Since the fuel tank has to be dropped to access the fuel pump, I would say that you should obtain evidence that the pump is actually the problem (evidence such as a low fuel pressure reading) before you invest in the effort to replace the pump.

So, I pulled the fender liner, replaced the Air filter (Which was dirty) and cleaned the MAF sensor. Had a solid two days of perfect driving. Had an hour drive to the airport the other night and the same problem reared its ugly head. The hunting/stumbling only occurs at intermittently at idle, and disappears after a restart. Once I reach highway speeds I'm sitting pretty. The car drives as normal. No fluctuation in RPMs and my gas mileage stays in to high 20s/low 30s. If the fuel pump was at fault wouldn't it cause issues while highway driving too? I also was not aware of the OEM jaguar only thing when I bought my air filter. I put a Beck/Arnley unit in. Is that really a huge problem source?

Watch fuel trims at idle both when OK and when not.

Check every sensor you can via live data for plausibility, again in those two cases.

Like JagV8 said, check your fuel trims. Look at this article to help make sense of what you're looking at/for:
https://www.autoserviceprofessional....for-you?Page=2


Possible relevant quote from article.
"Since the amount of air flowing through the vacuum leak doesn’t increase, the leak has less affect on air/fuel ratio at higher speeds and loads, so LTFT will come down as engine speed goes up. When you look for vacuum leaks, don’t forget the various “calibrated vacuum leaks” like crankcase ventilation, the evaporative emissions (EVAP) purge valve and, if equipped, air-shrouded injectors."

So where are we with replacing parts and verifying the original complaint and codes are still relevant? Also still curious about the voltage after sitting overnight.

Pressure high, low and sensor out of range all point to towards the sensor, the wiring or a control problem in how the sensors input is interpreted. Just shutting the car off and restarting it to me points at how the input is being interpreted.

Maybe a dumb question since I haven't researched it but how does the 4.2 control fuel pressure? Mechanical dump valve like so many other early gen vehicles or control the fuel pump speed like later model stuff.

Ah crud. My OBD-II scanner is not advanced enough for any of that. Can you recommend a model for me?

Replaced Air filter and cleaned MAF sensor. seemed to have fixed the problem for a couple days. Gonna throw the new fuel pressure sensor in tonight. I'm definitely leaning towards it being a sensor issue, the shutting off and restarting thing gave me that idea as well.
I'll have to wait until tomorrow night to get the 24 hours at rest battery reading.
To my understanding it's the latter with a pump controller (C2C35672)

elm327 should do but there are dozens or hundreds.

I have the Icarsoft LR2 and it's OK but not stellar. You can compare a couple displays but it's difficult to set up and the read out is in text only so you are stuck with what it reads when it reads it which is confusing for a fluctuating idle etc.

Page 1645 of the X150 S/M is interesting in its description of how the ECU talks to the pump driver.

We can probably conclude that it is not a fault with the pump though, correct? When I've had pumps fail in the past it effected highway driving as well. The symptoms also seem to go away after a long drive.

Battery at rest is 12.47 after sitting for a few hours. I'll report back with over night readings tomorrow. Gonna check the sensors on my new OBDII tonight

Well this got stranger. I'm still a novice with using an OBDII scanner for diagnostic purposes. (this is pretty much my first car made after the cold war). All the codes were the same as before. I used the OBDII to check fuel pressure and it came back hovering around 36 psi. Even though the car wasn't having the fault. The engine was running normally and smoothly. There's no way that's possible, so I swapped the fuel pressure sensor out for a new one and now it's hovering around 44psi, even at 2500rpm . Couldn't figure out how to get to the fuel clipping menu yet (just got the device tonight). I also couldn't get the car to start acting up. I reset the CEL with the OBDII and it did not turn back on even though the car was only at 44psi supposedly. So what now? Could it be a weak fuel pump? I'm stymied.

I would look into an actual pressure gauge and do some comparison testing. I assume there's a Schrader valve somewhere on the fuel rail to tap into.

Isn't it possible the sensor is the culprit and was providing bad readings to the ECU?

It's a feedback based closed loop system. ECU looks at power demand inputs and decides what pressure to feed the injectors then tells the fuel pump driver how hard to drive and then looks at the sensor to see what's going on. Somewhere in that chain of events there's a problem.

ECU
Pump Driver
Pump
Plumbing
Sensor
Wiring between anything

It can't be the sensor because I swapped that out and I'm still only getting 44psi which is too low.


ECU
Pump Driver
Pump
✓Plumbing (I can't see this being the problem in a car this new)
✓Sensor
✓Wiring between anything (This may be a preemptive check mark, but the wiring in the hostile environment of the engine bay that I have checked has been fine)


Oh man, the other three options look costly. Would a new ECM need to be married to the car by a dealer? I'm gonna go ahead and order a new pump driver because I did end up finding one really cheap near me.

Will do. I'll try and acquire one tonight

Quit wasting money on parts until you have some idea what's wrong. Until you actually know what pressure is at the rail everything is still suspect. You could have corrupted data from the sensor or PWM problems from the ECU to the driver. From experiences garnered from the forum over the past year all of this can still fall back against the battery. The X150 is a fickle beast when it comes to electronics and controls. At least you aren't paying a shop to misdiagnose it yet.

Did you read the section of the service manual on the fuel system?