It's may be the tfi module but it is not likely. They usually work or they don't start. A scan tool is probably not going to help you much here if you're engine light isn't on as 90% of obd I codes turn it on. If it kinda bucks and jerks down the highway at medium rpm with you're foot lightly on the throttle it is likely a weak spark problem.
(This is because the combustion pressures are highest in this conditions and the fuel mixtures are very lean which makes the mixture harder to ignite so if you have a spark problem it will show up at this time. Also if the engine shakes at idle but smooths out as you rev it up suspect spark.)
If it runs very good at low rpm but doesn't miss until higher rpm (but not dependant on load) it may be weak valve springs. If it only misses under heavy load but performs better under light throttle conditions suspect fuel pressure/restriction problems or plugged exhaust.
A couple other pointers, If it doesn't have cats it's likely not a plugged exhaust, If you suspect plugged exhaust take an infared temp gun and monitor the exhaust, if there is a restriction heat will be built up until the restriction and then dissapate rapidly after (be careful as factory mufflers have engineered cooling systems built in) You can also plumb a resriction gauge into your O2 sensors bung to verify, You will also have very little exhaust flow from the tailpipe.
A couple things to do if I didn't have access to a scope is to remove the distributor cap and check the cap contacts and rotor for corrosion. ( be sure to number your plug wires as they will need to put back in the proper firing order) Also verify base engine timing. You will need to take out the shorting bar to check timing, If the engine seems to run better if the timing is retarded or advanced alot more than it should be suspect valve timing is out. (the timing chain stretched and or jumped a cog). Also pull all plugs and inspect.
You're spark plugs are the tattletale of an engine and will tell you alot. If they are white they are running lean, If they are black they are burning too much gas or oil, If the electrode is worn it's time for new ones, If one is wet that cylinder is missing. Check the resistance of all plug wires (a good rule of thumb is 3000 ohms per foot), Anything that will cause driveability problems will deviate substancially from that. I have seen twice now on chevy's were two cylinders next to each other in the cap fused together and caused crossfire (you could not tell by looking at the cap in any way). So now I ohm each cylinder to every other one to be sure. Be wary that OBDI is very primitive and will not even set a misfire code nevermind inform you of which cylinder it is.
If your diagnostic path leads you to believe fuel is the problem, buy a gauge they are relatively inexpensive and trust me it is alot easier to rule out first off than to wonder if it is a problem. Zip tie it to you're wiper and road test under misfire conditions to ensure proper fuel pressure. Also If you're plugs looked white it is possible you have a vacuum leak. A high idle and hissing noise is usually apparent with a vacuum leak. The source can be pinpointed by introducing a hydrocarbon (ie butane, propane ether) in suspected leak areas and observing engine for rpm change, If this occurs you have a vacuum leak causing it to run lean.
I have seen a few times where ect (engine coolant temperature) sensor is open causing the computer to think the engine is below freezing, The computer has a very rich mixture incorporated into it's tuning for cold start up and your engine will run like you left a choke on an old carbjob.
If you suspect you're maf is causing the problem unplug it. You're pcm/ecm will resort to failsafe fuel trims where it actually runs half decent and will not cause a miss, it may have a rolling idle but runs fine WOT. Hope this helps you. I better get back to work.
Certified Master Technician
Also FORD certified
-Auto Trans, Electrical and Electronics,Manual Trans and Differential,Diesel Engine Performance and Repair, Gasoline Engine Performance and Repair,Suspension, Climate Control and Brakes Specialist.