I bought a 90 XJS v12 with 29,000 original miles that had been sitting in a barn for 10 years. Body and interior are excellent but almost every harness under the hood is damaged. I've read Jaguars have several mechanical issues. Should I spend the money to replace the harnesses and hope everything else is good or should I do an LS swap? I would like to get this running without spe doing a ton

 

Harness issues are common. The heat from the V12 and wire technology from the time make for crunchy wires.

When you say "I would like to get this running without spending a ton"...what's a ton? You can expect to spend 7 to 10K for a LS swap as a DIY endeavor. Is that a ton?
LS swaps are great I'm starting a turbo one soon. My advise, get the V12 running. The pigtails for the injectors and other harness part can be sourced for a few hundred dollars. Once the car is running and road worthy you can sort the suspension, steering, brakes, fuel system and other minor system. All useful details if you eventually decide to do an LS swap. Some details like a rack and pinion replacement are a bit easier with the engine out. The V12 experience is something every car guy should have even if it's brief and fleeting. Get it running and driving and have some fun then see if you love it before you do an LS swap.

Keep in mind LS swap, even the basic ones can really overwhelm the chassis. Once you get above 350 HP, issues present that need addressing. Axle tramp is a biggie. Better wheels, performance tires, stiffer suspension all may be needed to get the most out of an LS swap.

 

Harnesses are not too expensive, as far as actual mechanical issues this is false, biggest failure points as you've seen is engine electrical wiring and british rubber thats 30+ yrs old. Mechanically I trust my XJS more than my Superduty diesel pickup.

 

Keep!

 

By far, the simplest, cheapest and quickest thing to do is keep the V12.

 

If you really want to put an LS into an XJ-S, PLEASE do it to a high-mileage one and sell this one to a Jag person. PLEASE.

 

An easy and inexpensive way to find out where you stand would be to just pull all of the spark plugs and do a compression check. It would cost you nothing but your time. That will tell you right away if the engine is ok internally or if it has major issues. I know your wiring is pretty chewed up (I saw the pics) but if you need a little coaching on how to get the starter alone working, call me. Happy to help.

Andrew
Jaguar Specialties

 

Are you doing this or are you having it done?
How much of a mechanic are you? It’s a 30 year old car so it will need work. Cars from that era just aren’t as reliable as newer cars.

If you want the 510 horsepower V8 I’ve seen Jaguars with them. Look at the XKR. they are newer faster, lighter weigh less corner better, and start at around $10,000 Recently a little cheaper with some sharp dealing.

Here’s the deal. The LS can make more power but the old 350 makes less power than the Jaguar V12 but neither make that heavy car that much faster. The Jaguar is a heavy, car, over 4600 pounds. It also has a very high speed rear end 150 mph is possible with either the Jag or the Chevy LS ( but not the 350)

Assuming you buy a new LS crate engine and have it professionally installed you will have a lot of money invested. You see it’s not just the engine. It’s also a transmission. The Jaguar transmission won’t fit the Chevy. New alternator, A/C, power steering, starter, Now you can buy used out of a junkyard to save some money. But that complete package is needed and it might not fit the barrow engine bay since a V12 is 60 degrees ( and a narrow V ) with the spark plugs on top but Chevy has spark plugs on the sides.

A Jag V12 is very narrow compared to a 90 degree V8 like a Chevy.
Open the Jaguar’s hood and see if you can get your hands down under hot exhausts to change the plugs? That’s with a narrow V12.
You will also have to buy new wiring and ECM plus new exhaust system.

What about the tachometer? You’ll need a different one for 8 cylinders than 12 , oh and will the speedometer work with a different transmission? Well the whole gauge package might be different. Same with A/C , etc. etc etc.

One final question who will work on the result? Chevy dealer? Jaguar dealer? Maybe you plan on doing it yourself but what about the next owner?

The vast majority of Jaguar V12’s I’ve looked at over many many decades are mechanically perfect but some suffer from ham fisted mechanics. Usually when they fail it’s due to a sensor or rubber vacuum hose OK sometimes it’s wiring but you’re gonna replace all that no matter what you do. The only problem is a Stock Jaguar won’t work on a Chevy and Chevy won’t work on a Jaguar. Sounds like you’re in for some custom work and that’s not going to be cheap especially if you want a proper wiring diagram so the next guy working on it doesn't spend a lot of your money trying to figure what goes where and does what.

A compression test will tell you what you have.
the Jaguar crankshaft is a massive forging of EN 40 steel and then hardened. big massive bearings. Way bigger than Chevy. Yeh, race car stuff. It weighs 78 pounds. Compared to the Chevy cast crank that weighs about 25 pounds ( they vary, as lite as 22 pounds as heavy as 27 pounds) but most are either castings or sintered Iron. With Chevy Only special Racing cranks are made of EN 40 steel and are hardened.

it’s your choice but please know the truth. Chevy car dealers have service departments too. That means they break down too. Not everybody is a Chevy fan There are a lot of Ford’s sold, as well as Dodges etc.

 

Some good info here but I wanted to add some comments to clarify.

1) Will everything work as before?? Yes, it all can if you use the right parts. For example, for anyone using our LS kit (or small block Chevy kits too) all of the Jag gauges read correctly with the new GM engine, the shifter works perfectly with the new GM trans, the AC will work better because we switch to modern (and larger) parallel flow condenser, and more. Every aspect of how you operate the car remains the same. And this is all done without touching the instrument cluster itself, not modifying the shifter, etc.,. I have always been a big stickler for that- all of the changes need to be under the hood- keep the rest of the car standard.... No aftermarket gauges, etc..,

2) Width- The GM V8's are actually narrower than a fully dressed V12 so access to do nearly anything is easy. Spark plugs and other maintenance are a snap. And there is a lot of room in front of the engine for water pump changes or accessory work. (I just changed the water pump on my 88 XJSC LS1 and it took 20 min)

3) Weight- the Chevy small block will shave 300+ lbs off the front end and going to an LS with a manual trans pulls over 400+ off. That changes the handling/feel of the car, but also the impression of power. Pulling 300lbs off a car like this is like adding 100hp by doing nothing else. And there is nothing wrong with getting the car to a 50/50 weight distribution (front/rear)- the car handles much better than before, as you would expect....

4) Power- Chevy small blocks can make anywhere from the 290hp base crate engine up to an easy reliable 450 and everything in between. LS is similar- the base LS truck engines (4.8/5.3) come in at the 290's, with the LS1's at 305-345 and LS2/LS3 at 400 to 430. That compares to the 5.3 V12's 262hp (300hp for the 6.0L V12, but we're not talking about that here) and of course do not forget the weight consideration from #3 above. That is not to be minimized in the pushing around of these heavy cars...

I wanted to share this info just to make everyone had the correct details, not to convince anyone to do anything in particular. To each his own.

Thanks

Andrew
Jaguar Specialties

 

Thank you for your input.
A few questions.
my workshop manual page 7 says that the Jaguar manual is a deduct 106 pounds. From given weight. You deduct 200 pounds?
Next: you say the Chevy weighs 400 pounds less?
I’ve shipped enough Jaguar V12’s to know they weigh 730 pounds dry plus crate. So you are saying a Chevy LS weighs only 300 pounds?

Finally if I show up with a XJS what does the complete swap with everything working properly cost? Out the door? Is there a warranty ?
Who does the warranty work?

 

Just to further clarify, an LS1 weighs 430 lbs. Also be aware that all of the LS accessories are aluminum, the radiator we use saves probably 30lbs over the stock V12 radiator, and there are a few other areas that probably save weight. And I bet the T56 assembly (trans/clutch/flywheel) probably saves 50+ lbs over the T400 with TQ and full of fluid. I;ve been able to verify the general some total of these differences on an actual vehicle scale.

The cost of any of these conversions depends greatly on who is doing the work, what engine and trans you are using (ie, good used package vs crate engine), and to what level the work is being done (ie, retain most of the XJS stock exhaust or build a new system, and things like that). Most shops that do this will also provide a warranty of their work (if they don't, run away). Note- we do not do any customer work here- only design, development and testing of the the kits and parts we sell. For more detailed info on any of this, please contact me offline.

Again, not trying to convince anyone to do anything, just want to be sure the correct info is out there....

Thanks,
Andrew
Jaguar Specialties

 

Hello Ashfro

Just to put my 2 cents here,
When you buy a older type car that has not been cared for - you will run into issues. If you have the thought of replacing a motor because of wiring issues and that will solve all your problems, that may be a bit optimistic.

For me - I am a simple person, and the main difference between a V12 and a early V8 is you have more room under the bonnet, which means you don't have as much fear factor when you lift the bonnet. To wire a early V8 is simple(ish)
You start putting a more fancy V8 motor in, they get more complicated and the fear factor grows again. And then you need to wire the fancy V8 to suit (which to me is not simple)

So - before you decide - pull the plugs and check your compression, see what condition your motor is in. It doesn't cost you anything and lets you think while getting your hands dirty on her

If you want to see what can be done with a V8 in a XJS, watch Kevin Tetz "|hands on cars". It’s a build video of converting v12 Xjs to ls engine. It’s on amazon prime as well as his Face Book page.

At the end of the day for me - if it needs a conversion to keep it operational - then I am all for it

Cheers
Steve

 

Good advice-

Yes, the Kevin Tetz build is a good watch. He used all of our parts, BTW, and there even is a short section there where he and I are having a conversation on the phone about Jaguars and LS conversions. There are several aspects of that project that don't normally go into typical conversions like BMW seats (he wanted them) and aftermarket gauges (we always keep the stock gauge package with full function, but in his case a sponsor provided the new gauges for free). In the end, that car made some 398 hp at the wheels running a 6.0L LS and a 4L80E (4 speed overdrive) transmission. He was just in touch with me the other day and is still having a great time with the car.

Andrew
www,JaguarSpecialties.com

 

Often it comes down to money.

If the engine you have is defective your choices are to fix it, rebuild it, replace it with a confirmed good used one.
Or start all over with something different. ( Chevy or Jaguar V8)

I can see the appeal of a newer engine in an older car We hot rodders have been doing it since WW2.

I think your first choice is to figure out what you really need. Bad sensor? Bad wiring ?Bad engine?

Then shop around for the best solution. If it’s the engine There are all sorts of good V12’s languishing in Junk yards with a good engine. Most of them in my experience in fact. With very low demand they often sit for decades. But tend to have very low mileage. When I was selling them I’d ask $750 and be bargained down to $500 plus shipping. Now the market is closer to $1500 but there is still bargaining room. Try car-parts dot com ( realize the V12 is the same in both the sedan and the XJS )

There are more Chevy’s to be sure. But with a Chevy you have to do a lot of things to make it work properly. What is the price of a good kit ( you’ll need a lot more parts than just replacing the motor. The cost of the labor for the kit and finally once everything is finished who is going to Keep it running? The Chevy dealer won’t want a Jaguar and the Jaguar dealer won’t work on what is called a Lump.

Maybe extra power is your attraction? Jaguar has been building a V8 with a very good reliability record since 1998. Horsepower up to 510. And instead of being called a Lump by Jaguar enthusiasts it’s called an upgrade.

 

That's real well preserved coupe, I agree that you should sell it to an enthusiast instead of lumping it. The real low mileage cars are where the money is. Mileage is really not an issue, mine has a bit over 100k and it runs well. I don't think that there are very many high mileage cars out there. Usually some problems develop and the owner decides that they aren't going to spend the money to fix it. If they think they would rather sell, they find that the cars are not easy to sell and not worth very much money so they just let them sit. Luckily your car was sitting in a building. Most end up sitting in driveways, sideyards and parking lots. Where they deteriorate over the years of neglect. So your car is a bit of an outlier. Don't ruin it.

I agree that you should do a thorough assessment of your engine's condition. It might be that the wiring was initially damaged by rodents and that's why the car was abandoned when it would no longer run properly. It didn't take much to sideline these cars as they aren't worth much and cost a lot to fix. I recently had a shop do an inspection on my car. They handed me a ten thousand dollar estimate! Half of that was for replacing the rack and pinion, front brakes and steering components. The other half was for replacing the cam cover, oil pan gaskets and rear pinion seal. They didn't even address the front suspension bushings, which was the reason I took it to them! I obviously am not spending ten grand at that shop.

My biggest beef with the car isn't the motor, I've replaced the plugs, ignition module belts and other small items. I've also replaced the transmission myself, lying on my back in my garage. It's the front suspension. The engine has to be jacked up and the subframe dropped to remove the lower pivot arms to replace the bushings. The springs require a lot of extra care, not like the 60's and 70's American cars I've been used to working on. I haven't decided if I going to fix it myself or just sell the thing. That's how these cars end up sitting for years!

I'm not trying to discourage you, these cars can be worked on by the hobbyist, it just takes longer than a professional. My Dad, who worked on his own cars to save money but was not an enthusiast, gave me some really good advice. He said, "if you want a faster car, just buy a faster car." If you want a fast Jag, look for a nice XKR. Good luck and have fun.

 

Very well said. The only thing I will add is not only should you have more than an average amount of mechanical skill to do your own work on a Jaguar.
If you expect to get a semi decent rate of return for your labors.
It’s also probably best that you have more than an average set of tools. Tools of quality more towards Snap On than bargain tool bin. Oh, and leave the metric wrenches alone. ( they won’t fit)
With regard to the tools themselves. Even Snap On tools wear. Take advantage of their lifetime warranty and if they are sloppy have your dealers replace them. Yes I know there are good quality tools other than Snap On, There are even some Harbor Freight tools that are good ( and some not so good)
But loose fitting wrenches tend to cause rounding of nuts and bolts. And that not only looks sloppy, It screams of amateurs.

 

My 2 cents -- with the caveat that it's your car so do whatever you want -- is that a LS swap doesn't make any logical sense.

With a low mileage barn find like that, there are really only two paths:

1. Just get it running with as little fuss as possible, so that means dealing with the wiring and keeping the V12.
2. Since it is such a low mileage car, doing a full stem to stern bringing up to snuff, and the engine itself will be the least of your concerns with a full going over. There is no point wasting the time and money and not keeping the car original. The only XJSs appreciating enough to justify restoration are low mileage originals without modification.

IMHO, the only XJSs that are good candidates for an LS swap are ones that are cosmetically great, but with a blown engine. And the only way you can you blow up the V12 is to neglect it so you would have to be concerned with what besides the engine was neglected.

I think if you focused first on just getting it running, it will be kindest to your wallet, and then you can decide what to do next. Good luck!

 

I hate to sound like a party pooper but, as nice as it is, this XJS it seems to have a rust problem. Look at the area immediately forward of the left rear wheel. And lack of coachlines suggests it has been repainted.

Still nice but perhaps it might not be quite the gem that everyone thinks

Cheers
DD

 

A blown up engine should not call for a swap.
I used to see good engines for as little as $500 and checking car-partsdot com they sell for as little as $400 to $3000 with a warranty
a little advice, assuming yours didn’t burn, buy one that has. You can swap your wiring injectors, manifold right over. I see them all the time for $4-500
Jaguar engines just don’t wear out. Yes rubber hoses leak and sensors fail. All easy stuff to replace. But the normal is no measurable wear in the bore even well over 100,000 miles.

To be fair I’ve only taken apart a little over 50 V12’s and probably peered into another few engines at various shops.

 

I have a 1990 model year XJS and it never had coachlines. Someone posted a JCNA document on the forum that says MY '90 had factory coachlines, but I know mine never had them and I've seen numerous '90s that didn't either. I know officially '91-'93 did not have factory coachlines, though some dealers added them. Either it was a mid-year change in 1990, or the JCNA document is incorrect. So it might not be a repaint based just on lack of coachlines.