My 1986 V12 XJS will start up fine and run fine for about thirty seconds to a minute before dying out by what seems like stalling. I have replaced the inlet regulator and all 12 spark plugs, as well as bought a new fuel filter and fuel pump. Beyond this I have no idea what could be causing this issue and I hope someone here might know because I am at a total loss as to where I should look next.

 

Important as well to replace the outlet FPR (the one on B bank inlet manifold at the front, ie LHS of the engine looking forwards from the cabin). This is the one that controls the injector pressure. The two FPRs are NOT the same and are not interchangeable.
An important question:
When it stops, will it restart? Depending upon this answer, further test can be suggested. Fear not the guys on here will, via you, get it started!

 

No, I cannot restart it once it shuts off. The outlet was my next part I was looking at it and your reply only helped solidify that decision for me. Thank you kindly

 

OK
If it continues to die after you have renewed the LHS fuel pressure regulator, test for spark as soon as it dies. If no spark report back.

 

Could it be the swirl pot under the fuel tank?
I seem to remember that they can get partially blocked, allowing enough fuel to bleed through to start and run the engine briefly before it stops as the line dries out.
Once the fuel trickles through again the engine will start.

 

Well I messed with it tonight and fuel is not the issue at all. There is spark after I shut it off and now it just refuses to stay running for longer than 30 seconds at a time when it starts

 

If there is a strong spark at the plugs when it has stalled and will not restart, then fuel is the issue, which is not to say the fuel issue may not be caused by an electrical one in the fuel injection system.
I sugest that you pull one of the front two injectors from the manifold when it has stalled (practice this first, the hoses are long enough to wriggle the injector free without removing the hose). Then with a helper turning the car over on the key and you holding the injector nozzle in a jamjar, (i) see if it is spraying fuel and if not (ii) see if it is clicking, even if not spraying fuel - you should be able to feel if it is clicking.

 

Another idea:

Ground the orange wire to the fuel pump relay. Doing so will cause the pump to run whenever the key is "on"...as oppsed to being controlled by the ECU. If the engine now stays running you know you have a problem with the ECU.

Cheers
DD

 

another thing I discovered today was that the coolant temp sensor is bad. The resistance is reading way too high for it to be functioning correctly when the engine is cold so that also likely plays a part in whatever is going on.

 

Well, your next move is obvious. Replace it and post back ! Check the socket while you're there!

Cheers
DD

 

I will be going through this weekend and seeing if I can’t clean all of that up and get it somewhat cleaned out. This is also likely contributing to my problem. Thank you for the suggestion.