I've run into a very recent issue with my otherwise flawless daily driver XJS and I wanted to consult the wizards before tearing into it, or tossing more money at my local Jag shop.

'New' injectors
New injector harness (Jaguar Specialties)
New feed side FPR (not bosch so brand is suspect)
Old but working return side FPR
ALL ignition related components are "new" to within 3 years.

All Vac garbage deleted, smog crap deleted, basically followed Grant's advice to the letter beers included, and now I'm running into an intermittent slow or hard start that I can't seem to narrow down. The starter turns nicely, the lights don't fade or flicker it feels fuel starved trying to start but if you key on the pump you can hear it gushing back into the tank, and there is zero noticeable performance issues while driving or idling it still rips up the highway no real change in fuel economy that I can measure.

The issue doesn't seem to be correlated to engine temp or 'hot start' as its totally random - some times on like a light switch before I'm done turning the key other times its several cranks, or cranking it for a few moments twice to get going.

One on-going issue was a consistent exhaust detonation or gunshot on the passenger side while at high speed (high vac?) but with the foot off the gas, given that everything is relatively new I suspect a spark plug but who knows? Its always that one side. Could one failed spark plug cause an otherwise random issue?

Just to be clear so far this hasn't been a total no-start, she goes but I'm used to the light switch reaction so this concerns me.

 

I assume the car is a Marelli? (If it were a Lucas I would say replace the amplifier).
The first thing I would do is remove the A bank fuel pressure regulator altogether, and replace the B bank one with a new one. My experience with generics from Manners has been they are absolutely fine. This could be the B bank FPR getting iffy and changing it eliminates fuel aspect.
This actually sounds more like an electrical rather than a fuel issue, so I am totally ignorant about Marelli ignition; but common sense says replace the amps, coil, and carefully check the wiring.

 

Hi Vancouver XJ6

I had a very similar problem on my 1990 V12 'Cherry Blossom' with the intermittent 'Backfire' and it seemed to take forever to track the problem down

Which in the end turned out to be an internal 'meltdown' of the 'Thermal Fuel Enrichment Switch' together with a leaking FPR on 'B' Bank

As an aside the 'Thermal Fuel Enrichment Switch' is filled with Wax! which I would never have known about, were it not for a chance remark from 'Grant'

When the Wax inside this melted, it was dripping Wax all over one of the Injectors from above but as I didn't know about it being Filled with Wax I thought it was the Injector that was leaking

Owing to the Climate here in the (UK) the 'Thermal Fuel Enrichment Switch' isn't really needed, so I just cut it out of the mix but instead of Solving the Problem (apart from stopping the dripping Wax!) it just sent me in the wrong direction

Though just as soon as I replaced the FPR on 'B' Bank (like Greg said)

It was all 'Sunshine and Roses' as the 'Backfire' (no doubt caused by the leaking FPR on 'B' Bank) suddenly went away and even after being Stored all Winter, I had 'Light Switch Ignition' when I Started her up for the First time this Year

You can read all about 'The Saga' on my 'Cherry Blossom' restoration thread: See Link Below (about 1/3 down the page)

Sweet Smell Followed By A Massive Bang!

 

Glad the beers made a difference, best addition to any tool kit.

Coupla thoughts.

1) Ignition switch electrical section is going iffy, age related. Easy fix, although very fiddly. Place the switch INSIDE a freeze bag BEFORE opening it up, as there is a SMALL spring and pawl inside, and depending on the beers, they will go AWOL in less than a sip. Clean all the LUCAS goop inside that switch, relube lightly, and reassemble.

2) Multi pin plug of the EFI resistor pack, inside the engine bay, on the inner guard, RH side up near the radiator. Unplug the loom, clean the plug, remove the resistor pack (FUN, NOT), clean the socket, then clean them again, and lightly lube before assembly. I use Lemon Juice as a cleaner, and WD40 stuff as a washing agent and lube.

3) The B bank FPR, as mentioned, is always suspect, age again is against them now. The RH can be eliminated, as Greg says.

4) Marelli stuff, so wait for Warren, he will catch up soon.