1989 jaguar xjs v12 fuel mixture

A good chance that your car does NOT have one. OBDll was not required by law until 1996.
Some manufacturers had there own systems before then.
I found the following web reference:

"Sorry fellas, the 1989 XJS has no onboard diagnostics at all. It only has a red feedback monitor hookup [with a blanking cap] back near the fueling ECU in the boot. It was used for mixture setup and that's pretty much it.
The trip computer fault code readout didn't arrive until 1992."
The trip commuter readout he refers was a way to get codes (after certain steps taken) to show up in the trip computer display.
For OBDll compliant cars .... the port has to be within 3 feet of the driver with full access .... usually under the dash
Helpful?

https://parts.jaguarpalmbeach.com/p/Jaguar__XJS/Fuel-gauge-sending-unit-fuel-sender-unit-fuel-tank-sending-unit/49193626/DAC11188.html?partner=googlebase_adwords&kwd=&origin=pla

The sending unit is in the gas tank...(its #9) ..good luck

Usually means the sending unit in the gas tank has failed.

Jaguar Sensor, XJ6, XJ8, XJS, XK8, S-TYPE, X-TYPE, XJ40 ...

www.jagbits.com/jaguarsensor.php
FITS: 1993-1994 6.0 XJ12 AND 1989-1993 V12 XJS FROM VIN 157118 ON, 1994-1996 6.0 V12 XJS FRONT DAMPER .... LNA1622AA - MAP SENSOR ... C42166-USED - USED SENSOR COOLANT TEMPERATURE FOR FUEL INJECTION

Thread: 6.0 V12 XJS High RPM When Warm - Jaguar Forum.co.uk

www.jaguarforum.com/showthread.php?t=54266
Mar 3, 2013 - I recently bought a 1993 6.0 V12 XJS and when it is started from cold it runs ... to 1300 RPM so it wants to surge forward when braking which is strange. ...Classics · XJS 3.6,4.0,5.3 & 6.0; 6.0 V12 XJS High RPM When Warm ... You might want to adjust the aux (extra) air valve to bring the idle speed down.

Doubtful the valve is "stuck". More likely to be burnt. It happens. Do a compression check then pull the head off and replace the valve that is losing compression.

I would get complete set of maintenance records for that one. 250k is alot for a jag especially a V12. That one is either running on borrowed time or has been meticulously taken care of.

jaguar owners club.co.uk will be able to assist (if its just the glass get some cut locally.use double sided carpet tape to stick it on over the old))

Replace the crank sensor under engine. 90% of the problems is here.
Einar Norway

Obviously, fuel and oil should never be in the same spot at the same time. You have a dead cylinder and maybe a stuck-open injector. The black smoke is from a too-rich fuel mix, so the cheap things to look for are in the fuel injection system. Look at the fuel pressure regulator as well. Good luck, and don't forget to relieve the fuel pressure before you do any work on fuel inj. engines!