Hi all, I know there are plenty of posts about this topic, but this one has me stumped…
I have recently acquired a 1991 xj40 and been having fun electrical gremlins to troubleshooting regarding my fuel pump.
Been reading a number of posts and done a fair bit of problem solving at my end, but haven’t quite found the exact solution for my issue yet so figured I would try my luck reaching out direct.
I replaced the fuel pump with an aftermarket one a few weeks ago. All seemed OK until last week. My issue is that the car starts fine when cold, drives fine too. Once warm, if I park up and turn off ignition then it won’t start again…until sufficient time has passed such as 30-45 min…then it will start again…
Sometimes if I drive it for longer the cool down and “recovery” time before I can start it again is much longer, hours and hours.
I have put a voltmeter to the wiring in the trunk that leads to the fuel pump. When car is cold and I turn ignition on, I see brief spike from 0V to 12V and then back to 0V. This matches the audible sound of the pump priming. If I start the car then I can see 12V running continuously. I therefore know the relays and wiring right up to the tank are ok.
When the car is warm and I turn on ignition, I still seen the spike in voltage from 0V to about 10V then it drops back to 0V, but this time there is no priming noise from the pump…just silence. I even then tried to connect 12V from an external battery directly to the fuel pump, and it wouldn’t run.
You’d think the pump is dead, but then why would it suddenly start up and run/drive fine the next day???
This tells me there might be some kind of safety device somewhere between the exterior wiring to the fuel tank, and somehwere within the fuel pump assembly, and for some reason it is stopping power getting to the fuel pump until a certain amount of time has passed.
… But I am not sure what this could be???
Any ideas or suggest with be greatly appreciated 
Many thanks