Hi all,

Car is a '90, XJS 3.6 manual.

Have an occasional jerky misfire that has developed today between 1000 and 2000 rpm, car just hesitates for a split second and seems worse when the car is warm.

About 4 weeks ago I replaced the distributor cap and rotor arm as both seemed burnt especially the arm and the car was getting hesitant to start. This fixed that problem.

I noticed that the old distributor cap had white discolouration on every internal terminal as a band about 2mm thick, 2 mm from the end of each terminal. After the misfire started today I checked the new cap and found exactly the same banding. Thinking this might be the cause I removed the white layer with fine emery cloth. The car seemed better but not totally cured. After only 5 miles I had a look inside the cap again and found that the discolouration / banding had returned.

Is it normal for the internal terminals of the distributor cap to develop white banding? If not does anyone have any idea what is causing it and could it be related to my misfire?

Any other causes for the misfire as described in the first para?

Thanks for reading.

Cheers,
LeeP

 

How old are the plugs and HT leads? This sounds like resistance building up in the high tension circuit.

 

Thanks Greg,

Unknown on both counts so at least 5 years old and probably a lot longer. Sounds like my next port of call.

Cheers,
LeeP

 

Greg has sorted that one.

YES to your ask on that banding. The tip of the rotor does NOT touch thse terminals, it misses by what is known as an "air gap", so spark "jumps" that gap to get to the leads and then sparkers.

ALSO

Have a look inside the HT post of the coil. My XJ40 beasts, had green growth in there, and that really upset Her Highness.

 

Hi everyone and thanks for the responses.

Now cured, so, just in case anyone else has similar symptoms.......

My Jag manual quotes the acceptable range of resistance in K ohms for each HT lead. All of the plug leads were within spec but the king lead from coil to distributor had a resistance almost double the acceptable max despite looking in good condition. Bought a length of lead, connections and covers from the Green Spark Plug Co. made up a replacement length and all good with no misfire and smoother running.

Wondering if I should replace the coil as a precaution as both the primary and secondary resistance is slightly higher than the manual quotes.

Cheers,
LeeP

 

Yes, replace it, and the plugs if you have not done so. It is a mistake to piecemeal replace the HT circuit components when they are old..

 

Excellent info here...

Just wondering, and I'm excited because the leads are not budget breaking, what is that resistance value. I will go thru and test my HT leads, the coil wires and coils are new, plugs are new, cap is new, but the 12 leads are of unknown age and we're gifted to me from an xj40 back when my journey with ol Cherry Pie began. What numbers would I be looking for?

 

JJJ
Forget the measurement, buy a new set! If they are OK, they soon won't be.

 

Hi Jay,

I'm sure Greg is correct regarding replace everything but it is always nice to know the cause of an issue.

My Jag manual is only for the 3.6, 6 cylinder so no info on the 12 cylinder. However, I've listed below the figures for the 6 cylinder and the lead lengths on mine. Assuming that the acceptable resistance is proportional to the lead length you can hopefully work something out. Lead lengths in mm from my car and resistance ranges (in brackets) from the Jag manual in k Ohms.

0, (coil) 310 (1.8 - 4.1)
1, 540 (5.6 - 13.2)
2, 530 (6 - 14)
3, 640 (6.6 - 15.5)
4, 820 (7.7 - 18.2)
5, 880 (9.5 - 22.3)
6, 905 (9.1 - 21.4)

Hope this helps.

Cheers,
LeeP

 

Hi Greg and Lee. Top O the morning.

I should and will just go ahead and get new leads. I make myself crazy on the fence sometimes. Considering everything I've done, to hold out on spending 50-70bucks on something is plain ol dumb. Specially in such an important system. I ended up with a toast ignition amp and replaced the dizzy that had the same white residue and what looked like arching you described, Lee. Also found a spot in the copper wiring to the coils and amps that had to be producing resistance. I need to do the leads!

Thanks for taking the time to POST those values. I will check the values just to see when/while I am replacing things.

Cheers. And as usual, thanks a bunch!