As some of you may know from a previous thread, I took ownership of a 1980 that turned out needing an ECU. Well I also discovered cylinder 2A to have 0 compression. All the others were near perfect. I narrowed it down to being a valve issue by basically doing a leakdown test and all my air continued to free flow right out of the A bank exhaust no matter what stroke that piston was on. I decided to tear the head off and found that the valve seat had dropped, but didn't come over the valve, but instead just hung with a side of it sticking out. It not only gouged the piston pretty good, the motion of the valve still going in and out caused significant unrepairable wear around the hole the valve sits in.

Anyway, before I scrap this car completely, I've found a 1990 xjs with a good running low mileage v12 in it thats being parted out. If I got the engine, the harness and ECU, would it be a straightforward swap?

 

The engine itself would be a pretty straightforward swap. The problem is the rest of it. The 90 is a Marelli car. Has a whole different ECU for fuel, and a seperate ECU for ignition.

 

Do you have a donor car or just the drive train?

 

Ok so as long as I got both ECU's and harness I'd be good to go?

I don't have either yet, but I found a complete donor car of which all the parts are available. Just depends how much I can get the whole car or the parts I need for. I wanted to make sure it was pretty straight forward before dropping any money on it.

 

I'm only a couple of months into ownership into a '88 XJ12 so i can't comment on the swap.

Just how bad is that piston gouge, its hard to guage from the pics. If not structurally compromised, unless i was looking for a major project, I'd try to live with a new head on the A bank and a scarred piston. I saw some heads on ebay the other day, that just seems to be an easier opton. Put a new head on and then check for variance in compression on this cyl. to determine if its viable.

I'm just ramping up on the V12, I'm sure more knowledgeable sources will be able to provide more specific info. I do appreciate the pics, and I now know what the dreaded "dropped valve seat" looks like. I can see some gouge marks from a pry bar on the heads. I've heard all about how hard it is to get the heads off on the V12, any insights into your experience would be appreciated.

Thanks

 

The Piston I think could be ran as it is. It's not structurally compromised. The trouble is finding a head for a pre HE car. The heads that are on eBay are all from the later year cars where the head design changes so they could run higher compression. I don't think they'd work well on this car. That's the reason I'm considering doing a whole engine swap.

As for actually pulling the head, it really wasn't too bad of a job on this particular car. The real pain came from the very top head nuts that are literally placed right under the intake ports, thus requiring me to remove almost everything from the valley of the engine to get at. And they are placed such that they cannot be taken off with a socket. Only a wrench can get them. The head was indeed stuck pretty good and looking back, I should've covered the end of my pry bar with tape or a rag or something to avoid marring up the head surface. But since the head isn't usable anyway I guess it's ok this time around. I did have to remove that side engine mount and Jack the engine up, which still didn't provide enough clearance for the rear exhaust manifold. I ended up using a pry bar under that manifold to twist the engine further to get the clearance I needed. Also one of the AC lines runs directly over the head where the cam sprocket is and it had to be bent up and out of the way. But overall, not as bad as some of the other horror stories I've heard that it could've been.

 

It would mechanically, but if you do this, it would be best to take the whole donor car if possible, as the engine electrics, ignition system, timing and ignition curve etc etc and ECU are completely different. The ECU loom may also be different (runs from the boot to the engine bay in one long uninterrupted wrapped cable).

The gearbox and final drive ratio may also be different as may be the speedo drive system, early XJSs were mechanical speedos. I would swap out the entire drivetrain, refurb the rear axle while it is all out, and carefully install all the engine loom and other required looms (ECU, speedo) before reinstalling the engine gearbox as a unit.

On the other hand, second hand "flathead" cylinder heads are frequently for sale on UK Ebay and from UK breakers, and it would be a great deal simpler and cheaper to get one sent over to you.

Greg

 

I agree with Greg. I spent a month trying to figure out my Marelli car when I bought it, as a non running mess. I have a pretty good grasp on it now, but with the two coils, the amps out on top of the radiator shield, the ECUs, I simply can't imagine this swap. Believe it or not, I had a aluminum head on a big block Corvette repaired once that was far worse than that. I doubt you will need to change any of the drivetrain however, as you could just keep the mechanical speedo.

 

Well, I heard back from the people with the donor car and they are wanting way too much money for just the engine and harness and ECU's. Let alone the whole car. I can't locate a head that isn't pre HE. Looks like my v12 Jaguar Dream has come to an end.

 

Have you called around about getting that head welded? I've seen some wild things done to badly dammaged aluminum heads

 

I bet you would be surprised. A good head repair shop could fix that baby. If it was like my E Type, a new set of exhaust valves, that one seat repaired, might not be near as expensive as you might think, especially if you can get hooked up with the right shop.

 

I agree. See if you can get it fixed. Nothing to lose but a bit of time and gas.

 

Well, on the recommendation of you all, I'm going to take my head around and see if it can be saved or not. Even if that doesn't pan out, the car can still be saved thanks to superchargedtr6 who has offered me a set of pre-he heads he has!! Such a great community and I'm glad to be a part of it!

 

I would also break down and put a piston in it. It has been hit pretty hard around the edge and may have seized the upper ring. A lot easier to replace now while the head is off.