1999 XJ8, 90K
Changed spark plugs awhile back and it seems at about that point a rough idle began. Have installed new knock sensor bank 2. Now showing code for bank 1.
The car is VERY sensitive to the octane of the fuel and will let me know when a gas station has sold me lower octane gas than I paid for, by going into restricted performance till I add new fuel or add an octane booster.
I installed Bosch Platinum Plus FR8 DP plus at .42 gap.
Could these not be the best plugs for the XJ8??
Thanks,
Ken

 

There's a good thread on plugs for these cars here: https://www.jaguarforums.com/forum/x...vdp-faq-52159/ That should answer any plug questions you have.

It sounds like you may have damaged the knock sensors or wiring to these, based on the codes you've gotten. I'm not sure why your car should be so sensitive to octane, as the computer is supposed to self-adjust for octane. Did you double-check the gap (and recommended gap) before installing the plugs? Jaguar states the gap should be 0.035-0.039 on AJ8 4575 (iridium) plugs: https://www.jaguarforums.com/forum/a...park-plugs-jpg

 

Thanks Mrlovt,
I checked the gaps today, they were .42-.45 so brought them to .35-.39. Also found an air intke leak which I repaired. Still have rough idle but the CEL went out which had the knock sensor code. Added some fuel injector cleaner and will see if that helps.
I do have another vacuum or air intake leak as when I punch the gas while in park I can hear a small sucking noise from the engine area. The noise is similar to a carbureted car when you punch it in park.
Also have traction/stability/gearbox issue to address in another post after I have investigated further.
Regarding the octane, I agree with you that it should not do what it does, but it is what it is. I can fill up and within 5 miles go into restricted performance mode if I did not get 93 octane(even though I paid for it). I then can put in octane booster and within 5 miles restricted performance goes away. Or if I did not put much gas in I can pull in to a Shell(who I trust), add 93 octane and within 5 miles all is good again. It has been that way since we purchased it several years ago.
Again, thanks

 

No problem. Eliminating your vacuum leaks should help quite a bit. By the way, that spark plug gap was for iridium plugs, not sure if it's the same for platinum ones.

The CEL for knock sensors can be intermittent, and the issue is not uncommon on these cars. Mine lights up with restricted performance after a heavy rain and goes away after a long drive. Somehow moisture (condensation or humidity I suppose) is affecting it, though I've cleaned and used dielectric grease on the connections. I have replacements on hand, just haven't had time to do the job: https://www.jaguarforums.com/forum/x...ine-faq-45963/

If it's any consolation, I've always kept 93 octane in mine (91 isn't available in my area), and ethanol-free when it's available. I had a bad experience (burned exhaust valve) once in my '91 XJ6 on 87 octane at high speeds, so I'm not taking any chances. Of course the two may have been unrelated, but the peace of mind is worth the extra expense.

Let us know your progress!

 

Hello,

Has anybody experience with Beru spark plugs?

Thanks

 

Be aware that most off the shelf octane boosters will increase the gasoline's rating by only .1 per can (from 91 to 91.1 for example). This is a very ineffective and expensive way of doing things.

I believe that most Jags on this side of the pond were set up for 91 octane, so even if you were slightly cheated at the pump when buying 93, the gas still should have been acceptable. I'm not that familiar with your era of car, but am surprised that low(er) octane gas would set off the restricted performance light.

 

Right....let's cut the crap!
Ken...these are the correct plugs for your car :-)
NGK PFR6G-13E set at 52 thou.....none of this 39-40 thou rubbish!
The alternative is NGK Iridium IX BKR6EIX-11 also set at 52 thou

These are the plugs fitted as OEM....nothing else will do period!
The 52 thou gap was decided on by Jaguar to erradicate intermitant idling problems.

Any thing else is just ill informed bar room hog wash.

 

Rubbish or not, the problem should not exist with reasonable plugs set at a reasonable gap. The same goes for octane.

The Bosch wouldn't be first choice, but the problem will probably be something beyond the selection of plugs and the gap.

 

That as may be but these are the specifications set down by Jaguar themselves.
Would you feed your thoroughbred on cheap chaff and weeds?

 

Back to the subject I know there was a lot of threads about the spark plugs and that the NGK Iridium IX BKR6EIX-11 is what our cats love, but.... I recently noticed in the JTIS that the early AJ26 engines do have another recommendation showing NCA3850FA1 as a part number. After looking around it seems that these are NGK plugs as well but with a different gap.

So it seems that the previously mentioned iridium plugs are not what I need which could explain some issues I had. The question is - can someone tell me NGK part number for the plugs with Jaguar part number NCA3850FA1? And what is the right gap for the plugs on AJ26 3.2 engine?

The only picture of the plugs I found is this:

 

Estilian:
As stated earlier, if your car is that sensitive to plug gap or fuel, then there must be something wrong in the control. That said, I am as confused as you about what is now considered optimum plug selection. I have noticed no adverse operation from the original part numbers after 100,000 miles.

 

Right......stop pissing about!!

The correct and only plugs you should be using for this AJ26 engine is:

NGK BKRV5EVXA
Part No.Plu
Correct gap is 1.3mm or 52 thou.

Iridium eqivelanrt is:

NGK BKR5EIX-11....the gap is preset.

If it is an AJ27 engine....from August '98 to end of '02 , the correct plug is:

NGK PFR5G-13E.....gap as before 1.3mm or 52 thou.

Iridium equivelent is:

BKR5EIX-1

As has already been stated....it's not the engines sensitivity to plugs but you obviosly have air inlet system leaks which are being assessed as 'Rogue air'....in otherwords you have leakage upstream of the MAF which is being interpreded by the ECM as a lean mixture.
Check all your inlet trunking system.
You may need to replace the rubber seals for the MAF and TB.
Check that your anti-ressonance tube attached to main air trunking is not split.
Same goesd for part load breather and main load breather.
Part No.Plug Part No.Plug

 

Just a correction with regards to plugs and gaps, attached Jaguars specs:

 

There is no single spec as there were multiple specs over time and market.

There is also the fact that Estillian runs his particular vehicle at least part of the time on LPG.

 

Well....more fool him!

 

Yes....and all that is giving is the Jaguar part nos.
What i have given is the actual NGK part nos which are specific to these engines and are effective world wide other than markets such as Mexico

 

To add NGK part numbers for Supercharged engines, Jaguar recently updated the AJ27 S/C plug spec to - IFR5N 10 with a 1mm gap.

@xjay8 - I've already discussed an unmetered air leak/gaskets with Estillian.

 

The AJ84575 are actually IFR5N10, these have already been used from the 4.2 engines as far as I know.

 

so i know you guys are covering Bosch vs NGK and gap size, but my question is this, what is the difference between single, double, and 4 electrode plugs

also when searching for plugs i see some that say do not gap, gap is not adjustable

 

Hi,

Do you have any experience with BERU sparkplugs?

Thanks