My 1990 V12 XJ-S missfires under acceleration

The car starts and runs fine as long as I don’t push it, but when trying to accelerate and particularly when going uphill ( i.e. under moderate load ) it tends to cough and splutter, no back fire. So I back off and accelerate slowly and all is OK.
I have cleaned out the fuel tank sump tank and internal filter and changed the main fuel filter. I’m suspecting plugs, or fuel injectors but any suggestions as to cause and fix would be appreciated.


I’d be suspecting ignition breaking down under load.

What does the tach do during the misfire? Steady? Bounce to zero?


Thanks for the reply Robin. What would you suggest as breaking down - Plugs, plug wires? I guess I’d start there anyway.

Thanks for the reply Bob. Didn’t really pay attention to the Tach but I will next time and get back. Cheers.

Depends on what ign system is on the car. Much more knowledgable people on here that know the XJS inside out.

If it’s a 1990 v12 it’s Marelli ignition. If the tach is stumbling with the miss i’d suspect either the crank sensor (tdc) flywheel sensor (engine speed) or the silly thin coax wire in the harness on the left side of the fuel rail.
I’ve personally had the crank sensor cause an intermittent cut out, but not consistently under load as described here.
Let us know how the tach is acting. That will be a good clue as to whether it’s ign or fuel.

I have also had intermittent cutting out which I too found was the connection of the CPS plug to the very poorly located thin wires to the computer. I believe I have resolved this by giving the connection a thorough clean, relocated it slightly by pulling the CPS lead up higher and then taping the connection together as tight as I could. I bought a new CPS but haven’t fitted it as the connection seemed to be the problem. So so far I have ruled out the CPS as the culprit.

Hi Bob, I took it out for a decent run today. Ran it in 2nd gear mostly to keep the revs up and give the engine a good run at highish revs - (4000 rpm at 100km/hr). Acceleration in second was not too bad with virtually no hickups. When I ran it in normal drive and accelerated up a hill it hickupped pretty badly ( the more I floored it the worse it was - at one point it virtually completely cut out ). The tach however did not falter or jump about. I had put a bottle of STP fuel additive / injector cleaner into a half tank fill-up hoping this would help, particularly at the higher revs. Not sure if this has helped, but I am now suspecting fuel starvation under load.
Your thoughts?

Suspect- plugs, leads, rotor, cap.

OK thanks Foggyoo. That’ll be on my list as well.

The way in which the car almost stalled completely when I floored the throttle was very reminiscent of the effect of frozen fuel lines ( which we can get a lot of here in Canada, but I suspect not where you are :grin:) . This was why I think the injectors might be giving a problem as well. Unless it’s some kind of vacuum issue.

Ignition coil? Maybe not getting a strong spark.

Hi Paul: It does not sounds like a CPS or flywheel sensor to me. My logic: When one of those is failing, if it runs at all, the car will stumble like the key has been turned off and back on. This is because when the Marelli ignition computer looses CPS or engine speed inputs, it stops firing the coils, and it also stops sending a signal to the Lucas FI ECU. So everything is shut down. Due to the loss of signal to to coils, the tach drops quickly drops to zero.
I would at least inspect the cap and rotor for any signs of arcing. While I don’t think this is likely, you could be in the early stages of the “Marelli meltdown” where you are losing 6 cylinders due to rotor arcing. Obviously check the wires to the plugs, to both coils, and to the low voltage side of both coils. I hate to suppose, but it seems to me that since it runs at high RPM under light load that you don’t have an ignition problem. Most ignition problems get worse at higher RPM.

Assuming the cap and rotor and other ignition components look good, I think you are on the right track with fuel starvation. More throttle just means more air, then the FI is supposed to keep up from there. So it sounds like you are giving it more air and it’s not getting enough gas to go along with it.

You’ve already done the obvious with the main and sump filters. Maybe check the fuel pressure regulators, particularly on the left bank, for any signs of fuel leaking from the vacuum diaphragm port, which indicates a split diaphragm which probably means very low pressure at the rail.

Could there be water in the fuel? Maybe a can of Heet or other stabilizer to break up the water. That stuff is basically alcohol which is not great for fuel systems but one can won’t hurt if you run through a tank of gas over the next few days.

Ideally you want to get a fuel pressure gauge on there to get some idea of the health of the fuel pump. You can easily t into the main line feed on the right fender by the firewall, it’s just hose clamped on.

You didn’t mention how the car had been in the months leading up to this. Driven regularly? Other work recently completed? Other “unrelated” symptoms?

Others more knowledgeable about FI problems than myself will surely chime in.


Hi Bob. I had the Marelli rotor problem a few years ago - my A bank shut down. I fixed this with the installation of a new rotor and filling it with silicone as described in this forum. No problems since, but I check my exhausts every time I run the car just to be sure. I have run the car very infrequently over the last few years due to moving house and it is always stored from November to May during the Canadian winter.
I have a new set of plugs ready to go in so I 'm thinking I will get on with that anyway and check the distributor at the same time. Your points on the fuel system are well taken so I will be checking that as well.
Looks like I have a few things to get on with, many thanks to you and all who added their thoughts.


I am sorry, I didn’t read all replies. If you have had this problem before, and fixed the ignition cap and rotor, have you check the exhaust back-pressure? Fully clogged system is unlikely, but you may have partially blocked system, which may explain why the engine sputters under heavy load.

In any case, the already suggested by Bob fuel pressure test is probably going to tell you if you’ll have to chase fueling issue or this will ultimately prove to be a breathing problem.

Good luck!

Hi Steve, my exhausts seem to be fine - both giving ( to the eyes, ears and hand ) similar output pressure. I’ll be working on plugs, leads and fuel system as Bob suggests in the coming months, whilst checking on the distributor again.
Thanks again for the input.

FWIW, my preferred wire set for V12 w/ Marelli ignition are Bosch 09365. They are a little hard to find but they fit well, work well and last a few years. They include both coil wires.
Here’s my fuel pressure gage. Nothing special, many have done same:


Thanks again Bob.