Misfire 1990XJ40 Sovereign


Just recently developed a misfire on my 1990 XJ40. Car only done 45000 and is immaculate, I only use it occasionally as my 2nd car though it has a run at least once a week.
The problem is on start up on idle there is a misfire, if I increase revs it gets worse though as it warms up the problem disappears, I can then drive it normally increasing my speed without so much as a hiccup.
I can leave it for an hour or so and same problem, it did get to a point where it would not start, left for few days then tried again it started with same misfire and symptoms as described above disappearing on warm up, if I increase revs having just started up the misfire is louder, and more often until warmed up.
Any help will be appreciated.
Some photos

David …

How did you determine there is an actual misfire and not that the car is running rough for some other reason?

If it is a misfire then I would pull each plug and check for a strong spark. As a matter of routine maintenance I would also then replace the plugs, wires, cap and rotor, and coil. If you still have a misfire then you’ve eliminated the usual suspects.

What else could cause a rough idle … vacuum leak, fuel injector either clogged or not firing, valve problems, etc. I start here with a compression test.

My latest obsession - failing rotors! :laughing:

Change the rotor in the distributor - my own car had intermittent rough idle on startup then later failed to start. Rotor’s internal resistor breaking down will cause symptoms like this …however, a swap is a quick, inexpensive way to eliminate the possibility -good policy to have a spare anyway.

Thanks for this sound advice, I determined there was a misfire when one morning it did not start on 1st turn of key, it idled poorly and misfired on acceleration, even worse under load.

I will go through process of changing plugs, ht leads and rotor arm and take it from there, it did have service (included plugs changed) about 8 to 10 months ago probably only 2500 miles.

Thanks will change rotor, uncanny that fault developed so quickly and is intermittent

Check the inside of the plug leads for corrosion or verdigre I had a 2.9 that ran on 5 1/2 cylinders until it got warm. It was a build up on the number 1 spark plug cap and SP tip.

Thanks Robin gonna change plugs and leads as matter of routine service.

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I wouldn’t bother getting new plugs unless they’re all burned up - which I doubt. This engine is very easy on plugs, I only replaced mine after 50,000 km because a guy in Europe was selling new old stock OEM Champions for a song and I fancied getting some - otherwise there were no issues with the old plugs.
Cap, wires and rotors are cheap enough and lots of choice of brands so why not change those, good policy with winter on the way.

As Robin says, plug leads are more of a problem than the plugs - oh and don’t fit iridium plugs or anything fancy like multiple electrodes etc. These engines are designed for the very basic copper Champions or equivalent, anything else is just a waste of money and may be detrimental to the engine.

Thanks Larry sound advice…