Have you checked the coolant temperature sensor and crank position sensors?
There are numerous posts on here about them.

 

Hi Brinny

Just a couple of things you could try until 'The Wizard' returns

(1) Is the Spark Plug Wet with Petrol or Bone Dry?

(2) Are you getting Fuel at Pressure just before the Fuel Pressure Regulator on 'B' Bank as unlike the FPR on 'A' Bank the FPR on 'B' Bank definitely needs to work

(3) Have you checked the Fuel Cut Out Switch on the Passenger Side Door Pillar?

(4) Have you got the right Coil Plugged into the 'Correct' Bank of Cylinders?
The Lower Coil (The Red One) Plugs into 'A' Bank

The Upper Coil (The Yellow One) Plugs into 'B' Bank

If you get those the wrong way round She won't Start! Ask me how I know, or rather don't!

That should give you something to do until 'Greg' and 'The Wizard' get back

 

 

Hi Brinny

I had a very similar No-Start Problem with my XJS V12 'Cherry Blossom'

This was right back at the beginning of getting 'Cherry' back on the road, after a layup of around 14 years, where at the Time in question I knew even less than I do now!

There was only one thing for it and that was to send for 'The Cavalry' in the shape of 'Greg' and 'Grant' who between them must have forgotten more about XJS's than most people would ever know!

So to paraphrase a Quote from myself I had such Faith in those two Guys, that I clearly remember saying that:
"If they can't get her Started, then the Engine must be missing"

So all I did was to follow everything that they told me to do, which I might add wasn't easy for a non-mechanic like myself, who probably holds the Record for taking 3 days to change the Spark Plugs

Which was about as easy, as Building a Ship in a Bottle, with a Blindfold on!

But eventually we got there and She Started!

So if you want to see a Step by Step of everything we did, have a look at Page: 11 (onwards) of my 'Cherry Blossom' restoration thread and here is a link to that Page: How 'Greg' and 'Grant' Fixed my XJS with just a little help from me, as I was the one with the 'Spanners!'

I've said it before and I'll say it again, "What 'Greg' and 'Grant' don't know about XJS's, you don't need to know!

Have a look 'Brinny' something useful to you might roll out

 

You said back running after an overhaul, so does this mean it was working prior? If so make sure the ignition leads go to the correct place.

First the plug lead positions on a Marelli dissy are different to a Lucas, make sure these are correct.

Check the leads from the ignition modules to the dissy are correct, eg if you swap the inputs from the ignition modules to the coils the car will not start because the ECU will be firing the wrong coil. Same if you put the HT leads from the coil to the wrong input on the dissy.

 

 

 

Hi Brinny

With the help I got from 'Greg' and 'Grant' we worked through every component 'one at a time and slowly' and then it finally got to the Stage where we had almost run out of ideas

Except for a long shot so far away from any real reality, that it almost didn't seem to be worth trying which was swapping the Coil leads around from 'A' Bank to 'B' Bank

As when I was putting the Engine back together, it some how never occurred to me that which Coil Lead went to which 'Bank' would make any difference

This was really getting close to the last throw of the dice but when I swapped those Coil Leads around, She instantly fired up and ran like a bird

With the Benefit of hindsight, in a way I'm glad that I 'Screwed Up' as I don't thing I will ever make that mistake again!