I recently ran into an issue with my 1994 Ranger (4.0 5-speed manual 4x4) Misfiring, so I changed the spark plugs and wires. All was running smooth for about 5-6 hours. Drove it maybe 30 miles, not exceeding 55 mph, then it started running like crap again. I think I'll do a compression test on all cylinders to see if it's worn cylinder walls/ piston rings. I think I should also mention the truck is all original and the odo reads 255,655.

Edit: I left out a few details I think would help diagnose the problem... The spark plugs were almost all terribly fouled and were changed roughly 3 years ago. By "running like crap," I mean rough idle, shaking, and misfiring.

Any help would be greatly appreciated!

 

Welcome to the forum

1994 computer will need to have its 3 Capacitors changed sooner than later, just a heads up
The Blue ones seen here: Ford EEC-IV
Under $5 fix, if you or a friend can solder

Will cause rough running when they start to fail


4.0l needs a large spark plug gap, 0.054"
And they "eat" spark plugs unless you use Motorcraft, Autolite or NGK brands

Compression test is always a good idea, my 1994 4.0l with 400k still shows 150-155psi, last test was 2 years ago when my computers Caps failed, lol and I had the rough running
Changed the caps and its been running great ever since

Also O2 sensors, after 12 years or 100k miles, they will cause computer to run engine richer and richer for several years until they finally set a code and they will cause spark plugs to foul from the rich mix, and you could have bought at least 10 O2 sensor by then for the money wasted on fuel, lol
If your MPG is not that great and you don't know how old the 2 O2 sensors are change them