Pull any spark plug that's easy to get at
Wipe off the tip so it clean and dry
Put it back in, snug not tight
Crank engine over a few times
Pull out spark plug
Should be WET with fuel
If not then injectors are not opening
In 1991 spark is done with crank sensor and EDIS-6 spark module
This module sends the spark timing pulse to the Computer so it can time the injectors
That's pretty much all the Computer does, the fuel system
Each injector gets 12volts on a Red wire with key on, red wire just runs from one injector to the next
There would be just 2 ground wires for the 6 injectors run back to computer, called Batch fire injection
3 injectors per ground wire, computer grounds the 3 injectors to open them
The computer doesn't "share" grounds internally, so each ground wire for the computer must be ........grounded, lol
Computer pins 20, 40, and 60 are the main grounds
General use diagram here:
http://www.auto-diagnostics.info/ford_eec_iv