95 XJS V12 performance problem

ok a bit of a problem with the 95 V12 6.0, having a problem with poor performance, starts up fine and idles ok, but give it any throttle and she limps along and barely gets out of her own way. Engine is not miss firing, but it woiuld seems its a fuel problem, i am guessing a fuel pressure regualtor, or maybe a throttle position sensor? car has 77k miles on it. The FPR on the fuel rail doesn’t leak when i pull the vacuum hose off. How do i test the FRP and TPS to see if they are working properly and i read there were two FRP if so where is the other one located? any ideas? or maybe its something else? TIA

Sounds like a TPS out of adjustment. Only one FPR on your year.

Meter the TPS ref and yellow leads. Don’t disconnect the harness, go inside with your probes. Switch car to on but do not start. Should be .32 VDC. If not, rotate it. Three screw on bottom. Takes a bit of back-and-forth.

95 has only one FPR, on right front side of fuel rail.

Make sure the throttle shafts/butterflies are actually opening when you step on the case. The connections can get loose… youlll see the spindle move but the motion never gets to the butterflies.

Great thanks guys I will check that out

The voltage quoted is NOT correct for the 6.0L V12.
Your pot should read about 0.6V with closed plates (idle speed) and 4.5V at WOT.

To check FPR, disconnecting the vacuum line will only tell you if the diaphragm is ruptured and leaks. Whether the fuel pressure is correct, you’ll need to plumb a gauge, something like this:


The 36CU will flag a code if something is seriously wrong with the engine management. Do you have anything displayed on the trip computer?

yes i have FF14 but that only came on when i broke a vacuum hose coming off bottom of intake manifold that goes to the back of the air filter

I am puzzled. I am not aware of ANY vacuum line coming from the intakes and going to the air-boxes. Can you provide a picture?

In any case, FF14 is NOT related to vacuum and could very well be the reason your engine bogs down when warm – it is likely flooded with fuel.

FF14 is telling you that the CTS (coolant temperature sensor) circuit has a problem. Please refer to the description of the DTC your 36CU flags:

Has it had the Marrelli Mod, start the car and see if the exhaust is cold on the right side.


Here you go, I just check the computer and there are a bunch of codes being displayed now 23, 17 18 11. I have ordered a new cap and rotor and plug leads and spark plugs as they were a bit crusty on the inside and it’s time

SAV = supplementary air valve. Brings up idle speed up a notch when the A/C is putting load on the engine.
The vacuum hose can be plugged and the potential leak should not be related to any of your codes (but I do wonder if the ECU is working fine – FF11, 17, 18 are critical for the operation and unlikely to pop-up all at once, unless something is seriously amiss – what is the history of this car/problem?)

its a local 2 owner car, that was regualry serviced by a jagur specialist, i notice some rodent crap on the intake manifiold and one of the plug wire has been nibbled on, has 78k miles on it, but i am going to change plugs, plug wires, dist cap and rotor as see where that put things

So, you got the car in the condition…? You don’t know how it happened? Has the car been sitting (probably, if the rodent chew the wires in the see)? How long? Has the old gas been drained? Fuel pumps, filter, FPR – all of this was already mentioned. Dan also brought up the Marelli meltdown – cat converters on the A-bank (passenger, side assuming you’re in the US) are not solid blocks…

If I were you, I’d start with the basics, confirming that you don’t have serious engine problems. Changing the plugs and wires is a good idea, but if the root of the problem is not addressed, the new plugs will have to be replaced pretty soon.

Good luck!

no the car was running fine, i have put about 700 miles on it, then i started get the problem, fuel is fresh and good, going to change out the filters incase some crap has been dragged up, chech to see what wiring has been nibbled on. Thanks for the luck i will need it !!

That’s odd, but the beats has left the factory 25 years ago (mileage is pretty much irrelevant at this point) and many things can go wrong.

Check the wiring while you’re waiting on parts! Ohm each one!
If the ECU doesn’t get TPS, CTS and MAP signals, it won’t do its job.
Confirm you haven’t suffered from a partial cat. converter meltdown (best method is to test the exhaust back pressure, but the quick test for even and hot flow from both tailpipes which Dan suggested earlier is also a good indicator).

Fueling – you’ve got two pumps in the tank. There are reports (search the archives) for bad connections. Verify no corrosion at the terminals (look carefully on both sides – signs of overheating?). You may want to swap the connectors for the two pumps to see if this changes anything (one of the pumps receives much less exercise as it kicks only above 2800-2900 rpm, chances are it will work as it should in case the other one is marginal).

BTW, where are you located? There are only a few of us (6.0L V12 owners) that regularly post on JL. What are the last 6-digits of your VIN? The cars with VIN past 198335 will be having the Teves IV brakes (vacuum boost) which is more desirable than the earlier version III (mine is a few hundred numbers ahead of the break-point) .

i am in Los Angeles 198971 is the vin digits so looks like i got the better brakes, its a coupe so really rare here in the US, supposedly only 59 V12 coupes for world market 1995 MY, and only 29 V12 coupes came to US so quite a rare old beast. I sold them new here in Pasadena and we could not give the V12s away, everyone wanted the 6 cylinder cars. The sticker on the V12 coupes was around $72k from memory and Jaguar had to give us $10k rebates to sell them ! The convertibles V12 were $82k.

here she is old “COOL V12” !

Nice. Jim in Alabama has one just like yours, but in black. Very close VIN too…

BTW, the production numbers are often misquoted. I have the 95 production numbers in my avatar, but one must remember that model year in the US and elsewhere have different meanings (my car is MY 1995 in the US but it is not one of those 78 2+2s (apparently what we call convertible is not the same as what Jaguar classified it as) that rolled out of the assembly line in year 1995 :-)) Judging for the last 6-digits, your coupe is likely a late 1994 production.

For the year 1995 - here are the numbers produced (total worldwide) produced numbers are in brackets.

XJS 4.0 fhc XJ88 7X ( 793)

XJS 4.0 two-plus-two 7K (3943)

XJS 6.0 fhc XJ/27/87 9W ( 59)

XJS 6.0 Convertible XJ97 9J ( 11)

XJS 6.0 two-plus-two 9L ( 78)

total number of 4.0 L XJS cars produced in 1995 = 4736

total number of 6.0 L XJS cars produced in 1995 = 148

total XJS produced in 1995 = 4884

I was told a few years ago that of that low number of 95 V12 coupes, only about 43 were still around and on the road. Others demised due to wreck or neglect