Hi guys.

I have a 2004 X-type 2.5 V6 4x4 with returnless fuel system and I´m having a strange fuel pressure problem. I read many posts about fuel pressure problems this car has a reputation for, but no one seems to have the same problem as I do.

The car starts normally with fuel pressure around 4bar, runs completely fine for 10 - 20s then the fuel pressure dorps suddenly to 0.8bar (ECU fuel pressure modulation drops to about 15-16%). The car stutters and dies. I can somewhat keep it running with throttle, but the modulation and fuel pressure both stay low (although they move around) and the engine struggles to keep running. When I connect the fuel pump directly to 12V, the pressure rises back up and the car runs normally at idle (for obvious reasons I only tried this at idle), so the pump itself should be OK. I proceeded to change the fuel pressure sensor and the fuel pump driver module but it didn´t help and the car does the same. I´m beginning to run out of ideas what else could the problem be?

I assume that it isn´t the fuel system at all and it may be a completely different electrical issue... Did anyone come accross this kind of a problem? Or do you have any suggestions what can cause this behavior?

ECU diagnostics shows only P0191 error.

 

Juro, from what you are describing, it is sounding like you have a failed fuel pump module. What you are seeing is when the car is cold, it all works fine. But, once you start the car, some component in the fuel pump module is overheating (sounds most likely to be a transistor that directly powers the pump). This causes the pump to run on essentially reduced power, causing it to run slower and at a lower pressure. You can prove this by getting the car up and running, letting it go into this failure mode, then turning off the car to them attempt to restart it. Because the electrical component would still be hot, the car should not start because of too low of fuel pressure. If the car restarts, then we need to relook at what is bad.

 

Hi Thermo.

I did try this and it restarts fine, just to stutter again in few seconds. I also changed the fuel pump module for a different one and it does the same. The module I swaped in was not new, but I doubt that two modules would have the same failiure.

Also when I watch live values in ECU diagnostic software, it shows that the first few seconds that the engine runs fine, fuel pressure modulation is around 70% and the fuel pressure is around 4bar. Then the modulation as well as the pressure suddenly drops to 15 - 16% and around 0,8bar. And the engine starts to struggle again. That´s why I suspect that the fuel regulation works fine, but there seems to be something that causes the ECU to drop the PWM regulation to such low values. Could maybe a faulty air flow meter tell the ECU that the volume of air is low and the ECU drops the fuel pressure? I think something in these lines.

 

When it starts up, I assume that it can run is some kind of a "choke" mode, not caring about values like the air flow meter... and then when the "choke" switches off the ECU starts to regulate the fuel pressure and that´s when the problem occures.

 

It does have in effect a choke mode. It'll start OL (open loop), check various sensors (ECT cf IAT to get a feel if the engine's hot, O2 sensors to see if they're hot, etc) and go CL (closed loop) as soon as it can.

Does the reduced pressure effect happen much sooner with a hot engine?

I take it the fuel filter was replaced fairly recently...

 

Hi JagV8.

Yes, the interval is much shorter when the engine is hot.

In fact I didn´t replace the fuel filter because it seemed to me that it would be an electronic issue rather then a mechanic one (as the pressure seems to be fine once I feed in 12V to the pump). But I will replace the filter asap, just to be sure. But does the ECU have a way of knowing the filter is clogged?

 

Excerpt from Jaguar on new fuel systems =

The fuel pump uses the same principle of operation as the on the X-TYPE where the ECM monitors differential pressure across the fuel injectors and the injector pulse width and duration to accurately calculate the fuel quantity being delivered to the cylinders. It uses this to demand a specific fuel flow rate, which it communicates to a fuel pump driver module located in the right hand side rear wheel arch.
The ECM use a frequency of 150 Hz (PWM) signal during average conditions, varying its
duty cycle between 4 and 50% to control fuel delivery rate or turning it to 75% to turn off the pump.

Now the location of the fuel pump driver module is not the same as this is from the XK explanation. The description and specs are correct.

 

Juro, from my knowledge of the fuel delivery system, you have 1 of 2 issues. I would say that if you have a device that can monitor manifold pressure, I would tell you to monitor that. Start the car and watch the pressure. What you should see is the pressure start out at 0 in HG (or water), once you start the car, it should climb to around 20-25 inches of vacuum and stay there, only dropping when you step on the gas. If you see a good vacuum, then odds are your problem is ECU related. If you see the vacuum not rise to 20-25 inches, then you have either a bad mass air pressure (MAP) sensor or the wiring associated with this sensor is bad (bad wiring should be giving you an error code, a bad sensor I think would give you a P0171 or P0174 code).

 

First of all, happy new year everyone.

Guys, thank you for your elaborate answers, you are being enormously helpful! The fuel filter should arrive tomorrow or on friday so I´ll replace it asap. But I will continue tomorrow with the diagnostics and try to meassure the manifold abosulte pressure and compare it to sensor reading. I´ll let you know the resaults.

 

It can't check the fuel filter is good or blocked other than if bad enough it'll flag a code for something (which may or may not mislead).

 

First of all, thanks to all of you guys, the problem is solved and the engine is running fine. This morning the fuel filter arrived so that was the first thing I replaced... with no resault, the engine was doing still exactly the same. So I physicaly meassured the vacuum in intake manifold while checking it with the live values in the ECU and it matched, so I assume that the MAP is working correctly. Desperate, I read some more on the internet and I found some topic in which a guy described that when you unplug and plug back in the battery, that the ECU will automatically recalibrate all its sensors. So that´s exactly what I did. Unplugged the battery, waited for a while and then plugged it back in and behold, the engine is working correctly. Hallelujah and WTF at the same time

 

After a while check you get (pending) P1111. If instead P1000 then look which monitors are unset and hunt the cause(s).