Thank you for sharing your experience. I’m sure many of us will be referring back to this thread as time goes on.

 

UPDATE
Success: The hot starting difficulty (eg after a stop of 15 minutes to more than an hour with a hot engine, bonnet closed) that required about 4 seconds of churn with foot hard down on the throttle before the motor started, has been completely eliminated with the 40mm hoses. The engine starts immediately hot or cold. I put this down to the reduced hose length eliminating vapour locks.
To be clear, if the bonnet was left up, so no heat buildup under it, the hot starting was always perfect. I am as sure as I can be that somehow the shorter hoses have made the difference.

Failure: For some reason that I cannot be sure about, my new crimped-connection loom did not perform well. Sometimes it was fine, sometimes the motor showed clanking symptoms of lean knock on a smartly open throttle, particularly on a cold engine. At a steady state and at a slowmy opened throttle it was fine.
I replaced the loom with an OEM one, and all perfect. So I knew it was a loom problem, not an injector problem (remember that I had fitted refurbished, but old, injectors in this saga).
I can only deduce that the crimped, rather than soldered, connections throughout the loom do not quite flow the very short current pulses as they should. My friend Michel and I have rebuilt the loom using soldered connections to distribute the current and the return signal between the four batches of injectors (A1/A3/A5; A2/A4/A6; B1/B3/B5/; B2/B4/B6).
The car loom has a single permanent 12v wire and a single return (earth) wire to service each batch of three injectors. The earth wire goes to the ECU which switches the return to earth for the required spray duration to activate the injectors. Thus, inside the injector loom this single pair of live and return wires for each injector batch must be sub-divided into three pairs so each individual injector in the batch is connected. In our rebuilt loom, the joins required to do this have been soldered, rather than crimped, as our first effort was.
The loom with be tested tomorrow, stay tuned.

Additionally:
I wrote earlier in the thread that I wanted to use more modern connectors between the car loom and the injector loom. To test this out, before taking the drastic step of removing the OEM rubber moulded plug from the car loom, I have rigged up the test rig shown in these photos:

On the left an OEM injector loom connector (eight wires, four live and four returns) labelled to show which injector batch they service. On the right, Deutsche connectors, one for each bank, each Deutsche connector has four pins, one +/- set for each batch of injectors.

The connectors are labelled A or B, according to which bank they service, and one side of the plug handles the odd numbered injector batch, and one the even numbered batch.

A bank odd numbered batch wires this side of the plug

B bank even numbered batch wires this side of the plug


Wires from the OEM male plug carefully labelled. It is all to eay to lose track of what goes where otherwise.

The idea is to see if the new soldered-connections loom works as it should. If it does, and after a week or two of prolonged testing, I shall unplug the test rig, cut off the OEM female plug on the car loom, and fit two Deutsche plugs dirctly to the car loom, thus allowing the new loom to plug directly into the car loom using Deutsche connectors, eliminating the rig ringed in red below:

 

Bonjour Mr Greg. I'm not sure if this applies to your car but by happenstance I was reading the V12 Engine book publication S58 and read about the Fuel Pressure thermal vacuum valve that lets the fuel rail build maximum pressure once the engine is above an unspecified temperature. This feature, and I quote: "prevents vapor formation in the fuel rail at high under-hood temperatures and purges the fuel rail for hot starts." Not sure if this is even installed on your car but might be worth looking into?

 

Mark
Many thanks for the suggestion. I did have this on my car and it made no difference at all when working or when disconnected!
What the device does is to bypass the vacuum signal to the B bank FPR (the one that controls rail pressure) if rail temperature goes above a certain level. This change makes a marginal diffrence at best; indeed I am pretty sure that Roger Bywater has written that this hot starting difficulty was something Jaguar knew about and for years never 100% reliably fixed.
But now, problem solved! And as many XJS V12 HE owners will undoubtedly be overdue replacing their fuel rail flexibles, when they do, just shorten the flexibles to 40mm and all solved. This fix was done decades ago by Grant Francis in Australia, and it works in that heat too.

 

UPDATE 2

Fitted the homemade injector loom this morning. Nearly functions right, but not quite. Still just a suspicion of lean knock on sharply opening the throttle from tickover. Replaced OEM loom and perfect. So something still not quite correct.
Before junking the whole shebang I have one more test. I shall remove the Deutsche connector and solder the OEM plug to the loom wires and retest. I have a supicion that the non-OEM connector, or even an extra connector, is the problem. One way or another, we shall see.
In the photo below, I shall solder the wires link-ringed in blue together, thus mimicing the OEM loom precisely.


If that does not do it, then I admit defeat.

 

Is there an update about the wiring loom , or understanding why it did not work correctly?

Later j

 

Well two things. One, I bought a new Autosparks one, because I have to have the car reliable.
Two, I have a feeling that because the injector electrical pulse is so very short, the loom cannot function properly with anything but a very perfect connections. So I have removed the deutsche connectors and replaced them with soldered connections to the old OEM plug. So now my new home made loom is exactly the same configuration as the OEM one: ie OEM plug and all soldered connectors to the injectors.
What I have not done yet is fitted it to test it! When I do, I will post the results.