Every now and then (once in 50 km) my car, a jag X-type 2.5 model 2002 is suddenly stalling, losing power completely and fuel pressure is lost: fuel pressure drops within a second from 380 kPa to 200 kPa and below (150kPa). The figure in annex shows the phenomenon: in about 1 minute the fuel pressure drops about 10 times and recovers as soon as accelerator pedal is released. Pressure comes back to 380 kPa and once again the pressure is lost. If I do not release the pedal the pressure does not come back (not shown in the figure).
The engine light does not come on during that pressure drop issue. After about one minute the pressure drop issue stops and the car continues driving normally during the remainder of the trip (30 km or longer). Most of the time the issue shows up on a slope (quite often on the same slope), when accelerating, but sometimes also on other occasions.
Car starts normal: no issues there. I notice that code P1000 is no longer switching into P1111, where this was before always the case. I performed a hard reset of the electronic system: removed both battery cables for about 2 hours and finally short-circuited them to remove any voltages from the electronic system and went through the recovery procedure (windows, idling, varied trip).
I have that problem already since half a year: fuel filter has recently been changed. Spark plugs and ignition coils as well.
I plan to replace the fuel pressure sensor, but don't know whether this is a good idea.
Anybody familiar with such fuel pressure drop phenomenon? Jag dealer has no answer for this problem and does not find any in the jag data base for that car.
Can anybody help with this problem?

 

#1 = fuel pressure sensor (after fuel filter, you replaced)

#2 = beginning signs of a failing fuel pump, particularly if it occurs with lower fuel in tank (like around 75-90 miles left on "range"/"dist. to empty". You can read my saga on thread "my kitty died today" if interested.

 

When you say pressing gas pedal, how far down are you pressing?

It's possible the fuel pressure sensor could be faulty, but have never seen on the X-Type in my experience to go bad. However considering the age of the vehicle, it could very well be a culprit. I have seen quite a few fuel pumps go bad though, which will involve removing the tank from under the vehicle if deemed the issue.

Do you get any other codes?

 

When the problem occurs, I don't push the pedal far down. Maybe I should do that next time. When there is no problem, which is usually the case, I can give full power (4000-4500 rpm) and the car follows my foot.
Since about 1,5 years I sometimes find codes P0831, P0333 and P0430 (catalytic converter bank 2), but these don't show up when the car is having that pressure drop problem.
This winter (temperatures below 0°C) I noticed that when just started, the car had problems staying alive (rpm dropping to 500) and showing an empty hole between 500 rpm and 3000 rpm. It then was almost impossible to drive away with the car. Once underway the problem disappeared. The mechanic has cleaned the throttle house and since then that problem has disappeared: when cold the rpm go up to around 1.200 and gradually come down to around 700 rpm as motor warms up.

 

I replaced the fuel pressure sensor this morning. Everything went fine: the car drove excellent for 50 km. When returning home after a warm start the fuel pressure started to jump up and down and dropped even below 1 bar (see picture below: the x-axis shows the time). The problem started 7 minutes after a warm start and began after a stop at a traffic light. I had to pull off the road since the motor did not respond anymore to the gas pedal. The problem lasted for about 5 minutes and then stopped. I then went on the highway climbing a steep slope of about 1 km, no problem at all. I take that road a couple of times a week: the problem most of the time shows up at the same spot or maybe I should say at the same time after starting the engine when still warm. For the ramainder of the trip home I noticed no problem anymore( speed about 120km/h) with road going up and down and pressure stable around 380 kPA.

 

Problem solved!
I removed the backseat of the car and started measuring on the output of the fuel module that powers the fuel pump in order to find out whether the issue is due to the fuel pump or to the electronic steering.
Yesterday I made the same trip (35 km) as I do twice a week. I now measured in real time the fuel pressure and the voltage of the battery as well as manually the voltage on the fuel pump. On the trip to my destination I did not notice any problems, but if you look at the voltage graph (red curve), you notice small voltage drops; however these had no effect on the driveability of the car: the voltage on the fuel pump varied between 8,5V at idle and 10V when accelerating.
A couple of hours later I went back home and on the same spot where problems occured before the car stalled and struggled again and I had to pull the car from the road. I noticed that the voltage on the fuel pump dropped to 6-7V and even to 0. As soon as I lifted my foot the voltage came back to 8,5V but dropped again as soon as I tried to accelerate. Together with the loss of voltage on the fuel pump I nticed voltage drops on the battery going down as low as 12,1V while accelerating (blue graph). This time the problem lasted for about 10 minutes with in between periods where the car responded normally to the gas pedal. So this showed that the problem was not the fuel pump but the electronic steering of the pump. I then went back to my destination, since it was impossible to continue my trip.
From my observations I concluded that something could be wrong with the battery: although the battery was tested by a mechanic the day before and it passed the test.
I then switched the battery (my son had a spare one) started with a test trip: no battery voltage drops anymore, not at the battery nor on the fuel pump. Battery voltage started out at 15V according to the Jag specs for about 5 km and then switched to 13,2-13,4V. I went back home and everything went perfect for the last 35 km.
PS: the reason why the issue always occurred at the same spot or better after about 5 km is the fact that the battery is charging at 15V and then switches to 13,4V: drops of a couple volts at 15V have no effect on the fuel pump voltage, whereas at 13,4V this can be problematic. So apparently one of the cells of the battery now and then failed causing the voltage drops and the problem. The problem with the engine light started about 2 years ago, but that light dissappeared by itself after another 100 km or so and had no effect at that time on the driveability of the car until 5 months ago.
So end good all good.
Lessons learned: before spending money on a defect that only occasionally occurs, monitoring the battery in real time can save you a lot of misery and money.