I have a 1990 XJS in great condition with only 122,000km (80,000 miles) on the clock.
My problem is that she will cut out intermittently for no obvious reason, and sometimes (only sometimes) restart, but with difficulty. This can happen both cold or hot.
I have checked the fuel system from end to end and no issues there.
Occasionally she will not start at all, and occasionally she will happily run a couple of hundred miles with no incident… When running everything is absolutely 100%, although after a hard start she is not running on all 12 for a few minutes then picks up.
I have checked all fuses, and can see no signs of where there could be a worn wire or intermittent short.
Recently I removed the ignition protector relay, and lo and behold she immediately started then cut out a few minutes later.
Then, on advice from a post on here, I tried removing the fuel cap, in case the tank was not venting… there was a large rush of air into the tank, and she immediately fired up without any problem, only to cut out again a few minutes later.
As I had previously had trouble with the horn sounding intermittently, (and had re-routed the horn to a separate switch, rather than on the steering wheel) in desperation I isolated the horn circuit in case there was some weird short.
All the straight-forward things like fuel and battery connections are good.
Any ideas folks?
My go to on a Marelli car that is cutting out with the tach dropping is to check the crank position sensor and the engine speed sensor. Also the dreaded white coax wire. Check the archives, Myself and others have written a fair bit about how these work and how to check them.
(Robert Moore 90 XJS V12 Convertible Marelli)
Sounds like a crank position sensor issue combined with the other issues you have already discovered.
White coax on firewall was a huge mystery on my 90 convertible.
All other conductors in the valley were also suspect after I found coax insulation crumbling on either side of the splice point.
Conductors from factory can’t handle the heat after time.
Inner conductor will short to outer braided shield intermittently with no rhyme or reason due to vibration and hot/cold expansion and contraction.
Repair and relocate splice point with additional insulation.
Probably no need to purchase spare ignition and fuel control modules as I did for troubleshooting.
Never a problem again since this simple repair.
Very similar situation happened to me on Saturday. Driving back to Portland, Oregon from Seattle. Car started to intermittently hesitate on the interstate. Pulled off at a rest stop. Alternator belt was a little loose and had some oil on it, tightened and cleaned that. Started backing out and it died. Wouldn’t restart. Wiggled the throttle position sensor wire connector. Started up again but gave me a FF44 code. Disconnected the battery. Checked the connectors and wires for oxygen sensor. Reconnected battery and started up and has run fine since. I want to say the throttle position sensor connector pins coming slightly loose was the problem, don’t know for certain.
I had a CPS fail on a 1995 XJ6, it would run but not start. The CPS is
many (thousands) turns of wire & has a resistance of a few thousand
ohms. A ohm meter would show high, almost infinite resistance if
defective. An inductance measurement would be better as it could also
have a shorted turn.
The Jaguar XJ-S Repair Operations Manual (ROM) details how to test the CPS with an oscilloscope with the CPS on the car. When I had problems with our 1990 XJ-S starting several years ago I asked a friend who owned an oscilloscope to help me test it. He tested both the CPS and flywheel sensors (they are the same identical part) with his oscilloscope and they both performed to specification per the ROM. It turrned out that the CPS was not the problem but that all of my recently serviced fuel injectors were stuck closed. Go figure.