I've developed an issue with my XJ6 where if it sits overnight or for about 8-10 hours it just cranks but doesn't start. The fuel pump has been replaced, thinking that was the issue but it's still there.

Once the car is started, it runs fine and will turnover first try if it's shut off.

This has happened twice before I replaced the fuel pump and both times, after letting the car sit for about an hour or so, it starts up just fine and it seems like everything is normal until the next time it sits for more than a few hours.

I've got some ideas as to what it is but I want to try and pin point it. I'm thinking it could be injectors or fuel pressure regulator? Would cold temperature (50/60f) effect injectors?
Injectors came off of a used car 84k miles ago and fuel filter was replaced about the same time. I don't think Fuel pressure regulator has been replaced.

 

I think the regulators have been known to leak down, leaving you with little or no pressure at the injectors. Probably a decent place to start.

 

Just replaced fuel filter and now can verify fuel is passing through the filter but there is no fuel going into the regulator (checked by loosening the connections to see if fuel would leak but nothing)

Between the fuel filter and regulator is there only the "rochester valve" (black round ufo looking thing)? I've only read a few things here and there about those but not exactly sure what it does...

 

EDIT:
Confirmed fuel is flowing through fuel rail now. Check to see if I was getting ignition spark and now I'm not. Would a faulty crankshaft position sensor cause this?
How much does the rpm needle move when cranking if the sensor was working?

 

You should see 200 rpm or so, maybe 300, I never really paid close attention.... and mine lights off very quickly..

 

Yeah I've never paid attention to it and with the XJR it starts up quickly so I couldn't tell. But when I start cranking the needle hit's the stopper but then goes back to 0 and stays there

Just confirmed it was the crankshaft position sensor. Would cold weather or sitting for a about 8-10 hours cause the no start? or was the sensor just on it's way out and intermittently working?

 

I cannot answer your question except to say that any electrical coil device (which includes the CKPS) can have intermittent failures due to temperature.