This is a topic we have had various posts about recently on here and on the Series III forum. This post is to spread a bit more definite info about the OEM systems that were used to try to help vapour lock problems that make hot starting more difficult.
There were two distinct systems used on the Pre-Facelift cars (what is on the later 6 litre facelift XJSs I do not know for sure; but the parts diagrams on Jaguar Classics seem to show the electrical version might have been fitted):

1. A vacuum based system that when the fuel rail temperature rises about a certain level, removes the vacuum to the B bank Fuel Pressure Regulator (FPR). This has the effect of raising the fuel pressure in the rail, in the hope that this will purge any vapour more quickly once the engine is turning over on the starter motor.
2. An electrically based system with no contact with the FPR. It is this second type that is the subject of this post.

In both systems there is a special fitting that screws into a threaded bung on the fuel rail on the B bank side. This bung allows the fitting to contact the fuel rail; but not the fuel itself, it is just a theaded item welded against the outside of the rail. The fittings that go into this bung are as follows:
Vacuum operated fuel rail hot start assist gizmo part number = EAC5086 and EBC4600, which looks like this:

Electrical operated fuel rail hot start assist gizmo part number = DAC10335 or DAC3474, which is no.13 in this diagram and in photo below:
https://parts.jaguarlandroverclassic...al-switch.html

Now the story: I recently fitted a new Coolant Temperature Sensor (CTS) as a bit of planned maintenance. Naturally once fitted, the loom plug (which being of the Bosch EV1 fuel injector type only fits one way) was out by 180°. So I twisted the loom plug round only to hear a dreaded "CRACK" as the insulation on the loom wires under the outer cover broke. Careful cutting back of the outer revealed that the loom wires to both the CTS and the Air Temperature Sensor (ATS), which share an outer cover, were pretty well cooked for the last foot or so of their length and cracked.
In undoing this outer it revealed the solid yellow wires to my electrical type fuel rail sensor were part of the same circuit as the solid yelow wire going to the ATS. In fact the fuel rail sensor was upstream of the ATS and the ATS 5v feed went to the fuel rail sensor, then to the ATS. So my little mind was grappling with how on earth the ATS could work if the 5v feed to it was cut by the switch on the fuel rail that only got activated when the fuel rail temp was very high.
After a bit of chat on the Franco/Oz hotline the penny dropped that the switch must be normally closed and a test confirmed this! Following on from this revelation we deduced how the elecrically operated hot start sytem must work:
It is well known that removing the ATS from the B bank inlet trumpet (ie disabling the circuit) enrichens the mixture slightly, not more than 10%. So the electrical hot start system works in the same way: fuel rail gets hot; thermostatic electrical switch opens; ATS gets disabled; mixture gets enriched by a few percent; hot start vapour lock gets removed quicker as injectors are held open longer than normal. Fuel rail cools down; thermstatic electrical switch closes; ATS reconnected; fuel enrichment stops.

Here are some photos:

This is what the electrical thermostaic switch looks like, it is normally closed until the fuel rail gets hot.

This photo shows the 5v solid yellow feed joined to two bullet connectors. These connectors fit into the electrical switch's bullet connectors seen in the background. You can just see the male bullet connector's yellow wire looping round and into the black-sleeved loom. This goes to the ATS plug.
What this also illustrates is how badly cooked the engine bay loom can be. I am re-wiring with silicone insulated wire!

 

Further to the hot start heat-soak vapour-lock problem, I have found that the following routine works well:

• three goes on the key to the ignition position allow the fuel pump three goes at priming the rail
• engage the starter and simultaneously push the accelerator pedal to the floor and hold it there.

Car roars into life!

 

My ATS (Atmospheric Temperature Sensor) caused me a 'Big Problem' as on my Pre-Facelift XJS V12 the ATS is a Wax Filled Thermostat, that one day decided to Melt and drip wax onto the Injector that was right underneath, which had me thinking one of the Injectors had developed a leak

As there was a Sweet Smell from the Engine, that was followed by a Massive Bang, where the cause of this seemed to take forever to discover until 'Grant The Wizard Of Oz' came to the Rescue and told me what was wrong and so I did away with the ATS altogether and then replaced it with a single piece of longer tube

Now the Car always Starts first time even if the Engine gets very Hot, where after going on a long run many of us just open the Bonnet/Hood up, in order to dissipate Heat Soak as quickly as possible

As most of you know that Engine gets extremely Hot, so just as an Experiment when someone said it was hot enough to Fry Eggs on there, I thought I would give that a try and it actually did!

You can read the Full Saga here: Sweet Smell Followed by a Massive Bang and Hot enough to Fry Eggs On!

 

Further to post 1 above, it occurred to me that as well as renewing the CTS it would be a good plan to renew the air temperature sensor, as that was fitted to the car by the factory in 1985!
The wiring and plug had been renewed, and as the ATS costs only about 10 UKP, why not? Anyway it came yesterday and I fitted it. Unquestionably the new sensor made a difference. When warmed up the engine note is crisper (indicating a leaner mixture) and the engine is a touch smoother and slightly quieter - and this is in the context of a very good performance in these areas before the change. The tailpipe colour is definitely slightly pale greyer after town running.
I deduce therefore that the old one was not working and I feel this is a cheap and worthwhile bit of planned maintenance.
If the ATS is disconnected, the mixture is 10% enriched, as as the new one is obviously leaning the mixture a touch, this adds evidence to the old one being defective. My feeling is that the new ATS is now properly adjusting for air temps and the difference is noticeable. Also, between the new wiring and new AT and CT sensors, my hot starting after a 20 minute heat soak bonnet closed, is definitely better too.