I have had my XJR now for 6 months and turned over 100k miles. All is good and so I thought to change the spark plugs. This seemed pretty easy for 7 of them, but the last one, back LHS (driver) is really difficult to get to. I persevered and pulled it and found that I had 7 of one type and one of the 'original' IFR5N type and 7 NGK TT. Obviously they have been changed before, good, but by some amateur who did not get the last one out.
All looked pretty clean and the gap was about 38-40 thou.

After replacing all of them with a fresh set of IFR5N, and living with it since then for 2 weeks, I am amazed that the drivability has changed such. The car always drove well, good economy, smooth, but had this hesitation just off idle. This was not the ZF gearbox lurch, just a slight hesitation just moving off idle.

Since the new plugs, this hesitation has basically vanished, all other characteristics are the same, economy, smoothness, start up etc. but I am pleased that this characteristic has been basically eliminated.

I never imagined that a set of plugs that looked as good as what came out would have had that effect.

 

Spark plugs just make sparks ... but fresh ones make the right spark at the right time.

The trick is to get the electrodes under a magnifier to get a good look. You can have good gaps but rounded off electrodes. Electricity likes sharp edges for a good gap jump.

 

Funny how that one-eighth plug could cause the drivability problem.
I'm sure you are right. It was put in by the plant and never renewed.
That cylinder's slight misfire probably caused the hesitation.
Good deal.