sorry, realized i put this in the wrong forum.
ok, my brother got a 97 ranger, 4o auto. was complaing about gas mileage, 14mpg around town. figured to do tune. replaced plugs, air filter clean, looks new, went to replace 02 sensor just due to age. got drivers side and post cat changed, but pass side stuck in pipe and didn't have time to drop pipe to get it out. since that truck now gets 17-18 in town, 22 hiway. BUT, when accelerating at or above 2500-3000 there is a major bucking, feels like a misfire. or maybe ttrans lockup and out but very quick. no codes are stored. i thought we got the air filter/intake tube on crooked, and leaking but not that. engine idles fine, runs ok at light throttle, but accelerate hard or hit a grade on the freeway and this thing just stutters badly. you can keep your foot in it and it will clear out once revs climb. or back off and stutter goes away. was hoping someone may have an idea. i was thinking maybe tps? but i live 200 mile from my brother so hard to diagnose. will see him on turkey day, so.... any ideas out there? thanks for any advise/info -. mileage about 128k, previous owner had "lower intake gasket" or something like that replace according to records in glove box. ing wires, etc look ok, but did not meter test them. have cleaned the maf, ck all intake bolts=ok, tried plugging egr vac tube, as recommended by ron d. no luck. any and all other suggestions welcomed. thanks,jp

 

Just went through the same scenario in my 2002 3.0. I read everything I could find on this forum, the majority cure was the plugs. I replaced the brand new Autolite with Motorcraft. The truck runs like a new truck again.

 

thanks, that is the next thiing to try this weekend.

 

Run some Seafoam through it before you change the plugs.