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You should run Motorcraft AGSF42FM spark plugs gapped at 0.054(they should come with that gap)
But in a pinch you can use Autolite or NGK, also gapped at 0.054, they won't last as long but not a big deal
The 4.0l OHV literally EATS other brands
The main thing is the GAP, 0.054, did I mention that,
You have a Waste Spark system
This means there is a spark in each cylinder at its TDC(top dead center), since this is a 4-stroke engine that means only 1 of these sparks is used when a cylinders piston is at TDC, the other spark is "wasted"
Its just a simpiler setup, actually first spark system ever used in the late 1800's
Because of this Waste spark system there are only THREE coils are in the coil pack, not 6
3 and 4 share a coil
2 and 6 share a coil
1 and 5 share a coil
That's why coil is wired that way
3 4
2 6
1 5
front
Its for the way the engine is balanced
There are 3 parts to the 1991 4.0l spark system
Crank sensor, its on the lower front of engine by the crank pulley, it reads a Tone Wheel(looks like a gear) tone wheel has teeth that rotate with the crank and that causes a signal to be generated in the Crank Sensor and TIMES the spark
EDIS-6 Module, this gets timing signal from crank sensor
It uses that timing signal to ground and unground the 3 coils in the coil pack
It also sends out the timing signal to the engine computer(PCM) so it can time fuel injectors
The coil pack, when ungrounded each coil sends out its spark, just like any ignition coil
If you are burning out coils then thats usually a spark plug or spark plug wire issue
This is an autonomous system, does not need engine computer to work
BUT.........it does get Load Advance suggestions from the PCM
So the PCM could cause mistimed spark and loss of power, and a 1991 PCM could be in need of 3 capacitors that only lasted 25 years or so, nothing is built to last anymore, lol, they are easy to replace
Pull out computer and open it up, you will see the 3 cylinder type capacitors, they leak and the pins corrode and break, wiggle with finger
But I would just replace them on speculation, cost is under $5
Other than the spark plugs and wires not sure what else to suggest?
Double check the coil pack wiring, I still get the 5 6 4 side mixed up, either at the coil end or spark plug end of the wire