At my last service, despite the recommendations against Iridium plugs, I was allured by their promise of better performance and longevity. I must say for the first couple of hundred km I was impressed, although this may have been a post-service placebo.
Regretfully, I started to find that after my cat had warmed up, she would randomly stall at idle or low revs at which point I noted the idle was also a little rough.
Being a little stubborn about it and not wanting to concede that my new fantastic, expensive Iridium plugs were the problem, I started thinking along the lines of a faulty coil? Idle Control Valve?. As I had just changed the Fuel Filter and cleaned the Throttle Body, they couldn't be the problem? Hoping I didn't have to follow these lines of investigation I swapped out the Iridium plugs for a standard set of NGK BCPR6E as listed by local parts store for my 95 XJ6.
Miraculously the problem went away once the standard plugs were fitted. I have no idea why or how the Iridium plugs caused this problem, as the ones I purchased were listed on the NGK web site as suitable for my vehicle. I can contest however that they caused stalling issues at idle that was resolved once removed and replaced.

 

My both AJ16 engines run perfectly with NDA iridium BKR6EIX.

 

This has come up more than a few times over the years in X300/AJ16 engine discussions. Some sort of spark plug sensitivity thing goin' on.

Some report very palpable problems with anything other than Champion plugs; others report great results with alternatives. Others report problem with Champions.

Personally, I drove my XJR/6 till 171k miles and used a variety of plugs and never had any problems with any of them.....nor noticed any difference in the way the engine behaved with any of them. I do recall battling a driveabilty concern (gruesome details now forgotten) to the point of abject frustration and, on a lark, changed the plugs, which only had maybe 10k miles on them. Problem instantly solved. I installed the same brand that I removed.

I think there's something else afoot beyond just the plugs themselves. I long suspected a relationship with the (somewhat notoriously) problematic coils and how different plugs respond (for lack of a better word) to them. I dunno; just musing, really.

Cheers
DD

 

I agree with Doug 100%.

My 3.2 (Big Block) ran sweet as a nut on the Standard NGK.

The NGK rep called at work and GAVE me a set of Iridium. He kept pestering me as to results, so I reluctantly changed them, and the standards went IN the glovebox, real confident I was, and at the start, same/same performance.

I run 98RON fuel only, and NO Ethanol at all.

At about 200kms in the stuttering started, and the performance dropped right off. I knew the engine, and knew what it was, so on the side of the road, I put the "old" back it, and continued to work. When he next appeared I threatened to bend him over , and "shove them", and I have not seen him now for 5 years, HA.

 

Same story with the Bosch ones here, only managed just over 100 miles on them before car started to splutter and die occasionally, then I went back to NGK, only to change them during following service to the best plugs so far the hotter champions RC12s.
I've tested iridium on two x300, both with original and still working coils at the time and neither worked fine, I now have new genuine coils but don't think I'll ever try any other plugs than the champions on AJ16. They are not easy to find(the hotter 12s) but definitely best plugs to use and recommend by Jaguar.

 

The AJ16 actually uses the NGK “5” series of plugs, which are hotter than the “6” series. That “6” series is essentially the original spec that lines up with the Champion RC9YCC plug. When the TSB issued the change to the hotter RC12YCC plug, many of the interchange plug references were never changed/updated.

That being said, if the car runs well, don’t bother with the change.

 

I installed a set of NGK Iridiums at 85,000 miles. Changed 'em 5 yrs later at 196,000 miles. Nary a problem, despite a coupla wells full of oil. I replaced them because I was refurbishing the cam cover. Just seemed the thing to do. I used base Champions because the NGK's were no longer on wholesaler closeout at Rock Auto where I'd obtained the original set for a dollar or two each. Don't recall the exact part number and didn't make a note of it in my maintenance app.

 

In my time at Jaguar, AJ16 engineers were never fitted with anything other than Champion spark plugs. RC9YCC for some markets, RC12YCC for some markets such as US, where the lower speeds/and loads minimised any risks of using a "hotter" plug, which gave better idle quality Lots of people and website may claim to offer "alternative" / "equivalent" plugs, but owners deviate from the Champion plugs at the risk idle quality problems. There is plenty of anecdotal evidence of a compatibility issue between the ignition coils and some specifications of spark plugs.