my 1998 xjs 4.0 jaquar will start but will not keep running ?

Check air filter and clean throttle body.

Check the pressure at the rails..

rail preasure is good ,and air filter is clean,thanks

Sounds like you may have some dirty fuel injectors.

Are you sure it's a 1998 XJS? In thought they quit making the XJS in 1996! I have a 1996 XJS and all info I have says it was the last year they were made.

Sounds like the fuel pressure regulator.

Check and inspect your crankshaft sensor for dirt and wear and tear, replace the sensor with a new one if necessary, Because of the age of the vehicle, I would just replace it.

Adding to my earlier response, replace the sensor not the vehicle. Your 98 XJS is great. good luck.

They quit making the Jaguar XJS in 1996. Where did the 1998 come from? I have a 1996 Jaguar XJS 4.0 and it was the last year.