Very old cars... lots wrong sure. 4.0L 6 banger? 5million sold?
how many miles on it. 100k 300 , 500k?
is this that funky RENIX EFI system (french)
no scan tools work on it!
no CEL lamp? in cluster like my 88? (horrid)
if yes, good luck fixing RENIX !

so when does it misfire.?
only at idle? or? when?
hot ,cold, both, idle, driving slow, fast, up hills what?
engine load vary
as does EFI modes hot and cold
so.... all that matters big time in diagnosis;

misfire can be>
bad spark
bad fueling
bad compression on 1 or all.

fuel filter clean would top least.
can burn fuel it poor spark
then fueling
EGR is missing on 90 engine, (a blessing if true, it cant leak)
on mine the exhaust manifold love to crack and 02 goes nuts
with that crack sicking air. bad to the bone that.
MAP dont fail on this car, very rear, and is top fuel injection device.
the MAP is used to calculate air flow, (indirectly) and if its reads wrong fueling will be wrong, accelerating,

water leaks to the combustion chamber will cause misfire
i never skip compression tests. on 1990 old dog engines.
burned valves> sticking? lots of ways for and actual engine
to mess up inside, cracked valve spring, on and on.
Vacuum steady 19" HG (plenum) not bouncing
clue 2?
if not look for engine problems.
those are basics, compression and vacuum.
gun the throttle see vacuum drop fast the release throttle and see
near 26inches fast. if not the CAT is blocked up or melted.
my car had brick cat that love to break up in 2 new 1988s.
repeats over and over. (and exhaust pipe under oil pan the clamp breaks allowing the cat to shake like a wet dog. and fail)
i designed my own clamp for cure.

misfire only means bad combustion, its not just about spark
at all.
spark is only one of 3 major factors.
compression good, fueling, and spark.
and no water leaks. (bad head gasket etc)

I got lots more on 4.0 L. owned 3 for 500k miles total. so....

May be a bad fuel pump delivering fuel pressure but not enough volume ... Getting a lean missfire because of lack of fuel?