Hello,
The VDP my wife usually drives was having a slight hesitation starting up when hot so I decided to check/ change the spark plugs. (The old ones were Iridium Autolight and not terribly old but I figured I'd change them anyway.) I installed some Denso Double Iridium plugs and what a real difference they made! The stumbling during hot starting went away plus the performance is rather noticeable; plus it's getting a bit better gas millage! At this point I'm thinking of changing the ones in Jag 3 but they are Denso Iridium about a year old and the car runs fine....still. Best, Dave

https://www.denso.com/global/en/prod...lug/iridiumtt/

 

What was the gap on the Autolites? These are finicky engines and plug gap is important. Autolites also work better in lower octane fuel motors vs the Denso.

 

I'd expect the original gap was .40 but they had been in the car for some time; at least 50K miles so the gap was closer to .44. Also, the tip was sort of rounded off which would account for the difference. I should have taken a picture of one but the trash man just emptied the can!
I would add that since installing the new plugs the car performance has been significantly improved in both noticeable and subtle ways. It is seems to be far more responsive to just the slightest accelerator pedal press. I will end up replacing the ones in J3 at some point....just too many things going on around here!

 

If you want even more throttle response, try cleaning the MAF sensor followed by compressed air drying.

 

Hi Dave,

Are the plugs you installed the Denso TT Twin-Tip 4707? Just curious if you've tried the OEM NGK plugs and if you were unhappy with them for some reason? I've always had good performance from them.

For your 2000 XJ8, the Jaguar part number is AJ84575 (now superseded to C2A1535), and the actual plug Jaguar supplies under both numbers is the NGK Laser Iridium IFR5N10 / 7866. The gap Jaguar specifies is 0.9 - 1.0 mm / 0.035-0.039 inch. The Densos might like that gap range also.

Here's the info from the Jaguar Vehicle Specification Booklet:




Cheers,

Don

 

When it came to my time to change my cars spark plugs I did not buy iridiums. I did not want to spend close to 200 euros for a set of spark plugs. So I did a lot of research and looked ouf for the oem plug heat value, thread lenght, thread pitch, seal type etc. After that I looked for normal spark plugs. Found out one. NGK BKR5ES-11

That is exactly same as aj27 or aj27sc iridium but good quality basic type plug. It is easily avalable and very cheap. I have driven with those plugs since installed something like 20tkm without any issues.
In my mind spending like 180euros for the plugs that aren't difficult to replace in a car which is driven only in summertime and like 5-6 tkm per summer is not wise. Especially when iridium plug is close to 18 euro a piece and basic plug is like 4-5euro a piece. And if basic plugs stands the normal replace mileage 30 tkm. That is many summer seasons for me. And if any sort of hiccup eats a plug or two. It is not a world end to replace one or whole set.