So, after 3 years of trouble free ownership, I’m now having a random no start issue on my '92 XJ40. It started with stalling/low idle when cold, and has graduated to occasional failure to start (whether cold or warm). Sometimes starting and running is PERFECT, unless it’s a day where these symptoms are present (very strange and intermittent). I’ve replaced the coolant temp sensor and am about to replace the IAC. I’ve always got the fuel pump “hum” from the tank, so I’m not leaning toward a fuel issue. No check engine light. My main question for those who have been down a similar road: will a failing ignition amplifier or coil allow “cranking” but no start intermittently? As well as idle issues/stalling at times? It always cranks and when doing so, the tach will hover at 200 RPM or so - which if my issue was CPS related, I’ve been told it would not do. Just trying to avoid throwing more parts than necessary at this thing.
Jimmy …
I believe the starter is wired directly to the battery and powered through the solenoid mounted on the starter. The starter solenoid is controlled by the ignition key. So just about everything can go wrong with the car and it will still crank (but may or may not start). Remember though that the car will not crank unless the transmission gear selector is in PARK or NEUTRAL.
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Since the car sometimes runs perfectly you can probably rule out something mechanical (valves, pistons, etc).
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Next on my troubleshooting list would be electrical which can be checked by testing for a good strong spark out of each spark plug cable. Also you want to listen to each injector for pulse clicks.
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Finally I’d determine if fuel at the correct pressure is actually reaching the injectors. Even if the fuel pump is working properly (and hearing a hum is no guarantee of that) you still have the fuel filter and fuel pressure regulator that could be at fault.
Try unplugging and replugging the ECU
Thanks - already did this.
Thanks! #3 was on my mind last night. The fuel filter is not old, so I would think it’s fine. The regulator however…
Try a new rotor
Larry …
RIGHT YOU ARE SIR ! That is absolutely step number 1 (since I’ve been burned twice by that how could I have possibly forgotten 
)
This is a good opportunity to reiterate that every XJ40 owner should have in their emergency kit, preferably in the boot (trunk) of their car, a new rotor and a new, or at least known good, crankshaft position sensor (CPS), as those two parts can fail and leave you stranded. How do I know?
My '94 Died At a Stoplight and Wouldn't Restart?
Both parts are easily field installable, the rotor with no tools required; the CPS requires an Allen (hex) wrench that should be in the at-least-minimalist tool kit carried in the boot. A new distributor cap should also be part of the kit, to go along with the rotor.
Mike …
Also just to be on the safe side I would add a new distributor cap. Rather inexpensive and like the new rotor and crankshaft position sensor it takes up very little storage space.
Just like the American Express commercial says, “Don’t leave home without it” … (or them in this case 
).
As noted in the last sentence of my post. Great minds think alike! 
Thanks guys. Back to the immediate issue at hand, both the rotor and distributor were replaced in 2018 by the PO (which was about 7,000 miles ago). Seems a little premature that they could be on the way out so soon, but I suppose it’s possible.
With work this week, I won’t have time to get back to the car until the weekend (which stinks).
Rotors are very susceptible to failure on the XJ40 and can go high resistance giving you a weak spark. This is because they don’t have a direct connection from the coil (centre) to the plug wires but have a potted resistor along the length of the arm which can fail. Distributor cap is pretty stable by comparison.
Jimmy, if you have mailing list mode enabled you have already seen this post and thread but in case you do not: Non start spark present - #26 by Mike_Stone?
Thanks Mike. I’ll give it a look-see.
Jimmy A …
After my Long-g experience with this car and the Jag Lovers forum I believe replacing the rotor/cap when troubleshooting a non-start situation is a no brainer, no matter how new they are. Considering how inexpensive a spare pair is, AND ALSO what other troubleshooting job can you literally do anywhere with NO TOOLS !! This has happened to me 3 times over the years.
Mike … Sorry I missed the very end of your post. But just in case it hasn’t already been mentioned always carry a spare distributor cap along with your spare rotor 
No worry. The important thing is to get the word out. Another advantage to having these items in the emergency kit is they can be handy for troubleshooting. Like in this case of Jimmy’s car - no indication the CPS is the problem, and it can’t really be tested with the equipment most of us mere mortals possess, but it is easy enough to pop in the spare.
I was thinking of laying in a spare crank position sensor for my 1990 4.0 XJ6. Moss does not list them, SNJ Barrett shows a DBC21441 at $38.14 and 5 others with greatly varying prices. Rock Auto aftermarket ones at WVE 5S1797 at $478.79 or PC 518 at $345.19. Except the $38.14 one these do not square with forum comments that this is an inexpensive part to carry as a spare. I’m not even sure which one I need. So I am confused.
John, this page gives you the equivalent part numbers for aftermarket units. Scroll down for fitment info.
Apparently changing round plug to square plug and vise versa is no big deal so that may give you a broader search range.
Personally I would give this Rockauto one for ~$30ish dollars a go and swap out the plug if yours is round. The thing is a two wire version which is probably the same as our 3-wire types without the shielding …it’s more designed for the x300 but It could be made to work I’m sure …price difference is staggering!
https://www.rockauto.com/en/moreinfo.php?pk=7326008&cc=1177567&pt=7196&jsn=399