It's taken me about 5 years but I finally have what I consider a perfect idle. It always seems like she was rather disappointing at idle. After a lot of work over time she finally feels like she's not even running when I am at a stop.

First thing I changed was the coils. I changed the original coils with aftermarket coils. Those failed at the 6 month mark. I replaced those with more aftermarket coils which failed shortly after. By early 2012 I had a clear understanding that the coils have to be Made in Japan type. Got 6 new of those which now have been there 2 years with no problems.

But the coils was only part of it. For a while there I ran NGK iridiums and Boch plugs, which she did not care for at all. Then I ran Champions RC9YCC's which where better. Finally started running the plugs specified for the AJ16 engine in the last jaguar tech bulletin; the Champions RC12YC's, but gapped at .30 instead of .37. This made a huge difference.

I also changed all the injectors with new old stock parts . That also made a difference. I also cleaned the MAF and changed the CTS and EGR Valve.

But the cruddy cam cover kept on leaking oil into the new coils and plugs. And the used one I got to replace it did the same thing. Eventually I also replaced this with a new old stock cam cover that was harder to find than King Arthur's sword. The dry plug wells seem to have brought the car to the point it's at now. I really though that she would never get there.

Just posting this because I thought it was impossible to get to this point.

 

Congrads Mark on getting that all figured out. You have spent a lot of time working through this so happy to hear you got it dialed in. The AJ16 is an amazing engine and like you said when running correctly is as smooth and quiet as the V-12. I love sitting at a light and except for the tach. showing the engine is running I cannot feel or hear a thing-we'll except for my stereo blasting one of my favorite songs-LOL!!!!!

 

The specified gap has always been .035 for the plugs. Where did you get .037 from?

I've read many posts on multiple sites where it seems Champions should be used. Some people defend NGKs and even Bosch, but there are too many posts, like yours where people claimed to notice an improvement when returning to the cheap copper Champions. Apparently even an upgraded Champion plug (platinum, iridium, etc) gets shunned.

I'm glad you resolved your issue. I'm surprised to hear that you could not keep oil out of the spark plug wells! You know I recently had an issue where I lost a spark plug which I suspect was due to oil built up in the cylinder! Glad you got that resolved!

Can I ask why you went with new injectors over sending the injectors out for cleaning?

 

Hey Vee-

Look at TSB 18-56 on the TSB section--it calls for a .38 gap......with the RC12YC's....running .30 is working very well for me. IMO, the hotter plug with such a large gap will shorten the life if the coils considerably.

The reason I got new injectors is because I found them for a price I could not refuse. Otherwise I would have probably had mine cleaned.

 

Mark,

Congrats on restoring your Jag's purr

As is often the case with elusive issues, they may have multiple problems!!

As you changed/cleaned many components, it would be interesting if you could rank them in order of significance?

From your post it seems that the plugs made a big difference but the coils not as much? (apart from reliability) I realize it is somewhat subjective but may be helpful to others to understand the effect each change had.

I can understand why the smaller plug gap would reduce demands on the coil but was wondering if you noticed any reduction in power or fuel economy after the change?

 

I've always stuck to the original specified gap repeated in TSB 12-54, which is .035.

It's odd that they recommended to open up the gap a little bit in order to address the rough idle. Even stranger that reducing the gap is what worked for you!

Everything has been running fine for me, so I will not monkey around with the plugs.

I would also love to hear the incremental result of everything you did. I am in the process of replacing out the boots, not the COPs themselves in an effort to prolong their life. When my spark plug blew, the corresponding boot blew up. The COP ended up weathering the storm just fine.

The original boots seemed pliable until I compared them to the new replacement. I figured it was worth the $15 per boot to see if I could keep the current COPs functioning. The OP has already replaced two of them. (He was nice enough to etch the dates of replacement onto the head of the COP.)

 

Mark, so glad you have finally got the old girl to idle.
Worth all the trouble, I am sure.
RagJag

 

Thanks for your replies.

The three things that made the most dramatic difference where definitely :

1-Coils-the originals where misfiring, the aftermarket ones would fade in between the time they would go totally out, so it would cause an extremely sloppy idle. When I replaced all six as a set at once with new OEM ones it made a huge difference.

2-The new cam cover. The dry plug wells have made an almost equally dramatic difference as the coils did.

3- The right spark plugs. Going to copper champions was also very noticeable. The car would vibrate so much with the Ngk iridium plugs I could feel my seat trembling at stops. As soon as so e champions where in there, that stopped

Everything else made small noticeable improvements, but not to the degree that the three above did.

 

Man, I remember marveling at the V12's idle at traffic lights. Even after owning the car for a while, sometimes I had to check the tach to make sure the engine hadn't stopped running. Truly a thing of wonder at the time. These cars are special, 6 or 12.

 

They are indeed. I ruined my V12 E type's starter right after I got it a few years ago trying to start it while the car was already on, which I did not realize because of how smooth she is at idle. Now my XJS is just as smooth!!

BTW-interestingly enough my 04 X type V6 is no where near as smooth as my older cats.....but the 13 XF v8 I test drove was super sweet and super fast. I would have traded the X type in if it wasn't for the XFs (IMO) bad looks :-(