Our '90 XJS V12 was running fine and suddenly dropped a cylinder or two, then began missing even worse, with pops out the intake and virtually no throttle response. Was able to limp it home, but it wouldn't go over about 2000 rpm. Any suggestions from the Smart Guys what to check for? Our Jag technician is booked solid for a month! Every suggestion much appreciated!

 

While waiting for Smart Guys I'd read the Kirby's book, there are described Marelli one bank failures.

 

Start swapping coil and ignition amp plugs and see if the problem follows, your car is a 90' so its the more flammable ignition system, but also the more simplified one (in my opinion) you can also remove the dist cap and look for wear and scorching marks but I'd bet its just a failed coil.

Also while doing any of these tests have a fire extinguisher ready and don't run the car for more then a few moments as you'll be pumping raw gas into a dead cylinder bank it gets pushed down into the exhaust.

 

That would suggest to me that it is NOT a Marelli one bank failure but could still be distributor/rotor issue.
Does sound like a timing issue. Can you run it for short time in dark with bonnet open see if you have a light show.

 

Last time I had a ignition issue one of the wires to a coil plug had frayed to the point of intermittent contact, the idle of the hot engine would move the wire just enough causing pops/banging and then low power. Jiggling wires in this case also helps narrow down if or where the connection is being lost.

 

Agree Vancouver, what l was trying to point out was the normal behaviour of one complete bank shutting down does not result in any loss of even running which is one reason it is often missed at the beginning.

 

Thanks for all the tips--now I've got some things to check today! My first thought was a bank failure, but it was running too rough and got worse. Again, Thank you all.

 

I'm a tad confused. Never a rarity, but seems more so as the years, a lot of them, go by!!!

I'm familiar with running on seven of eight. But, seven of 12???? Six of twelve seems possible as history on this forum.

Carl

 

Well, I couldn't accurately count how many were not firing, but the engine seemed weaker than with half firing, but hard to really tell. Initially, it just seemed like one or two were gone, as the car drove kind-of OK, but then it grew weaker. It made just enough power to get me several blocks back home, albeit slowly. Another symptom I neglected to mention was that the car began detonating badly--which it has never done before--further suggesting a timing issue. No fireworks when started in the dark garage this morning, but also no improvement!

 

Are you sure it's not fuel starvation?

 

Possibly, I suppose, but I would assume that the car would stop if it was running out of fuel (tank is half full). And it acted like it would just continue on and on if I were willing to put up with the misfiring and detonating. Guess I'll have the shop look at it!

 

i would change the plug leads, unless its allready been done

 

 

That's a good suggestion, and best of all, easily checked with a gauge. Thanx for pointing it out.

 

Losing one whole bank at idle is almost unnoticeable, I was chasing an issue years ago and disconnected ALL injectors on one bank, it was barely noticeable.

Pops out the intake sounds more like the timing has moved, which is possible in a Marelli car if the damper rubber fails as the timing wheel can move. This is where I would start looking. Line up the TDC mark on the damper and the wheel tooth should line up with the VR sensor.

 

Warren, the timing "star" is actually pinned and bolted to the damper hub itself so unaffected by rubber failure.