I'd like to see that TSB because I believe you have it backwards. The TSB was to increase the gap on the factory plugs to .052 (you left off a zero). Please see http://www.jagrepair.com/images/TSB/...plug%20gap.pdf

As previously stated, I choose a .047 gap due to installing an upper and lower pulley SC upgrade. Increased boost requires a tighter gap. Also, my Twin Tip Iridium Plugs are working swimmingly.

 

NOPE...I had it the right way round ;o]
The factory TSB stated to REDUCE gap to correct lumpy idling.....on AJ26 engines.
If I or some one else finds it, I'll post up ;o]

Well good Sir....I am forced to eat my own words and all credibility is shot to ribbons....I read it backwards....put it down to old age and the onset of dyslexia....well, that's my excuse and I'm stickin' to it ;o)=)

 

This is to clarify the recommendations from NGK for these two models, which I reported a few days ago. I listed them out of order for the two models in my post (1999 XJ8 and 2000 Vanden Plas) and cannot seem to edit it anymore. Also, I got new and different information on recommended spark plug gaps from the Jaguar dealer, so here goes again:

1998-1999 XJ8 base (non-supercharged AJ26 4.0L V8): NGK recommends the NGK PFR5G-13E (Stock No. 2761), laser-welded platinum, which comes pre-gapped at 1.3mm or 0.051" (rounded to 0.052" in their spec). Price is currently $10 to $11 online.
Apparently Jaguar has changed its spec for the X308 (1998-2003 XJ8) to 0.048" or 1.2mm, but that is within the gap adjustment range of these plugs.

3/2000-2002 Vanden Plas (non-supercharged AJ27 4.0L V8): NGK recommends the NGK IFR5N10 (Stock #7866), laser-welded iridium center electrode and platinum ground electrode, which is nominally 1.0mm or 0.039" (rounded up to 0.04" in their spec). Price is currently $10 to $13 online.

If the original Jaguar X308 Vanden Plas spec, at least for the 1999 model listed by th1akm above, was 0.9 to 1.0mm (0.036 to 0.039", or 0.04" rounded), the NGK IFR5N10 seems correctly gapped as-is. However, Jaguar apparently increased the spec to 0.048" or 1.2mm, based on my dealer's information. So I am wondering if these plugs should have their gaps increased, or whether NGK's recommendation is simply out-of-date.

Also, according to NGK, their BKR5EIX-11 Iridium, nominally pre-gapped at 1.1mm (0.044"), is a "downgrade" option -- not an "upgrade" as stated elsewhere in these forums. One difference is that it is not laser-welded. Moreover, its gap is different from ANY of the listed Jaguar X308 specs. However, the BKR5EIX-11 samples I measured were 0.039" out of the box. Based on the 0.036-0.039 spec listed above and many recommendations of this plug, I bought a set and had them put in my friend's '99 XJ8.

However, after calling the Jaguar dealer, who checked the VINs, I learned that both the 1999 XJ8 and my 2000 VDP now use a plug with a 0.048" gap. Could it be that NGK Tech Service are going from the original specs they were given? Or is the design of the plugs is more important than the gap, which can after all be adjusted up to 0.008"?

(I read it's OK to adjust "pre-gapped" platinum or iridium plugs as long as you use a special gapping tool that puts no pressure on the center electrode. But the Advance Auto Parts store did not have that tool, and 3 salesmen there had never heard of it.)

Assuming the current Jaguar spec of 0.048" is ideal (or a good compromise for smooth idle, emissions, mileage, etc.), the BKR5EIX-11 plugs I got were still over 0.008" smaller, at the limit of adjustment allowed. In any case, NGK recommends different spark plugs. I would think Jaguar's replacement plugs, if made by NGK, are exactly the ones they recommended to me, but with 0.048" gaps (and 50% higher in price).

And yet -- despite the incorrect gap and NGK's dark warnings about the BKR5EIX-11 being a "significant downgrade," my friend reports his XJ8 is "running great," with more power. Of course, he's comparing with the original 17-year-old plugs. (But before you conclude he was neglecting his car, keep in mind it only has about 64K miles on it.) Also, I put in better Shell gas and Chevron Techron. In addition, the cam cover gaskets had leaked oil into some of the spark plug tubes. The mechanic vacuumed and wiped out the oil before putting in the new plugs, but it may have been affecting performance.

 

All three normal spark plug gapping tools I have do not contact or put any pressure on the "center" electrode, I think your over thinking that aspect.

 

Don't forget -- the dealers don't gap the plugs. The TSB is to check and adjust the gap with a customer concern. Jaguar did not want to pay to replace the plugs. Manufacturers do this for two reasons -- if they pay to replace .. they all get replaced. I bet the check and gap was no money maker for the dealer .. they only did it if a customer complained.

Also -- NGK could be gaping the plug differently for Jaguar.

I never play with them

 

I changed my plugs, and used BKR5EIX-11 Iridium cause they are easy to buy here.
Straight out of the box into the engine, no gap-adjustment, no thinking, nothing.
Just the correct torque, and there she goes .... super smooth ...

You guys have way too much time on hand, go out and drive, don't overthink too much .

 

Hi there.
I found the notation at the beginning of this thread regarding the "Ignition plug Corona discharge" very interesting.


Since its abbout "coronas" and "discharge" i do wonder if this may have any impact on the radio as interferance?


Does different types of sparkplugs make any difference in this matter?


The spark gap?


Thankful for any ideas regarding this matter.


/E