Just my .02¢ Do what you feel is right doing but I am using Bosch + 4 in my 99 XK8 (2 sets) from 76,000 to 122,000 and +2 in my 00 S-Type over 20,000mi on the plugs and they never let me down. When I step on it, it never wants to know if I want to do that it responds. Sorry Steve!

 

Just replaced my plugs.

Noticed oil in some of the plug openings, cleaned it out.

All the plugs looked normal, the prev. owner used autolite double platinum.

I used a .040 gap.

Hope it improves mileage.

Not sure how oil got in some wells.

 

I can tell you how - head cam cover gaskets It's a good idea to change them.

 

I have run Jaguar restoration businesses for many years. Although we have dealt mostly with older vehicles, all the running business and personal vehicles are newer Jaguars. I have replaced probably 30 sets of plugs in XJ8s and XK8s. Most of them have been with the NGK BKR5EIX-11. I have never seen an "out-of-the-box" gap of less than .040. I just measured all the stock plugs we have and they are the same. Just curious where the "out of the box .031" came from. Maybe NGK sends different plugs to the colonies? Otherwise this is a very interesting thread. Take care.

 

Any word on Twin Tip Iridiums that will fit an X308? I'm looking at replacing my 99 XJR plugs soon,...

Otherwise, what's better? Iridium or TT Platinum?

More specifically NGK PFR6G-13E vs NGK BKR6EIX-11 IRIDIUM IX

Thanks!

 

Anyone put these bad boys in their Cat yet?

Denso: IQ20TT / DEN 4707


Iridium TT; Gap 0.040; Long # IQ20TT; Iridium TT - Ultimate Endurance and Power, 0.4mm Iridium and 0.7mm Platinum Twin-Tip Technology; 8 Req.

Denso Iridium TT Spark Plug: IQ20TT 4707: Advance Auto Parts

 

Yep. New Denso plugs two years ago. Work fine. ("After all, it's just a car" Danielsand, ca 2012)

 

So I picked up a set of the IQ20TT and am installing them tomorrow.

I measured the gap with a wire spark plug gauge and am estimated the out of the box gap to be approximately .038"(between .035-.040).

Adjusted to approximately .047"(between .44-54) for my XJR.

This gap size was chosen as I intend to install some performance upgrades soon so this tighter gap will be desired,...

Apparently there is a AJ26 TSB that calls for a .052 gap to fix rough idle (to be performed at regular service interval).

 

Just to add my 2 cents...I am an aircraft mechanic and a Jaguar owner (1988 XJ-6 and 1998 XJ-8 ). I highly recommend buying a "good" torque wrench when installing spark plugs. Don't buy a cheap $20.00 torque wrench....go to NAPA or a good tool source and you will likely spend about $125.00 for a decent analog tool. I am required to send mine off every year to get them certified. I put zinc based anti seize on all my plugs. Get a small artist brush and carefully paint a little bit on the threads, being careful not to get any near the threads on the bottom of the plug. There is no need to cover all the threads...just a bit on each side will spread out when you install them. If you goof up...rinse the plug off with brake cleaner and start over. We in aviation have issues with plug resistance going too high on certain brands of plugs. Since a new spark plug costs about 25 dollars and most engines require 8 or 12 plugs, we are careful when cleaning, gapping and installing used plugs. On my cars, I remember the new plugs tested about 10K ohms each when I installed them. I then assume this is normal for this brand of spark plug. I have seen high resistance readings in aviation plugs as high as 300K ohms, which caused misfires . I would not worry too much if the plug resistance was in the neighborhood of 10K to 25K ohms on these cars. Since most of us are not driving around above 20,000 ft. Good luck...

 

You need to read this

Link Use of Anti-Seize on Spark Plugs/Lug Nuts - Jaguar Forums - Jaguar Enthusiasts Forum

 

Dear KAITHM,
It is always a pleasure to read the opinion of an engineer. I am replacing the plugs in my 99XJ8 with NGK iridiums 5464. However, here in Colorado we have an altitude environment that ranges from 5000 to9000+ ft above sea level. Should I consider a different gap for this application?

 

That poster has not been here in over a year and I don't believe he had any expertise of his own in the field.

Correct spark plug gap is not affected by altitude. Set them to the recommended gap indicated by Jaguar and enjoy the car.

 

From NGK......

Barometric Pressure/Altitude

• Also affects the spark plug’s tip temperature
• The higher the altitude, the lower cylinder pressure becomes. As the cylinder temperature decreases, so does the plug tip temperature
• Many mechanics attempt to “chase” tuning by changing spark plug heat ranges
• The real answer is to adjust jetting or air/fuel mixtures in an effort to put more air back into the engine

 

Exactly- and the engine controls constantly adjust the air fuel mixture as we drive.

 

Despite the passage of time (5+ years now in Sept. 2016), the comments by ka1thm above are extremely helpful as I search for the correct spark plug for two Jaguars - a 1999 XJ8 base and a 2000 XJ8 Vanden Plas. I can report that the myths, confusion and misinformation about the correct gaps and "pre-gapped" plugs persist in the market, at least among car parts salesmen.

After checking Jaguar specs, NGK's online fitment guides, and three different part stores, I followed the majority recommendation here and got the NGK BKR5EIX11 (Stock No. 5464) iridium plugs. These have a 1.1mm (0.044") gap, which seemed a close enough match to the 1.0-1.1mm (0.035-0.039") stated above. Also, at Advance Auto Parts, the clerk measured a couple of actual samples at 0.039" out of the box, so I eagerly got a set of 8 for about $7 each.

HOWEVER, this morning, after installing them yesterday and feeling satisfied with myself, I found an email reply to an online question for NGK Tech Support. The BKR5EIX11 turns out to be slightly wrong, at least for the North American 1998 & 1999 XJ8.
In their email, NGK's Tech Support in Michigan states: "Thank you for your inquiry. IFR5N10 (stock#7866 is the actual OE plug for your XJ8. They come out of the box pre-gapped at 1.0mm (.040"). BKR5EIX-11 will be a significant down-grade for you. They come out of the box pre-grapped at 1.1mm (.044") We recommend the OE plug IFR5N10 for the best performance."

Also, according to NGK's online guide as well as another email from them, the OE spec from 3/2000 thru 2002 for the XJ8 with the AJ27 engine is also different. NGK: "For the 99 XJ8, the actual OE plug is a Laser Platinum PFR5G-13E (#2761), which comes pre-gapped at .052". Going to the BKR5EIX-11 would be a downgrade, and would need to be gapped up to .052" since that plug comes out of the box at .044" (1.1mm)."

So the OE fitment for the 2000/2001/2002-model-year XJ8 with the AJ27 engine was changed. The gap increased to 1.3mm or 0.052". (Maybe it was to smooth out idling, as stated in a '99 Jaguar TSB, which applied to pre-'99 models.) Note: I didn't confirm with NGK whether the OE fitment for the 1998-1999 VDP's, or if the UK models have different specs from the NA XJ8's.

Anyway, the difference in spec gap vs. the measured 0.039" is more than the 0.008" adjustment range allowed. So I am planning to replace them with the recommended PFR5G-13E's ($13 ea), especially if there is any noticeable problem with power or mileage. Also, since the valve cover gaskets were leaking and need replacement soon, and the mechanic said it would be just another $50 labor, I'm inclined to bite the bullet.

 

For a start there is almost as much crap out there about plugs and choice as there is oil threads.
Chuck all that out the window!
The only plugs to use are NGK (per OEM) PERIOD!!
The X308 was originally fitted with a plug gap of 0.52" but a factory bulletin was issued to reduce the gap to 0.48" due to owners having problems with lumpy and erratic idle, most likely down to poor quality
North American fuel.
My X308 '98 AJ26 has used NGK BKR5EIX-11 Iridium plugs for the last ten years and gap set at 1.2mm (0.4788").
Plugs are replaced at 80,000klm and car now shows 215,000klm. ;o))

 

There's another thing that can be thrown out the window as a myth. Fuel in the US or Canada is in no way inferior to anywhere else.

 

Hehehee....I knew that would solicit a response ;o]P

 

Unfortunately, there's some that will believe it cause some random guy on the internet said so.

 

Yes, indeed!