Trying to help my coworker figure out why his Dad's XJ6L won't run. Apparently they haven't been driving it in a few months and the battery went flat, they jumped it and can get it to run but will die after only a few seconds. I am doing this third-hand here so I apologize for the lack of real information, but I am trying to buy this car but don't want to buy a non-runner. For some reason they think it is related to the security immobilizer but I doubt it. I have asked him to replace the battery with a new one, and check the fuel pressure at the injector rail, although not being able to get my hands on it personally, and knowing that my coworker is not mechanically inclined I am not sure he knows what to do.
Anyway, any thoughts on a possible cause? Any real reason to suspect the security system? Are fuel pumps known to go bad on these? Car has 125k miles on it.

 

If it starts and then quits, it's not an immobilizer issue, more likely the fuel pump. Old pumps tend to fail when the car hasn't been started for a while. The 6 doesn't have a schrader valve to test for fuel pressure, so it will be necessary to loosen the nut at the end of the rail.

 

Fully charge the battery before trying anything else! in theory the alternator should keep the car running when you jump start it, but since it has to charge the battery and keep all the electronics running properly might be hard if the battery is flat dead. Also when you first jump the car what is your voltage ? should be 13+ if its any lower then that you either have a bad alternator or the belt is slipping. My xjr had a loose belt before where it doesnt charge at 13+ at initial startup so it basicly dies right after jump start when i disconnect the cable

 

I'm going to go out on a limb here and say quit doing ANYTHING with it till you get a good battery in it. Then ask if you need anymore help. These aren't run with the battery removed cars, they require a good battery to perform well if at all.

 

Yes, this is exactly what I told him to do, don't rely on jumping an old battery. Again, my hands and eyes are not on this car, he lives on the other side of Houston from me which is not a trip I want to make just to look at a car that doesn't run.

As to the fuel pump, where is it located?

 

Unfortunately, the fuel pump resides in the fuel tank, and YES, it is a minor PITA to replace it. Check the archives for tips.

 

Ok, a pump inside the tank isn't a deal killer, I've replace similar before. If nothing else, its makes checking whether or not it is running a simpler proposition as one could just put an ear to the filler opening while someone else turns the key to the ON position and listen if it primes the system.