I just swapped out my spark plugs in my '96 XJS. (I
replaced them with the same Champion RC12YC that were in
there) I ended up finding two cylinders with a substantial
amount of a black powdery substance. This is before I
removed the existing spark plugs, so it would share the same
space as the coil boots.

I'm assuming its from the spark detonations in the engine.
Should I be concerned that some of this dry material is
finding its way out of the spark chamber and into the coil
boot cylinder?

Car seems to drive well, with 19MPG typical. I'm hoping
that by switching out the plugs, I may have solved whatever
was causing this problem.

Thanks.
--
1996 XJS 2+2 Convertible with 4.0L AJ16

 

The rocker cover they put on the AJ16 is prone to corrosion. The result is a fine black/dark grey powder. It is especially common in the coil wells/cylinders.

Is the epoxy coating on the rocker cover blistering or broken anywhere?

 

not only is that corrosion, but its caused by the coils misfiring to ground through the boots to the cam cover thats causes the electrical oxidation of the base metal. Best thing to do would be to remove the cam cover. bead blast to clean and either paint or powder coat it and reinstall. I would also replace the coil boots

 

Holy crap...I don't think I can do that? Where can I go to get something like that done?

It sounds like Im supposed to bead blast and repaint the interior of the cylinders where the coils are...

There is paint flaking off these, and other, cylinders.

 

no you just remove the cam cover, and take that to a powder coater. Call and ask ifthey can bead/soda/walnut shell blast to clean and prep. then powder coat. If you can change plugs you can unbolt the cam cover, that engine is super easy

 

I wouldn't be so sure.

Unless the boots are cracked, I find it highly unlikely that there would be any arcing to the cover.

When i first noticed the corrosion in my cars, I did suspect arcing. HOWEVER, I found the exact same powdery corrosion under the blistering coating on the outer edges of the rocker cover, and obviously there couldn't have been any arcing there.

I'm sure your problem isn't as sinister as Brutal might have you believe. You said the car was running ok, and didn't mention any misfires.


First things first: Were there any cracks in the rubber coil boots, or the blastic casing on the coils themselves?

As for re-coating the rocker cover: Yes, it's probably a good idea to have it done to prevent further corrosion, but consider the following:

- Are the coil wells (cyliners as you call them) still sound? There is a chance that the walls may have corroded right through, as I observed on my fathers AJ16. You might have to have the ends of the coil wells rebuilt, or think about replacing the whole rocker cover.

- The corrosion is usually in deep veins. If you have your rocker cover re-coated, make sure they do a very THOROUGH sandblasting job, otherwise it'll just continue to corrode under the new coating.

- I'd think twice about powder coating. I was looking into it myself, but I saw reports of rocker covers warping due to the high temperatures involved. An epoxy paint would be a safer option.

 

There were absolutely no cracks in any of the coil boot casings. I was expecting to see some cracks and was surprised to find none.

There was some paint flaking in pretty much each of the six wells. Some were worse than others, but it all looked like it was heading that way.

I have a welding shop right by me, which seems unusual. I will ask if they do this type of work. I'm sure they'll know who does if they do not.

 

Agreed. A high-tempurature one at that.

The engine cam cover on this car is prone to corrosion because it is made of aluminum.

 

you dont have to have cracks to have misfires through a boot, plus you dont even have to have a direct arc, but those wells have no air flow through tem and have alot of electrically ionised air. grab a coil some time or plug wires that area good with a engine running and see if you dont get shocked. I have many times over the years. And the covers are more a cheap pot metal than a good quality aluminum

 

The fact remains that the same corrosion is commonly found on other parts of the rocker cover, leaving the same powdery residue. Parts that cannot be subject to arcing.

 

I think the concensus here is that I need to remove the cam
cover and have it bead blasted and repainted with an epoxy
paint to prevent this from happening again.

Would a generous smearing of dielectric grease in the cylinder
wells work as a more cost-effective solution?

Would it be a good idea to coat the underside of the plastic
cap with the grease?

 

I'm not sure... My gut tells me the grease would probably end up pooling around the plugs, what with the heat and vibrations etc.

What plastic cap?

 

Eh, you're probably right. The heat would melt the grease down and set it to gather all around the spark plug.

I was referring to the plastic lid that covers all of the coils, but as I type this I realize that's of no concern.

 

I agree with Brutal, I would change the boots. Seems like we go through a lot of these. May even save you a set of coils later. $100 now may save you even more later. Last I checked the cam covers were on inter-galactic backorder through Jaguar. If you go used, do be aware of warped covers. There is a lot of truth to that as well.

http://www.motorcarsltd.com/LHE1510AB.KIT.html

Powder coating vs. epoxy paint: six of one, half dozen of the other. (Powder coating looks cool!) I think the paint would do the same job just as well and probably less expensive. I would think if the walls were too thin, the shop would tell you before blasting. That is their business.

 

I really cannnot see any gain in replacing the boots unless they're damaged, and in my experience the coils will crack up well before the boots. That rubber seems particularly resilient.

Another hidden cost if you end up removing the rocker cover:

Unless you have already replaced them very recently, you'll have to buy new seals for it. This includes the large rubber gasket that sits around the rim of the cover, the six small round gaskets that seal the spark plug wells, and the (I think it's sixteen) tiny gaskets that seal off the bolts that hold the cover down.

The rubber in those is likely to have gone hard and brittle, and it will not re seal if the cover is removed.

 

Yes, I've been checking out whats needed is in terms of gaskets. I'm sure I need six rings for each of the plug holes, as well as the large gasket for the edge of the cover, but I don't believe I need the sixteen little gaskets for the screws. I think that was for the AJ6? Its been difficult to find a solid list, I've just tried to piece together what I can from the different sites that sell these gaskets. None of them are definitive.

Anyways, winter is a good time to get this done. The unavoidable snow will force the XJS off the road. I work in the construction industry, so I know someone who can do the bead blasting for me. I may even be able to have him powder coat it for me as well. I'll let you know what he's going to charge me if anyone's interested.

 

Thanks for this! I plan on using these next time I do an oil/plug change.

 

No, AJ16. And sorry, it was 13, not 16.

You need the following for a rocker cover gasket renew:

1x http://www.britishparts.co.uk/.sc/ms...COVER%20GASKET (main gasket)

6x http://www.britishparts.co.uk/.sc/ms...%20PLUG%20SEAL (spark plug tube seals)

13x http://www.britishparts.co.uk/.sc/ms...20COVER%20SEAL (bolt seals)

And possibly 6x http://www.britishparts.co.uk/.sc/ms...0COIL%20GASKET (coil gaskets)

Although it would be pretty easy to make the last gaskets if you have gasket paper.

You'll probably get a better deal at motocars ltd etc. - I just linked to that site because I already knew where everything was.

 

Wow, that helps a lot! The gaskets for the coils are actually a hard, rigid material. Easily reusable if not replacing a coil.

As for the rest of that stuff, it sounds like you got it all. I like Britishparts.uk, I've bought from them before. I'm always amazed how fast they can get me my parts...all the way from the UK!!!

 

I've attached a photo of one of the spark plugs I've removed from the engine. I haven't previously seen burn marks on the spark plug body like this before.

Is this indicative of the conditions assumed in this thread? Is this normal?

I'd also like to add that there was no dielectric grease used to seal the coil and the plug.

Thanks.