well just the other day I noticed my 1991 xjs v12 running strangely a little rough and weak

yes the a bank is not firing

cap and rotor look really good

previous owner replaced cap rotor plugs and fuel lines

i just replaced coils and no change, then swapped ignitors and no change

any ideas ?

 

Two ignition coils?

 

Just had same problem. One of the two coils had failed.

 

I had the same problem recently and it was one of the ignition amplifiers. I have a marelli system and they are position on the front valence above the cooling pack and are mounted onto aluminium plates. That is assuming that you have this system or equivalent that has amps.

If you replace the amplifier make sure that you use thermal compound to attach it to the aluminium plate.

If you have cats then be careful as fuel will still be injected and if the cats are hot they will then overheat and get damaged.

 

Is the 1991 a marelli? Don't those have separate systems for the left and right bank?

 

yes it has two new coils and I swapped the left and right amplifiers

bank A still nothing

i have a new cap on the way ..... the one on the car looks like new but maybe it is bad

 

There is the known issue of burn through on the rotor arm so I would replace that at the same time.

 

That would be my thought as well. If switching the plugs on the amps made no difference, then the next fault is the coil(s), as they are triggered by the amps. If it isn't a coil, (those can be switched as well), then the ONLY thing that is after that, that controls an entire bank of cylinders is the rotor button.

 

If it is a marelli car, check for spark on the non-functioning bank. If you have a spark, then check the injectors are working. I think, repeat think, that the marelli cars have separate injector circuits for each bank.

greg

 

Remove the distributer cap and remove and inspect the rotor. Burn through of the rotor is a very common problem. My 90 is a Marelli equipped car, so your 91 is as well.

 

ok now I am embarassed I thought I had a spark problem ................ but NO

my brother brought over a spark tester and I have spark then I pull plug A1 and it is dry like no fuel

so is it possible to have fuel on the left side but not the right ???

we also found what looks like an air flow sensor dangling out side the air intake tube on the right side .... i have the aftermarket K and N air filters

so is it possible that if the switch or sensor is broken that it would shut off the fuel to the right side ???

it looks like the the printing on it says "otter v12"

 

I'm just winging this one in out of left field.

I recall reading about someone having a similar sounding problem, but memory kinda fuzzy about which version of the V12 they had.
Problem turned out to be a throttle linkage connection that had come loose. One side was working, the other side not.

I could never be that lucky, but maybe you will!

 

Cannot help with the sensor, but which side do you think is the 'right' US driver's or US passengers? If US driver's then it may well be the air temp sensor which should be plugged into a hole in the inlet trumpet and connected to the loom with an injector-type plug. It will not stop the bank running though.

Secondly, see my post above, if no fuel it is either the loom or possibly the power resistor (see sticky at top of the list to fix). I am no Marelli expert; but I am pretty sure that each injector bank is separately driven on your engine. Someone will chime in.
Greg

 

I'm not a Marelli expert, but can assure you that the fuel, and the ignition have nothing in common. That was the problem with the Marelli cars from the beginning. The Marelli sends a engine speed signal to the Lucas ecu, but that's it.
What I bet you have, is a problem in the power resistor. It would shut off fuel to a entire bank. There is a thread on here that details how to refresh it, get it going good for little to nothing except your time.

 

The cool air sensor on the driver side can cause this issue. Has done it twice on my V12 1991 XJS

 

Hi tdfxjs

The Resistor Pack can play havoc with the firing of the Injectors and all it sometimes needs would be its Plug and Socket given a really good Clean, with either Vinegar or Lemon Juice followed by Contact Cleaner

The Resistor Pack is located near the Front Headlight Nacelle on the drivers side on a UK Car

Ideally this needs to come out of the Car, so that you are able to clean the Pins of the Plug and Socket until they are 'Shiny Bright'

Its only held in by a couple of Bolts which are a bit of a PIA to get to but in order to do the job properly (which may save you buying a New One it does need to come out)

Here is the Link including lots of Photos

Cleaning the Plug and Socket of the Resistor Pack on a 1990 Marelli XJS V12