There are only two ways for gasoline to get into the intake from 1998 and up
Fuel injectors and Pulse Damper
Pulse damper is at the end of the fuel rail, it is a rubber diaphragm that absorbs pressure waves created when fuel injectors open and close, when Ford switched to 60psi fuel pressure this was needed.
1997 and earlier used 30psi pressure and had a Fuel Pressure Regulator on the fuel rail so didn't need a Pulse Damper.
The Pulse Damper has a Vacuum hose attached as a safety device, if Damper should leak then gasoline wouldn't drip on hot engine parts, it would be sucked into the intake...............stalling engine, lol, or make it run Rich and set rich codes
Yes, with key on all injectors get 12volts on the Red wire, Red wire just daisy chains from one injector to the next
PCM(ECU) has a Ground wire connected to each injector, it Grounds an injector to open it.
With key on PCM will unground all 4 injector wires(2.3l), so you can test each to make sure it is not grounded, if one is then start moving wires around to see if that wire is shorted to ground somewhere
Before pulling intake I would have pulled out all 4 spark plugs and then cranked the engine, if an injector was stuck open you would have seen alot of fuel spraying out that one spark plugs hole.