Yesterday I went to a doctors appointment. When I returned My 96 xj140 would not start. The engine would turn over but the car would not try to start. My wife brought me starting fluid and the engine started right up and idled fine. The car idles fine and runs smoothly with plenty of power but will not start without a small spray of starting fluid.
The 1996 XJ6 X300) did NOT have the AJ6 engine, but it has the AJ16 engine.
I think you are posting in the wrong forum!!!
My first thought was, the spark plugs are not hot enough. It is either the plugs themselves or it is not getting the correct message from the dizzy.
Coolant temp sensor acting up or bad connection
If your car was an XJ40 and had an AJ6 engine, I would say you have a classic case of fuel pressure regulator failure where the system is not holding fuel pressure in the fuel rail on engine shut-down and it takes a little while for the ‘priming’ pressure from the fuel pump to build up to the required start pressure.
Try the key cycling trick before engaging the starter - turn the key to position II - ON-OFF-ON-OFF-ON- five or six times and then crank the engine. If it fires then, the problem is as described above.
Perhaps the same applies to an X300 AJ16 also?