I’m so excited, tomorrow I will dig in and start working on my new 88 XJS for the first time. Gonna replace all spark plugs. Feel like I’m about to get to know a long term good friend, the V12. 
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Make sure to gap the plugs properly. And apply anti-seize compound to the threads.
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Will do. .023, .024, or .025? I’ve seen all three recommend
.024. All three are close enough. Just don’t leave them at the.032 they come out of the box with.
Greg,
I have removed and replaced the spark plugs a few times now on my two V12 Jaguars taking time to clean up the “V” and repair or replace broken items (like fuel injector connectors) along the way and rewrap wiring harnesses that have disintegrated over time.
I recommend that as long as you have all the plugs out that do a compression check to baseline your engine for future reference. If you are like me you won’t be able to get all the plugs out before the engine cools so the absolute numbers may not mean much but the relative comparison between the cylinders should be helpful to capture for future reference.
The last time I removed and replaced the plugs on my wife’s 1990 XJ-S convertible I also removed and replaced the leaky cam shaft covers and adjusted the valve clearances. I am glad that I adjusted the valve clearances since most of them were outside specs. The car had about 124K miles on it at the time.
BTW, what plugs are you using? Be sure to gap them to spec before installing them.
Paul
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Yeah, I was planning on a compression test. But I’ll have to do it cold. Until I can redo the entire fuel system and test out the cooling system, I don’t trust the overall condition of the engine yet, so don’t want to risk letting it run long enough to warm up. The car has been sitting a good 5+ years.
Regarding plugs, I went with the recommendations from Kirby and the forum, NGK 2238 TR5.
The first time I changed the plugs on my 94 V12, it really took me over 8 hours.
It was a long time ago but I think I probably first gave the “V” a stout cleaning.
My HT Leads, cap & rotor probably got replaced too and I really took my time.
Even though the cap is marked with the numbers I had to research A vs B side.
I didn’t even know where number one was until then.
That was the first time I had ever seen a V12 and I didn’t have a safety net.
After I got the HT leads off and started removing the plugs I hit a few glitches.
Some of the plugs were easy to reach & remove with standard ratchet & extension.
Others needed the full force of my tool boxes.
As I recall, the front left was the hardest to get at.
The AC Compressor was in the way & couldn’t be moved due to it being bolted to the block.
It also required a box wrench as opposed to the plug wrench to break free.
Also, there was no room to swing a wrench so it was painfully slow.
This was before I had all the sizes of GearWrench in SAE and metric.
Whoever installed them didn’t use anti-seize so added leverage was needed.
Because of that, one plug broke the plugs metal part at the surface of the head.
I was really lucky none of the ceramic dropped down the cylinder.
Getting around where the throttle pedestal was needed some tricky wrenching.
Over eight hours after starting I was finished, both with the job and my body.
Even the easiest plugs required hard stretching and uncomfortable poses.
All same bodily contortions and combinations of tools were needed for the install.
I didn’t know what to expect, and probably took a day to plan the job.
One piece of advice, use a rubber hose to start the plug into the threads.
Now I have a rubber tool designed for that and it makes everything easier.
It’s very easy to cross-thread the plugs and the head is soft aluminum.
Keep a thread chaser handy.
Even later on, it still takes me at least four hours.
If you rush this job, you WILL xxxx it up.
I found that removing the cruise control actuator and throttle pedestal and unbolting the A/C compressor and moving it forward (but not undoing any of the A/C hoses) made for better access to thr spark plugs. It is true that doing all that is additional work and adds time and complexity, but it made for better access to the plugs and clears things out to clean up the dirt and grease that accumulates in the V.
Paul
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Paul,
You are totally correct, I forgot about the cruse control and the throttle linkage.
I think when I was doing the job, I was too intimidated to move the AC compressor.
Remember I was a total greenhorn, and it meant loosening a belt.
I removed the cruise control and AC compressor and moved it forward, and removed the throttle pedestal and coil, all in 1 hour 15 mins. I could prob do it in 1 hour now that I know what tools work best.
Unbolting the AC is very easy, although I’m sure putting it back will be harder. Then I easily removed cap, wires and plugs in like 20 mins. So much easier with that stuff out of the way.
Mine were also tight, how I wish everybody used antisieze. Smashed a pinky two times. I find threading the plugs back with extension and fingers very easy, don’t need a hose. I then did a cold compression test, all were between 180 and 200. Not bad, since the engine runs really rough, and has been sitting for years. Plugs looked terrible, black and worn with a .035 gap. Could be originals? (Car only has 48,000 miles). Now that I see how much work it is to get to the V, I’m gonna take my time in there cleaning and protecting wires and replacing vaccum hoses before I put everything back together. One thing though, working in the V is a real back killer! But I’m intrigued working on this V12, what a unique beast.
The back does suffer. I have seen model railroad stands, sort of “U” shaped on its side. it has wheels on the lower leg that go under the baseboard of the model setup. That allows you to stretch out on the top leg and reach much further.
If you had a similar thing where you could lie flat out and work in the V it would make life easier.
A few more years from now I had better make one if needing V work.
Greg, before you put everything back together, consider modifying the throttle pedestal like the pic shows. Installation will be easier, and more airflow through the “Vee”.
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Here’s something else to add to the List of Important Things To Do. I discovered this when I replaced the plugs on my car.
Whenever you R/R the throttle pedestal and turntable, as you put everything back together and tighten all the fasteners there is a fair possibility that the turntable may not end up in the same exact position as it was before you started.This will change the position of the two throttle rods and that means that the two throttle butterflies will not start opening at the same time!
Slight adjustment may be needed! (Sorry to add to the list…)
I’m not a fan of actually removing the throttle pedestal.
Doing so is a sure way to un-adjust the throttle position sensor.
Unless you are prepared to do a TPS adjustment, leave it alone.
I didn’t, and even though it was more difficult, I was actually confident of the result.
Of course, removing the linkage is important and doesn’t undo the adjustment.
The links on each end of the rods are snap-in ball joints.
I didn’t disconnect both ends which was a mistake.
They kept getting knocked back in the way.
I was almost completely in the dark about what was or was not required.
That’s why I was more conservative in what I removed.
As it turned out my linkage needed to be adjusted anyway.
It was easier to do than expected.