Planning for some minor tune-up work, and wanted to verify the correct spark plugs for the 1994 XJ-12.

What is the recommended brand & model of plug, and gap?

I'll be replacing the wire set and distributor cap as well. Anything else I should plan to replace as maintenance while I'm in that area?

Thanks,

 

Go to rockauto.com and see all of the different spark plugs listed there for the car.
They all show a 0.035 gap.
BTW, replacing the lower ignition coil or perhaps both of them unless you have maintenance records that says this has been done recently is a good idea on these cars. The lower ignition coil sits down in the V of the V12 and gets heat soaked.
When just one coil fails the fuel injection system keeps throwing fuel into those cylinders (each coil runs runs 6 cylinders) and then it's VERY possible to get a fuel fire inside the catalytic converter on that side when this happens.
Also it is possible to burn through an ignition rotor and have the same effect of one bank of the V12 going dead while driving down the highway with the same result. A fire in the catalytic converter on that one side.

 

Thanks, any recommendation on a specific brand/model of plug that is known to work well?

 

Hi Al,

If my research is correct, the original plugs in your V12 may have been Champion RC9YCC or RC12YCC, but at some point the Jaguar part number superseded to EBC4021, which are NGK BR7EF/3346.

In 1995 Jaguar revised the specification for the V12s and changed the spark plug part number to NAB3850DA, which are NGK BR6EF/3177 (one step hotter than the BR7EF).

I'm attaching the TSB for your reference.

Cheers,

Don

 

Rockauto has the NGK 3177 (BR6EF) plugs in packs of 10 for $1.43 each and individually for $1.46 so a set of 12 plugs would be $17.22 plus shipping.

Personally I would want to go with a longer life plug but these are the factory spec plug according to Don.

 

Thanks everyone, perfect information!

 

I thought conventional wisdom on here, and maybe the cd-rom, was to gap the plugs at 0.025 rather than 0.035 for both the HE 5.3L and the 6.0L V12 engines. You may want to search the XJS forum for additional comments on that. I've always gapped to 0.025. I use BR6EF plugs.

 

The reason for this "mod" was to reduce the chance of the spark burning through the rotor.
I.E. provide less of a gap for the spark to jump across and thus reduce the spark energy to fire the plug.
If you do the research on the V12 engine list on Jag-lovers.org there are also various other mods suggested for the Marelli equipped V12 cars distributor in an attempt to prevent this same problem from occurring.
This rotor burning problem was only an issue on these late V12 cars with the Marelli ignition system.
This issue is also covered quite extensively in the Kirby Palm XJS help book that is available from various sites on the inter-webs.

Here is a web page with some of the text from Kirby Palm's XJS Help book that speaks to the various issues with the Marelli ignition used in the late V12 cars:

Marelli Ignition issues

Here is a post from here on the Jaguar Forums that talks about some of the same issues:

Marelli ignition precautionary measures