XJo90,
Lets go back to basics, you need spark and fuel to make an engne run. How do you check these on our version of the XJ since we can not read error codes, since the computer does not store them, and there is no customer way to read the computer.
I highly recommend you get the factory manuals for you XJ, they will pay for themselves. If you need informaiton like today then sign up at
www.alldatadiy.com and get a subscripition for your XJ. Either source should tell you how to check fuel pressure and what value it should be.
For FUEL you need to verify you have pressure in the fuel line and that the fuel pump does work.
For SPARK try to barrow a timing light and hook it up to #1 cylinder, actually any cylinder would work, and crank the motor. If it flashes then you are getting spark. If it not them my first supsect is the Crankshaft Position Sensor (CPS). With our XJs their are two versions of the CPS - standar and high altitude. They are the same price. The high alt gives you more advance which means more power don't worry you have a knock sensor so you will not have any pinging. Go to the dealer to buy it, either way I'd get the high alt - more power for no additional money that a no brainer
If you do have pinging then time to replace the knock sensor. Some people NAPA sells them but I know I got a new one at the Jeep dealer.
Lets say you do have spark that could confirm you have a bad camshaft position sensor. It is burried inside the distributor and is a Hall effect sensor. It tells the computer and 3 injectors fire fuel. The key question is "do you smell gasoline after you long cranks?" if you do then I'd say the camshaft positon sensor is good, it also called the sync sensor. If this sensor is bad then time ot install a rebuilt distributor to get a new sensor, either way you may have warn bushings. On my 69 Camero with the 230 straight 6 we changed the distriburor at 80,000 miles and 150,000 miles. So distributor bushings wear. It more important on a car with points and condensor but still is important for an electronic controlled engine.
Do those tests if you can but like 5-90 I think it most likely the CPS, doubt it the sync sensor. Either way with If you have over 100,000 miles on the engine then I'd consider a new/rebuilt distributor as preventive maintenance.