I've own two V12 XJS 2+2 Convertibles for awhile, a 1994 and a 1995, and once before had a 1994 4.0 2+2 Convertible. We really liked the car even compared to our V12's and loved the awesome gas mileage. It got as high as 30.8 MPG during a couple of tanks on the way home and always got in the 20's, usually mid 20's on driving around our area which is mostly two-lane 55mph with some small city driving and always back up in the high 20's or even 30 again when we'd take road trips. I was just never crazy about the color but it was priced right at the time and in fantastic condition but due to the color we sold it later on. We like our V12's but wanted another 4.0 for that kind of gas mileage plus they feel a little more nimble in corners at speed.

Anyway, recently I bought a 1996 4.0 2+2 Convertible but on the ride home (about 1200 miles) the best I got was 18.33 and after 5 tankfuls that's still my best tankful and so far my running average is 17.87 with my worst tank being 17.53.


This 1996 engine looks a lot different than the 1994 model did and I know they did some upgrades to HP and Torque... Did they screw up the gas mileage that badly to get those extra ponies or is this car not living up to it's MPG potential for some reason?


It runs great though I have noticed every now and then (maybe once every 40 mins or so, no real pattern that's just my estimate of about how often it happens) the car will have a real quick "jerk", not harsh or anything but it's like it had a misfire then quickly recovered. You just barely feel it but it's there.


Also, what kind of gas mileage are you other guys/gals with the redesigned ('95/'96) 4.0 engines getting?

 

I purchased a '95 2+2 4.0 a few months back. I recorded 24 mpg on the freeway ride home. In the city now (in stop and go traffic), I'm getting 13-14 mpg with the AC on. I'm thinking that your mileage is a bit too low.

 

My '94 4.0 xjs delivers 17-18 MPG stop and go. Highway 28-30 a/c off in the 70-75 MPH range. 75 MPH hovers around 2300 RPM. My '97 XJ6 delivers about the same stop and go. Highway is about 1 to 1.5 MPG less at 70 to 75 MPH. 75 MPH hovers around 2500 RPM.

 

My 95 4.0 Coupe gives me 20 to 21.5 at highway speed with A/C on. This was consistent on 5 different 600+ mile trips. I am a bit more aggressive in local driving and get about 16mpg, Trans switch is always in sport mode and so am I.

 

Do the coil packs on the '96 4.0L need to be changed on a regular schedule (ie. get weak over time/miles) or are they just basically "good until they go bad" ...?


If they do need to be replaced is there any brand/model number that's better than others? Like better performing ones than what they originally came with now that they have had 18 more years to advance the technology.

Also is there a particular spark plug brand/model that's experienced owners of the 4.0L engines have found works particularly well?


I've already ran two cans of Seafoam through the car on the drive home, one tankful apart. Now on it's 6th tank since I bought it (and I run it down almost to the red area before I refill) so by now there is no old gas left in it. I plan on changing the spark plugs, fuel filter, air filter, transmission fluid/filter, engine oil/filter, and changing out the fluid in the rear differential...

What else on these '96 model 4.0L AJ16's need to be changed on a regular basis to keep them running strong, efficient, and reliable?

 

'Diamond' was the actual OEM manufacturer and those are the ones you want. Go to the X300 section and look for the thread 'Head's Up On X300 Coils'.

Replace only as needed, IMHO.

I'll let others comment on spark plugs

Cheers
DD

 

Oh I forgot to ask, does bad coil-packs throw a code on these cars?

One of the first things I did was plug in my OBD-II tester and checked and there are no codes.

 

Bad coil packs should indicate a misfire in that cylinder. p30x

Use Champion Spark Plugs RC12YC. Make sure they are gapped between .035 and .038

People have reported strange behavior with other plugs such as Bosch. Others have not had issues with plugs like NGK, however everyone seems to agree that the specified Champion plugs work well. Stick with those.

 

On 1995 AJ16 cars, at least, the OBD II system is reluctant to set misfire codes. I've had numerous coil failures with no codes. I have actually disconnected coils and no code was set.

Maybe '96 AJ16 OBD II systems were more fully developed.

Or maybe X300 versus XJS makes a difference here? I dunno. I'm speaking from X300 experience, of course!

Cheers
DD

 

I replaced the iridium plugs the PO had installed. On my drive home when I purchased the car I received several misfire codes and reset codes with my code reader. I had Hennessey Jag do a pre purchase inspection when I got it home and that is when we found the wrong plugs. Service manager told me to install the Champion RC12YC which cleared up the problem and improved my MPGs as I would have expected.

 

Well I just changed my plugs. All of my coil-packs say Lucas on one side but say Made in Japan on the other which I take it from what I've been reading that those are what i want.

The plugs was fairly new, there was a sticker under the hood actually listing when they was changed; miles and date, and it was about 15k ago so fairly new for good quality plugs. They was NOT Champions, they were NKG number BKR6E. They was all at about 0.039, they looked good and clean, the previous owner got a little carried away with the anti-seize compound on the threads and the dielectric grease in the boot (a little is good but he had way to much). I also notice they was snug but not really tight, at least not torqued, and #5 was slightly looser than the rest.

I set all the Champion RC12YC's to 0.035 and put them in, didn't need to add any dielectric grease or anti-seize since he kinda over did it last time and was still plenty in there.


One question though;

None of the coil-packs had gaskets under them EXCEPT when I got to #6 at the back, it had a green gasket under it, though it broke all apart when I lifted it up so now it doesn't either. None of the others has this in any way. Is this something they are supposed to have? Do I need to order some gaskets somewhere to put back under all of mine?

 

SouthernGypsy,

All my coils have gaskets LHF1719AA. See here:

Ignition Position Sensor, Coil/Plugs-4.0 Litre - Parts For XJS from (V)179737 to (V)226645 | Jaguar Heritage Parts UK

Paul

 

This thread is helpful. I have a 95 4.0 and my combined highway/city gas mileage is always in the 16-17 range. Too low, I think. I also get that occasional "misfire" feeling when I'm cruising on the highway. It's sporadic and I never notice it when I'm driving around town; I only notice it on the highway. Some days I don't notice it at all. I have also tried Seafoam but with no improvement. I'll see what plugs I have. They are just a year old but maybe the wrong kind. Wondering if it could be a bad coil(s) too.

 

That's exactly what ours is doing, though yesterday for the first time after being on the Interstate about an hour I exited an off-ramp and when I came to the top of the off-ramp and sat behind another car, after maybe 15-20 seconds the motor seemed to stumble a moment just sitting there at idle. Other than the occasional "misfire feeling" when going down the interstate that's the first time it's done anything else odd. This is after I changed all my plugs and checked everything else out that I could think of.

 

I am using NGK plugs but no misfire but I consistently get 17mpg city and highway mix. I am in CA if that matters.

 

I had a Chevy V8 that once had a misfire, ie it would 'bog down' or 'hesitate' when under acceleration. Had a hard time troubleshooting the problem. I didn't think it had anything to do with the plugs because I had changed them to Platinum Plugs (the type with the thin platinum insert located under the gap and tip of the ground electrode) only 1k or 2k miles previous to my misfire. Anyway, it turned out the insert had fouled up on one of the plugs. I replaced the platinum plugs with OEM type standard plugs with the much larger diameter insert and never had a problem with that engine misfiring again.