V12 XJ6 Conversion Build

Okay, thanks to a few people on this forum for the very valuable info I have officially acquired an engine and transmission for my Series 1 XJ6 to v12 conversion. Here are some pictures, I managed to find one in the Junkyard, I paid $700 in total for the Engine, Transmission, FI computer, and the Marelli Spark computer. I’ll be updating the thread occasionally when I have something of note to share or have some questions.

Replace that yellow fan. And you might have picked up the front suspension springs and anti-sway bar while you were there.

Best to take the full front suspension and get the 4-pot calipers and better steering!


What would I replace the yellow fan with? as for the springs I am not sure i can take them as they dont allow you to take in bottle jacks into the yard. i’ll have to take a look at the sway bar and steering though. I dont even have the series 1 XJ6 yet and these XJS popup in this yard everynow and then so i’ll pick the parts up at some point.

If you take the full front suspension you don’t have to mess about with removing the springs. They stay in the unit. You just undo the top of the shocks from the wing. With the caveat that I’ve not been under there for a while, mind you.

Search the archives for “V12 black fan” or “EBC4553”. Many of us have replaced their old cracked yellow fans (they were originally white) to avoid engine bay damage when they come apart and throw their blades. Yes, it does happen.


Ok good to know i’ll add that to the list of things to get. thanks for the heads up.

Once the engine is out, disconnect steering column, disconnect shocks at top as described, and disconnect four subframe mounts and the entire front subframe and suspension should fall out. If the wheels are still on it, you can probably roll it outta there!

Thanks1 It appears to be exactly the same as the XJ6. That is exactly how I dropped it for refurbishment on my 83 XJ. I would go back for it but I don’t have much room in my garage so it’ll have to wait.

Add the sway bar mounts; 4 bolts and nuts. These are handed, so make note of their position.


You want the F.I. harness as well…which is no 5 minute task to remove from the car. Or, at very least, the oblong connector to the ECU/computer. It’s similar in appearance to the connector for the Marelli ECU as seen in your pics

I converted a Series III XJ6 into an XJ12. Fun project!


I got both of the connectors, you just cant see the F.I. ECU plug in the picture. I made sure to leave about 8" of wire out of the plug to connect to. I am going to make a new simplified harness i kept the whole engine harness just cut it off at the firewall. I plan to delete all the smog stuff, cruise control and just have an ignition, fuel injector, and sensor harness to simplify the “rats nest” in the V.

Ok, I have a question, How does the V12 ECU know how much air the engine is consuming? I looked at the wiring diagrams for the Lucas FI ECU and the Marelli ECU and I can’t seem to see an AFM or MAP sensor the closest thing is an air temp sensor in passengers side air box. does it just guess how much fuel to give based on the throttle potentiometer and temp of the inlet air?

It’s a pressure sensing system, not an air flow based system. Incoming air is not measured. The ECU doesn’t know or care about incoming air nor where it comes from. Primary mixture control is via the manifold pressure sensor which is incorporated in the ECU. In essence, it fuels according to engine load. There’s a long vacuum pipe running from the intake manifold balance tube to the ECU in the trunk.


Oh okay, so it does have a MAP sensor its just inside the ECU. I am used to more modern systems that have the map sensor in the intake.

Ok so I have some good news and some probably bad/confusing news. The good news is that the engine has great oil pressure, just cranking with the starter it has a healthy 75 psi. the bad/confusing news is the compression tests. Ok so I did a leak down test first and here were the results

1a: 18% leak 1b: 10% leak
2a: 35% leak from intake valve 2b: 38% ish leak intake valve
3a: 10% leak 3b: 10% leak
4a: 20% leak 4b: 5% leak
**5a:**10% leak 5b: 5% leak
6a:15% leak 6b: 5% leak

besides 2a and 2b it looked all good. I figured I would do a compression test for fun just to see, the spec is between 200 and 240 psi. here are the confusing results that I was hoping someone could shed some light on.

1a: 150 1b: 70 —> cleaned spark plug threads and seat = 140
2a: 110 ----> 150 with oil in cylinder 2b: 70----> cleaned spark plug threads and seat =120
3a: 120 3b: 210
4a: 150 4b: 100
5a: 120 5b: 210
6a: 150 6b: 100

I am wondering if the lack of compression is dirty spark plug threads, when I cleaned 1b and 2b I saw a noticeable jump in compression. also maybe I am turning the engine too slow? seems unlikely as 3b and 5b had acceptable compression. Any help or theories would be very helpful!

I tried it again and got better readings on some cylinders. for example 1b without oil got up to 190! i guess the piston rings are stuck ish. the engine looks like it had sat for a few years at least. So i have added some penetrant to each and i’m going to let it soak for a few days.

Ok so I have let it sit and the readings have not improved on a few cylinders, I was already planning on refreshing this engine at the very least even if it passed all the tests so no big deal. Ok I know this topic has been beaten to death but as you know I got the engine with the Marelli system, I am aware of the Silicon sealant and distributor shaft trimming and have no issue doing that. Thankfully the engine has
the original genuine Marelli cap and rotor and not the terrible aftermarket ones that some have come across. What I was wondering is, should I go ahead and just convert the system back to the Lucas system and forget about it? Or have I scared myself reading all of these horror stories of fires that seem common? I plan to one day eliminate the distributor entirely and go with a MegaSquirt and coil system so either system would be temporary. One or 2 years at most. I am meticulous with maintenance so I hopefully won’t every run into the Marelli failure if I do stick with it. thanks for any input !

One more thing! I forgot to mention I probably won’t be running catalytic converters on this car (pre 1975) so I assume that reduces my chances of a fire too. it seems that the catalytic converter is what ends up getting got and starting the fire.

I wouldn’t go thru the trouble of converting, personally.

What makes the difference is awareness. You are aware of the potential problems and will take the steps to prevent them…which really is just a matter of upkeep and regular inspections. Properly gapped plugs, good wires, silicone mod for the rotor…you’ll be good to go, IMO. Pop the dist cap once a year for a look-see.

Others will offer differing opinions, no doubt :slight_smile:


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