94 XJS stumble issue not yet solved

I am still trying to solve the issue of the engine stumbling and stalling in my 94 XJS.
I will update for those who haven’t seen my earlier posts.
4.0L 94 XJS coupe, 50,000 miles +

Ran fine until a little while after I changed the fuel filter, which was probably original.

Has had the full 50,000 mile servicing and tune up.

New water temp. sensor, oxygen sensor, throttle position sensor, crank position sensor, cap, rotor, plugs, wires, valve cover gasket.

Tested to make sure they are okay: ECU, injector ECU, and a couple other ignition parts have been replaced.

I let the car sit for a while after bringing it home on a trailer from a mechanic that couldn’t sort it. Today I decided to put a battery in it and try to start it. After cranking it three times, which isn’t normal, it started up. I let it run for a couple of minutes and took it to 3000 RPM, fine. I dropped it back to idle and let it run. It ran for about five minutes, stumbled and stalled. It then wouldn’t stat back up. I have a fuel pressure gauge in line and it was bouncing a little but stayed right around 36 PSI while it was running. When shut off the pressure started dropping and I assume will go to zero in short order. The car has been gone through extensively with no cure to the issue. There is no one anywhere reasonably close with the needed Jaguar diagnostic equipment. I plan on removing the injectors and have them serviced and tested. Not sure where to go from there.

Any helpful insight will be greatly appreciated. That includes referral to someone that has the skillset and equipment to sort this out.


Have you tried a new rotor

Stephen, the fact you can start it when cold and it will run fine until it is warm eliminates a number of things.
You have had an impressive number of items replaced to try and fix the problem.
So expect the unexpected and try to find what does work until something turns up that does not work.
This not going to be easy but you always find the headache in the end.
Start with warm engine that has just stalled ( if you can get it going again in the first place ).

  1. Quick try, put icy water on coil and ignition amp - they can fail when warm.

  2. Pull a plug and stick it with HV connection on engine.
    Crank and see if you have a nice blue spark.
    You can use an old plug with same gap instead.
    Easier to see the spark in the dark, or with another to help in daylight.

  3. If ignition O.K., check injectors pulse on cranking.
    If not a sensor that initiates the injection cycle is suspect.
    You mention fuel pressure looks good.

  4. A temperature sensor that fails when warm can alter injection timing sufficient to prevent ignition of fuel.
    You can find the correct resistance for a warm engine and substitute.

  5. If you can start the engine again when cold try and get a temperature reading with an IR meter of a few things around the engine until it stalls.
    Good to know if the stall happens at a particular temperature.

  6. if the stall happens very much at a particular temperature, maybe stop the engine half way to that point and then see if you can start it again.

  7. Measure the voltage at the hot end of the coil while engine is running.
    Should be around 14V. Will drop after engine stops, maybe down to 12.5V after short while.
    Does it look like dropping when you get near stall ?

  8. What kind of gadget is on a 4.0L for extra air when engine is cold ?
    That might stall the engine but should not stop it firing up again.

There are lots of things to try before you need a diagnostic machine.
One good thing if the problem is 100% related to the engine getting to a particular temperature.
You can use icy water, or cold air blower, to cool items down and see if you can stop the stall. You can even put cold air or icy water on the ECU.
Something will make a difference, it has to be found.

I got caught out with an odd problem on the HE engine in my 1979 coupe.
It has an aftermarket EFI system that lets me drive along with a laptop and change fuelling and ignition advance on the fly.
I reported on this forum over a year ago that I had an annoying shunting effect around 1200 - 1500 RPM on light throttle. Hardly to be seen on the tacho, but I could feel through the seat of my pants the RPM was going up and down by about 50 - 100RPM every 2 or 3 secs.

I drove around for days with the laptop varying everything and could not nail it.
I do not drive the coupe often. One day I tried to analyse what, when and where.
I realised that there must have a change of something which had an unexpected consequence. Then it dawned.
I do not have the OEM aux air valve. I had fitted an electronic type which has a moving vane, driven by a temperature sensing controller. It is not the much more common stepper motor type. One end of this item was connected to battery voltage, +14V when running, and one end to the controller. Obviously with warm engine around 1200 - 1500RPM, although the vane was nearly closed off, the alternator output was dropping slightly which caused the vane to open a little more, that caused the engine to speed up slightly and increase voltage, that caused the vane to close off - etc.
Classic feedback mechanism in a system. Only did it in that narrow RPM band at light throttle. Not pleased it must have taken me 2 or 3 hours of fooling around with the laptop to achieve nothing.
It can take a while for the obvious answer to dawn on you.

Sounds like a fuel blockage. Sludge tank?

Jim 1984 XJSC 3.6L

That was done when the car was tuned up.

Thank you. I am going to try these things. I don’t think it would be a sludge issue if the fuel pressure stays at 36 PSI, as long as that is the correct pressure. I put another new fuel filter in it yesterday just to make sure the replacement I put in wasn’t clogged. It wasn’t. By the way. The original fuel filter I replaced was totally clogged. I couldn’t blow air through it but the car ran fine. I know that I will kick myself when this is said and done. These things always end up being something you should have thought of first.


I was just reading through my bill from the mechanic. In one of his test drives he disconnected the MAF and the car seemed to run better but nothing showed up as bad when it was tested.

Is the 36psi measured with the engine running?

Jim XJSC 3.6L

No sludge tank on the 94’s.

Yes, 36 PSI is with the engine running.

Sorry to hear you are having issues with your '94 coupe, too, Stephen. :frowning_face: I’m curious as to what’s involved in a 50,000 mile “service” for our '94s? I didn’t really know there was a protocol at that mileage (congrats on having a '94 with that low, btw :clap: ). Superblue has 80,000 on her, and I don’t know whether the PO ever had the 50K mile servicing done on her. :question:

And when it fails?


The owner’s manual has a schedule of service that is supposed to be followed.
I went through the entire car doing a total tune up, brake fluid flush, transmission flush, replaced any worn bushings including steering rack and trailing arms, belts, hoses, coolant, oil and filter, air filter, checked the brakes, removed the trans mount and had the entire thing powder coated, had the trailing arms powder coated, checked fuel lines, changed fuel filter, (which is where my problems seemed to begin) and I guess that is about it. Oh yeah, I also checked and did minor rust prevention where I thought it was needed.

Okay, I just changed the original injectors for a full set of remanufactured injectors. Wanted to start but didn’t. I pulled the wire from the number one plug inserted a long screw driver and set it up so that I could view the spark from inside the car. The car started and ran in that situ until I pulled the screw driver out of the connector and put it back on the plug. After that the car wouldn’t start. I then did the same thing with the number two plug. It almost started. I then gave up for the day. It now seems to me that it resembles a car out of time. By the way the spark was yellow orange.

Perhaps a weak coil and or poor connections on coil?


Thanks, I’ll check it

While you’re there, can you let us know what kind of plugs were used? Just making sure.

It sounds like you’ve isolated the problem to a connection between coil and plug! I know they are new, but there could be a defect. You don’t still have the old wires, do you?

I will check and up date this weekend.
I am in communication with an ECM expert and he feels that there are bad capacitors in the unit. I am going to send a spare ECM to him and have it checked and if necessary reworked.
I’ll update you all next week.



I only know one place that can successfully refurbish these things. Send me an email offline and perhaps I can compare notes… I’ve spent tons of time talking to different ECU specialists. Most are honest…

I am sending the unit to Module Experts in Jacksonville FL I spoke with Adam and he seems to know what he’s doing. I will report to the forum one way or the other.