If it has spark and fuel then it will start, unless there is no compression
Only way to check for spark, at the right time, is to spray fuel into the intake and then turn over the engine, 50/50 test
Been used since the first spark plug engines in late 1800's,
If you manually add fuel then there are two results:
Engine starts and dies, spark is OK but no fuel is getting into the engine
Engine doesn't start, spark is not working or not timed correctly(assumes compression is OK)
So do 50/50 test so you can get on the right track, spark or fuel issue
I think 1991 F-150 5.0l still used a distributor so it can be mistimed, 180degrees off
There were also 2 firing orders depending on the cam that was used, but even with wrong firing order a few cylinders would fire if timing is correct
Fuel pressure doesn't mean fuel is getting into the engine
You can crank engine over a few times and pull out a spark plug to see if its tip is WET, if not then injectors are not opening
Fuel injectors all have a Red wire, that red wire should have 12volts with key on
The computer Grounds each injector to open it
So test if Red wire has 12v with key on
You can't really test Ground wire on injectors, it happens too fast to see on a volt meter, you can hook up a NOID light between injector and wiring harness, and it will flash if computer is pulsing the ground on that injector while cranking engine over
The computer has a few Ground wires, they are not shared inside, so each ground wire needs to be grounded.
And metal in a vehicle is NOT a good ground because its metal
Engine should have the larger negative battery cable connected to it, for starter motor and alternator grounds.
But engine and trans sit on rubber mounts so don't transfer ground to frame or body
And body parts are all painted FIRST and then assembled, so no reliable ground transfers between body parts
There needs to be a ground strap from engine to Cab Firewall
A ground from engine to frame
A ground from engine(or battery) to inner fender and Rad support
Computer needs to be grounded to cab or inner fender