I have a 1994 XJS 2+2 4.0 US specs The engine starts quickly when cold and has a lot of power no black smoke .

I have replace Fuel Pump Relay Coolant Temp sensor Crankshaft position sensor. and Fuel Pressure Regulator My question is does the fuel pump have a check valve that prevents reverse fuel flow or is there a valve in the Fuel Module? The reason for my question is when vehicle sets for about an hour when hot I don't have Fuel Pressure in the Fuel rail. with engine off.
I have to crank the engine a few seconds maybe 7 seconds then turn key off then try again then engine starts.

 

I have a 95 XJS that had a similar problem. I replaced many of the components you did, with no luck. One day I was looking at my plugs, a couple of them looked like they had traces of unburnt fuel. I ended up pulling my fuel injectors and had them serviced. I used Jaguar Fuel Injector Service in Jacksonville, Fl. Turned out 3 of the injectors had too high a flow rate. I was able to locate a set that were matched, had them once again tested. After installing the injectors, I have no problems with a hard hot start. That is what worked for me.

 

EJ,

Were you also experiencing poor gas mileage?

My car seems to be operating normally, but I am getting 14-16mpg, which is substantially less than the usual 17-18 I was accustomed to (in the city). The car runs great and has no codes. I suspect the injectors, but haven't gotten around to taking them off and shipping them out.

 

Open your hood when you shut it down. All you have to do is crack it enough to let the heat out. You'll neber have the problem again. XJS has no hood vents like E type. Big engine and small space means severe engine heat. It gets super hot after shut down and evaporates fuel in the rail. My 91 would only sputter until I figurured it out. Jag never told their customers to "crack the hood open on your $65000 car", but they should have. It works wonders.

 

I am not sure about the gas mileage improvement, but the car does run smoother and has a little more pep.