mine tends to take a bit longer to start when sat overnight or a few days. I think it’s fuel pressure. If I turn the ignition on/off a few times to prime the pump, it seems to start a bit faster.
I assume the pump check valve is either worn or non existent (I have Walbro pump), I was thinking of installing my own check valve in the fuel lines in the trunk after the pump.
Series II and III XJ6 with fuel injection used the CAC2263 pump, which later changed to CAC4269 and then to CBC5657
The XJS V12 HE and 3.6 used the CAC4269/CBC5647 as well (until,of course, Jaguar went to in-tank pumps). The Series III V12 sedans also used this same pump
The Series III fuel injected sedans (6 and 12 both) sedans do indeed use a replaceable screw-in check valve, located in the ‘air bleed assembly’ which is located a couple feet downstream of the pump. Presence of the replaceable check valve does not preclude such a valve being incorporated into the pump itself.
I’ve never researched, but always presumed, that all electric pumps, Jaguar or otherwise, have a built-in check valve. Could very well be wrong there.
Cheers
DD
To be clear, you’re not priming the pump. You’re priming the fuel rail.
Anyhow…
Drain back will cause harder starting in the form of slightly extended cranking time. But, in my experience, the pump will recharge a depressurized rail in mere moments…like about 3 or 4 seconds.
If really extended cranking (more than 6 seconds let’s say) is needed then I’d say there’s a problem in the delivery system OR the extended cranking is due to some other fault.
(When I say ‘seconds’ I’m referring to counting off 'one thousand one, one thousand two, one thousand three" measurement 
)
Cheers
DD
Yeah, sorry, that’s what I meant.
In my case, if car is warm, having been running mere hours ago, it starts up in about 1-2 seconds.
Sitting overnight, it will take about 2-3 seconds of cranking.
Sitting for several weeks, it will take about 4-5 seconds.
None of those times would alarm me, one iota.
Carl
My old Alfa has a similar problem. It has a low pressure, mechanical pump, feeding a pair of DCOE Webers. Leave it for more than a few days and the petrol will make it’s way back to the tank (hopefully, not sure where else it might be going) Then it will take about 3-4 churns of 10 seconds, a fair bit of throttle pumping, and quite a lot of swearing before the engine decides to kick into life. I have an electric pump sitting on the shelf that would probably solve this problem entirely, by priming the system without having to run the engine for so long.
I’d estimate that it would probably take at least 4 hours to plumb this pump in, instead of the mechanical one, and get the electrics working. So, given my current usage of the car, it would take about 10 years before it was cost effective to replace the pump, rather than take an extra 25 seconds to start the car. Hmmm 
I’m with Carl. Nothing alarming there.
All of my 80s vintage Jags, and many other cars, have behaved that way. There might be a subtle fault or might be room for improvement but, personally, I wouldn’t spend much time chasing my tail to find it.
Conversely, my XJR (and other cars) have been very fast starters. There are probably explanations for this but I don’t know of anyone having enough time to research what they are
Amusingly (and sometimes annoyingly) my old 1995 Ford F250, which sees limited use and no TLC whatsoever, is an instant starter under any conditions and the engine is so smooth at idle you can’t tell if it’s running…something I’ve never experienced on any of my Jags despite my efforts and care.
Cheers
DD
I concur. Switch the ignition on, plug your phone in the craddle, connect the power lead, find your damn glasses, no not those, the other ones… by then the engine ECU is happy, the brake accumulator charged, so start the engine. It’ll fire up in a few seconds. Doesn’t much matter how long “a few” is, you’ll know pretty soon when it is too many.
Oh, I’m fine with my times too. I was just posting so the OP could compare.
Older engines aren’t as perfect as new ones, I guess (even my 90s Volvos will start in 1 second no matter how long it was sitting)
Plus, I assume this V12 needs a whole lotta fuel!
[Fingers in ears, La, La, La, I’m not listening]
Greg, don’t spoil the fun. Do not tell my wife, or my mother, or my bank manager what goes on at the petrol station.
My boss drives a diesel Hummer H2, he gets better mileage than my XJS!
My 1967 Sunbeam Alpine (dual ZS carbs) was a very fast starter. Starting it was like bumping a momentary switch. Just turn the key to start and release as fast as you can and it’s running. First hint I had that the battery was bad was when I did that and it not only didn’t start, it didn’t turn over at all.
Unfortunately, that was on the beach at Daytona – back in the days when you could drive on the beach. No problem; put the shifter in 2, put the girl friend in the driver’s seat with her foot depressing the clutch pedal. I got behind and starting pushing. After about three steps I yelled for her to drop the clutch and I never felt the car push back on my hands – it just took off. Single-person push start on soft sand.
Of course, the choke on that car was a knob on the dash. I dunno what was wrong with that idea. It always seemed to work better than the schemes we have now. Certainly better than an AAV.
Yes, go for a 4 week jaunt around UK and Europe every year - their summer and our winter.
Come back and turn the ignition key on the X350 with a 3.5L V8 with temperature about 10degC and get an instant start.
Both XJ-Ss I know will take 3 and maybe 4 cranks to start even after charging the battery overnight. Sure is not a fuel problem, I have a fuel pressure gauge on the coupe and that pressure comes up in a second or two. You are right Doug, must be something subtle.
To be fair, my 6.0L XJR-S benefits from a better engine management system than my 5.3L SIII XJ12 had. Consequentially, a “better” economy figure. But we’re talking here about a percentage change, an absolute value of a 4 MPG difference would not be significant to many people. Still probably quite a lot better than Harry’s XJ 5.3C though. That must drink faster than Oliver Reed at a free bar.
That doesn’t preclude a fuel issue. When you first turn the key to start, the EFI system tosses a charge of fuel in there to facilitate a cold start. If there’s no pressure when it does that, your cold start is going to take longer.
I don’t think that is the answer. I get the same symptoms with both the stock OEM system and aftermarket EFI system. Both will give a boost of fuel when the ignition is turned on, or maybe it is a boost on initiating cranking.
With the aftermarket system I can vary that boost from zero to lots of fuel, although i have never experimented. It would take a fair bit of messing around to change all the variables and reach a conclusion.
I doubt that it is a lack of fuel making the V12 hard to start after being dormant a month or so.
Anybody have a suggestion what could cause this ?
Thanks to all for the input above…a real masterclass. After working through the various options one by one and testing I think I have a culprit: poor connection on the B bank coil. It looked fine (and tested with appropriate voltage even on cranking), but a bit of wiggling highlighted a loose connection to the pin in the plug. I’ve jury-rigged it in the short term and have acquired a set of new connector plugs and pins…now I just need some high temp solder and painkillers for my back when I have to lean over the car to do the work. Can’t figure out if I’m too tall or too short…
It’s not you, it’s the Jag - no matter what I’m doing it always seems to be at the wrong height.
You do not need high temp solder, run of the mill solder melts way above typical under bonnet temps.
Nowadays you can buy crimp type connectors. If they are a decent design and you have the right crimp tool they do a good job.
You may still need the painkillers either way.
My XJS is the opposite. It struggles with slightly warm restarts. It’ll fire on a cold start almost instantly, but when warm it cranks a bit longer, then runs at like 500rpm for about 5 seconds before it figures out what it should be doing. It actually runs so slowly I can’t hear it over the noise from the starter motor.