Folks
I have the opportunity to buy a very tidy early 90s Daimler Double Six XJ40. Am curious about the practical differences to the 5.3 (except for the obvious 700cc).
Are these engines also prone to dropping valve seats etc?
All thoughts welcome.
Cheers
John
Probably most people here know more than I do about such things. I believe your early '90’s is an HE, so the heads would be different with the funky exhaust. I believe by the early '90’s the V12 was distributorless. As I understand it, most of the issues with a V12 dropping valve seats is related to a poorly maintained cooling system. If the car is clean and tidy and has maintenance history, then you should buy it. The 6 liter does get better fuel mileage which probably wasn’t one of your concerns.
Probably most people here know more than I do about such things. I
believe your early '90’s is an HE, so the heads would be different
with the funky exhaust.
The Jaguar V12 was H.E. from 1981 on until it ceased production. However,
the combustion chamber changed shape slightly with the 6.0, evidently to
adjust compression ratio.
I believe by the early '90’s the V12 was distributorless.
Nope. Didn’t go distributorless until its very last year, when the Nippondenso
ignition system was introduced.
The 6 liter does get better fuel mileage which
probably wasn’t one of your concerns.
Actually, many owners have reported that it gets WORSE mileage than the
5.3.
– Kirbert
Worse mileage than a 5.3, worse than this?:
My guess that would be true on a car needing maintenance. I know nothing about the intricacies of the Jaguar V12 fuel injection system, but I have experience with errors in the feedback loop on modern cars: like when the feedback gives a false high o2 reading which causes excess fuel delivery.
I’m no authority on the 6.0 engines but in the grand scheme of things they seem less problematic the the 5.3 version.
My impression…and that’s all it is… is the 6.0 engines are not prone to dropping valve seats. At least we don’t hear about it, it seems. Of course there are a lot fewer 6.0 cars running around out there and they’re probably not seeing much use these days.
I had a pal years ago who had two 6.0 Jags and they both suffered Marelli failures so that potential problem remains the same.
Cheers
DD
That’s a pre-H.E. Yeah, they were the worst fuel economy-wise. But the 5.3 H.E. was MUCH better.
My pre-HE is right on that number. Last few fillups:
11.96 mpg
10.54
9.67
11.13
…
BTW, I also have my window sticker, at my website.
www.efsowell.us
Ed, congrats on your anniversary - Mr. Stox was one of my favorite restaurants when I lived nearby!
Your window sticker wouldn’t open for me, I always like some of the interesting info you can find on window stickers.
Kirbert, just curious if yours is a 5.3HE and what mileage it gets in whatever measure you use Imperial, Gallon Liter.
Yes, we were sorry to see it go. Developers made an offer they couldn’t refuse.
Thanks for letting me know about the window sticker. I’ll fix it. The car was originally owned by Henry Segerstrom, recently deceased. I bought it from Segerstrom Farms in December 1980.
Ed
Not sure it’s fair to say any of the V12s are “prone” to dropping valve seats. It does happen but there is usually an avoidable cause.
Fuel consumption is largely determined by the right foot but average of late for my 6 litre is 10.4 litres per 100km. mixed country driving.
My own car was an '83 5.3 H.E. with a manual transmission installed, meaning it’s perhaps not representative. But those with 5.3 H.E.'s in OEM form will tell you that the cars typically get 16-18 mpg in US gallons, higher in Imperial gallons. The 6.0’s typically drop a couple of mpg from that, although you’d think they’d do better having one more gear in the transmission.
Never seen 18 mpg. These are my numbers:
http://www.jag-lovers.org/snaps/snap_view.php3?id=1469410491
Kirby forgets that the XJS with a 6.0L V12 has a 4L80e transmission but also has 3.54:1 differential. Thus, comparisons of mileage are not a good indicator of “thrust”
I’ve put a link to the original window sticker in my Webpage. It’s in the second paragraph of the Ed’s Jag page.
Ed Sowell
Thanks Ed. Interesting that your XJS was rated 1 mpg better in the city and 1 mpg less on the highway for the exact same drivetrain as an XJ12. And apparently an XJS and the XJ12 weigh the same 4,500 lbs with full fuel tanks.
I’m not forgetting. With the lower 1st gear, I’d expect the driver to use less throttle from stop in everyday driving. Maybe those 6.0 owners just have their foot in it too much!
The 4L80E has the same ratios as the TH400 in the first three gears. Fourth gear is overdriven.
'Tis the TH700/4L60 that has the lower first gear
Cheers
DD
:-)) I wish that be the case
The reported mileage is calculated based on over 800 miles on the highway. I stayed out of trouble, so I’d argue that being too aggressive on the gas pedal ain’t it.
FYI. the 4L80e is programmed for a take-off in 2nd gear. One needs to use the selector and switch to “S” (sport?) to enable launch in 1st
Steve, your 16.4 highway mpg is very good vs., your car’s 16 hwy mpg rating. Did not know that about the second gear start which gives it a Powerglide gear ratio for normal driving (and then overdrive on the highway).
Here is how your car can achieve 18 mpg or higher - use Imperial gallons 
The U.S. gallon is only .83 of an Imperial gallon.
I am told the fuel economy will improve if the top is up. Have never tested it – my convertible hibernates during the winter and I take it out of the garage for longer drives only if the weather is nice and the top can be folded.
A fellow is the UK with a 6.0L coupe told me he routinely sees 21 mpg (Imperial gallons)
21 Imperial would be 17.4 U.S. gallon, which is within a range of what could be expected: some examples doing a little better, and some doing a little worse. I think that a careful going over of your fuel injection could bring your car to the same mpg’s. I do not know how easily the fuel injection on your Jaguar V12 can be tuned, I like the tuner friendly OBD1 and to a lesser degree OBD11 of many later model American cars that allow for some incredible tuning that combines performance and fuel mileage (and emissions). I had a later model standard Corvette with a top end heads and camshaft package that would top out just past 180mph - and get 25 to 26 mpg in normal highway driving at 75mph.
