Hi guys,
Figured I'd share this, I bet there's one or two other folks out there who too have lost track of mileage since last spark plug change or, like me, fell for the Iridium plugs last for xxx miles line of BS!

I'd have to look to be exact, but I last changed my plugs about 60,000 miles ago. Recently I've been battling flat spots on acceleration, takes longer (more cranking) to start, rough idle, and let me describe the last symptom:

You know how if you leave a tire partially inflated, parked for months, there will be a flat spot on the tire? It'll take a few miles for that feeling to go away.

Well at times, especially when cold, it would feel like there were a dozen of them on a tire. Of course I'm describing the feeling while driving,, it has nothing to do with a tire. I didn't know if it was a mis fire or what, no codes.

Cleaning the MAF and plugging air leaks in the intake greatly reduced this feeling but it still occurred at times. Decided to change the plugs, even though these Iridium were supposed to last 90,000+.

I bought new plugs, gapped to 0.052 inches, and changed them yesterday. The old plugs were those Iridium plugs. Wow was I surprised, the electrode on each one was worn to a nub! The gap was now almost twice what it should be! And this was 60k miles?

Well what a nice difference the new plugs make. What a nice surprise.

Oftentimes its more than a year or two between doing the same job again. I admit I sometimes look back at my posts like this so I don't spend time reinventing the wheel, so this note is for me as much as anyone else who can use it:

The 6 bolts that secure the cover over the coils and plugs are 9/32 inch which probably means 7mm since everything is metric on these cats (9/32 worked fine ).

Same bolt, the side with air cleaner, center bottom bolt. No space for ratchet wrench. Just about enough room for the length of the 1/4 inch 9/32 ratchet head! I wound up locking the ratchet in a pair of vice grips, at slight angle in the jaws. Fits fine, worked like a charm (after about 30 minutes of searching for a tool to fit that space!)

John

 

What a coincidence. I just ordered a set of spark plugs from rockauto since my car is a few hundred miles shy of 80,000 miles. I figured it was time to retire the 17-year-old plugs. I bought NGK IFR-5N10 Iridium plugs. I plan on changing them this weekend. I wonder if I will notice any difference in how the car drives?

Also, shouldn't the gap be 0.040 inches?

 

They issued a tsb that changed it to.052

Let me see if I have it handy.

Oh boy yes it really improves the driving, get ready for some smiles!

John

 

In case photo is unclear:
https://www.google.com/url?q=http://...pFTF6YaDgAp4tq

I was happily surprised at the driving improvement! Don't forget to let us know what you find after you do it!.

John

 

Hey Glendaniel,
One or two heads up if I may.
1. Be careful not to over tighten the bolts for the spark plug ignition coil cover or for the ignition coils. The bolts screw into brass threaded inserts that are just pressed in.

I was standing there unbolting one of the cover bolts for a while before it hit me - this might be stripped! The bolts not moving (oh great!). Then I realized, its not stripped, the treaded insert was just spinning. The good new was i could just pull it out. I put a dab of glue on the exterior of the insert and just pressed it back in.

Thought about it and realized that I was probably the culprit by tightening the bolt too tight last change. The torque to get the bolt started for removal broke the insert free! Same thing happened with 1 or 2 of the ignition coil bolts too.

Not a big deal, but it'll at least save 5 minutes of your time spinning a bolt around if it happens to you this weekend:-).

2. I guarantee the locking clip for the ignition coils' plug will snap off when you breathe on it! Just don't let the debris find its way into the cylinder. But you don't need to go nuts securing the plug in place. Mine snapped off 60k miles ago, still firmly in place yesterday. Don't laugh butvwhen it happened 60,000 miles ago, I went nuts trying to secure a few of them with daisy chains of wire ties. Upon reinspection the other day the wire ties were a waste of time:-). Those plugs didn't move a nanometgrips.

Hope this saves you a minute or two mate . The hardest part is getting to the obstructed cover bolts. You'll understand what I meant about the vice grip.

John

 

Its a bit awkward to have them worn out so much as you say at just 60km, at least I have not seen something like that before, maybe something else is here at ;play as well causing this?

What iridium plugs had you used, and can you share a picture of the electrodes?

 

I know it says all V8 engines, but the date is from 1998, does this apply to the 4.2 engines as well?

John, I actually had the coil plug screw brass insert issue with one of the coils when I was doing the valve cover gaskets a couple months ago. I had no problem removing the coil wiring harness plugs. In fact, those weren't even brittle on my car 500 miles ago and I found a way to remove them safely. You gently pry the tab upward with one hand using a small flat head screwdriver while with the other hand you tug on the plug. Once it starts to shimmy loose you can release the tab and focus on pulling the connector. It's not the quickest method, but being careful with it means you don't break it. For the stubborn ones, I used a plastic trim remover tool that I placed at the edge of the connector and used it to push on the connector to help me have some purchase when pulling on the connector. I think I recall something functioning as a fulcrum for the plastic trim remover tool.

 

2006 XK8 with 121,900 miles....

Changed the factory plugs at 100,000 miles just because that action was listed in the service manual. I ordered the recommended NGK IR IFR5N10 iridium plugs from rockauto.com. The factory plugs still looked nearly perfect and performed flawlessly. So do the new plugs. I noticed absolutely no difference in performance. My long-term experience on other vehicles (primarily Lexus) is that Denso and NGK iridium plugs are typically good for 150,000 to 200,000 miles if the engine is properly cared for. In retrospect, I replaced the XK8 factory plugs far too early....

 

Avos, here's a photo. This one was the worst for wear, but this is why I posted

 

Spark plug types and gap requirements have changed with the year and engine type. (See attached)
Also the Iridium plugs have a much smaller gap requirement.
What Jon89 has reported is the usual outcome when checking correctly fitted Iridium plugs, they will easily last over 100,000 miles.

 

Folks, for the record I am not debating the efficacy or lifespan of iridium plugs. This was just a report on here's what happened here. Use it if you choose it :-).

John

 

Thanks, certainly doesn't look good, and would advise to find the true cause here.

Can you share as well the brand and type (ie partno)?

 

Maybe the ones in the car were NOT the correct ones.

I've the same 4.2 SC engine, with the iridium plugs, and like Jon89 mine appeared near enough the same at 100K miles as the new ones (but I changed them regardless).

 

Wow, perfect timing. I'm taking my CAT for Plugs and fuel filter change. Thank you for your support. I need to ensure my mechanic knows about not to over tighten. Ok, for my 2005 XK8 VIN X52A43421 what would be the recommended Spark Plug and its gap? The provide info shows 2003 and onward....would this be for mine? thank you, please keep the good work for us to enjoy out CATs

 

The 2003 model YR onwards information is correct for your car and all 4.2 XKs up to 2010 MY, including the XK150 AJ40

 

Due to the long maintenance intervals for the plugs they tend to bond to the head. If you do pull them put some anti-seize compound on the threads when you install the new ones. It can save you some headaches later on.

 

When I bought them, NGK stated the plugs are pre-coated with anti-seize and NGK said not to add any.

 

I had no trouble at all when I removed the factory plugs at 100,000 miles. When working with spark plugs, always make certain that your socket is completely and squarely on the plug before you turn it in either direction. You do not want to have to deal with cross-threaded plugs. Good tools placed properly in position eliminate that possibility....

 

Wow! That's the first I've seen that! Are the NGKs platinum? I'm a bosch platinum fan.

 

No, they're iridium and last far longer.