Jaguar XJS spark plug leads arcing and car 'misses'

[geeman237]

I have a July 1993 build XJS 4.0 AJ6 coupe with a very rare manual gearbox (for US spec cars) It has the front and rear full moulded bumpers and outboard rear disc brakes. It has 99,600 miles. On start up it sometimes puffs blue smoke (er don't all old Jaguars do that??). It has a mysterious misfire that I cannot trace. I hope someone can assist as where I live in the US does not have a very good Jaguar expert nearby! I changed the plugs leads a couple of years ago and bought a set of Magnecor leads as I had heard these were highly regarded. The old leads were the original Jaguar ones. Both the old and new leads show clear evidence of arcing with star shapped patterns of fine dust where they have arced. The arcs are present on all the leads at various points along their length. The car has a fairly random miss beat at idle and when on very light throttle at 20-30mph in third gear you can feel a very faint 'missing'. I have changed the coolant temperature sensor (not the temp gauge sender) in the thermostat head as I got a fault code for that last year. I have also changed the mass air flow sensor (between the air filter and throttle body, to no avail). None of the injectors appear to be leaking. I am at a loss as to where to start on tracking down cuplrits. The check engine light is NOT lit up.
I am thinking of changing the plugs and distributor cap (separately) to see if there is a change. Because of the arcing on the spark plugs leads I am suspecting an ignition problem rather than a fuel problem.
Today after a 30 mile or so drive I listened to it at idle. When I blipped the throttle it revved ok but could not find a steady idle and dropped to about 300rpm, but not quite stalling before revving up again on its own, then back down then up and so on. If I revved it then gently let off the gas it would settle to a steady 600-700 rpm idle.
I think these XJS models were somewhere inbetween old school electronic ignition and the new plug and play technology. The main Jaguar dealer in town really wouldn't be much help and the independant guy I use can't find anything obviously wrong.
Has anyone else had an AJ6 engined Jaguar have similar symptoms, and what are some of the causes of this plug lead arcing?
Thanks in advance.

[dan19evans]

i'd say its most likely just the HT leads that have split insulation.
try replacing them again, but personally i'd stick to genuine jaguar parts

[Pumaracing]

I would look at your spark plug gaps. The bigger these are the more load it puts on the ignition system. Back in the day most cars managed quite happily on gaps of about 0.7mm. That's wide enough to generate a spark that'll fire any normal air fuel mixture however the combustion process is an inherently random and chaotic affair. The completeness of burn on each cycle depends on such things as there being a nice combustible pocket of mixture next to the plug when it fires to initiate the burn and how the turbulence patterns in the chamber propagate the flame across it. These things vary slightly every cycle leading to tiny differences in combustion and power output which average out so you don't feel them or see them on a normal dyno but they exist and can be measured.

"Bad" cycles might only be 1% less efficient than good ones but in the search for ever decreasing emissions and the need for catalyst equipped cars to not have any unburned fuel going out of the exhaust, plug gaps and spark voltages have been steadily rising to try and make the combustion process more complete and consistent until it's not unusual nowadays to see gaps of 1mm or even 1.2mm. The same engine can even have different plug gap specifications in different countries just to try and meet emissions regulations.

This obviously puts a huge strain on the leads, dizzy cap and other parts of the ignition system and frankly makes sod all real difference to how the engine runs. A gap of 0.8mm or even less will fire the mixture quite happily and remove most of the load on the leads which will then last much longer.

Turbocharged engines are particularly difficult to fire due to the density of the fuel air mixture and can require even smaller gaps or the spark will simply not jump the gap between the plug electrodes. I recall a friend's heavily modified car that had an awful misfire on full boost that defied all attempts at tuning it out with fuel and ignition mapping which turned out to be nothing more than the standard specification plug gaps being a tad too wide for the boost of the modified engine. Reducing them by 0.2mm eliminated the misfire completely after months of fruitless attempts at curing it by other means.