. Car had an accident recently.
What could be my options to start from? Replacing the fuel filter? pressure sensor? air pipe? possibility of fuel pump?
Thanks

 

Sorry for getting into the old thread. I am new to forum, didn't find any link how to creat a new thread.


My 04 xk8 has same code p0193 with p1582. Car starts, revs and drives good but don't have the thrust feel of a v8 as my previous v8 jag had. Have to push hard throttle some time to touch the floor, the revs up but slowly. In first gears it's quick but then it doesn't give the feel of a v8. Have a little crack in air pipe. The fuel pressure on the scanner is 55 psi with slight fluctutaion between 54.3 - 55.8. When the engine is off the pressure is 70 psi. I checked the pressure while driving it and it's the same 55 psi with minor fluctuation. I am not an expert, just got a basic scanner and basic knowledge. The fuel gauge was also playing up, it's steady now for last few days. Car had an accident recently.


What could be my options to start from? Replacing the fuel filter? pressure sensor? air pipe? possibility of fuel pump?


Thanks

 

Any air leak such as a crack is going to cause big trouble so get that fixed (properly - needs to be 100%) and see if the P0193 persists.

If it does, next follow the advice in the thread.

 

For P0193, I would say start with a bottle of quality injector cleaner to see if you can get the relief valve to somehow do its job more consistently. Folks mention Techron. Read the directions, but do not go over the amount indicated. This stuff makes rubber swell like crazy, so more is not better. Start by going through a full tank (with appropriate quantity of Techron) and check for improvement. I believe the valve is "triggered" under high vacuum, so throttle closed, while decelerating, or going downhill (engine brake). Manifold pressure is available over OBDII. If you use TorquePRO with a cheap ELM327, you can record these numbers (fuel pressure, manifold pressure) while you drive and check these numbers later for correlation, keeping your eyes on the road.

Best of luck, keep us posted.

 

I will try the injector cleaner. Attached are the pics of the scanner showing fuel and manifold pressure. The issue comes when vehicle is driving under load like going uphill, have to press the throttle fully. Going downhill no issue.No issue if driving gently. Revs go up normal in park mode. Thanks

 

For that, have you looked into adjusting the "throttle" cable. It is the cable from the pedal to the sensor. The adjustment of the slack is done next to the brake booster, it is a "fork" design on a bunch of beads at the end of the cable. That cable needs to have very little slack. This is a common problem.

If it is really engine related, check the long term fuel trims, both banks. Anything over, say, 5% means it needs attention. Air leaks make the engine feel sluggish, but a code only triggers fairly high at 25%. You can also throw a new air flow meter, our cars use a very common model.

Best of luck, keep us posted.