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Minor service time yesterday, just an oil and filter change and a new set of spark plugs, simple enough. Didn't 'test' it due to running short of time.

This morning, I started the engine to check for leaks and levels before going out but I noticed the engine was running rough, after checking that all leads were connected properly, I pulled the spark plug leads off one at a time with the engine running and sure enough number 3 was the culprit, not firing.

Put an old plug on the lead and earthed out on the cam cover, yes sparks. Took the new plug out and tried that in the same way, yes sparks. A bit more head scratching, swapped spark plugs but number 3 still not firing.

It was only then that I noticed the electrical connector on number 3 injector was off! I expect it had dislodged when I was pouring in the new oil. Shame I didn't spot that earlier, it would have saved me an hour

Thats frustrating! Still, at least you were able to resolve. I am still scratching my head trying to work out if I have an intermittent issue with my left-hand rear bulb failure control unit. Yesterday AM I had a failed brake light bulb; just replace with a spare - sorted. Then en-route to Woodford Halse country fayre and car rally, the bulb failure light came on each time I braked for around 7 miles of the journey then cleared. No issues until the return journey right at end as we pulled into the drive; bulb failure warning light iluminating each time the brake pedal is applied. I have checked the bulb seating and have walked the car extensively whilst Sue actuated the brake pedal repeatedly; no bulbs out and both brake lights appearing with the same brightness to the naked eye, yet the bulb failure warning light is triggering as described. I did question the bulb integrity as I understand that power consumption can make a difference and enduce a false bulb failure warning (had this issue with some LED brake light bulb replacements so had to return to traditional bulbs).

Guess that I'll have source a replacement warning unit if these are still available (not checked yet).

Cheers, Rich

AFAIK, the bulb failure warning stuff measures the difference in current between the two sides of the car. So if you changed one bulb but not the other, it could sometimes cause enough of a difference to trigger the warning. Or at least, that was my understanding of the system on my 1981 XJ6.