Ok, My truck has been good to me 99% of its life...It now has 140,000 miles on it..here goes..it is a 5 speed manual transmission...It has recently started this weird stumble when accelerating..doesn't matter what gear..It first started in lower RPM run ups...since then it has become a problem when going highway speed and trying to pass someone etc...It will kick and buck and then spit a little and takes off...It almost felt like a clutch problem ( it is not throwing codes) but I had it checked...worn a little..but fine.. I dont have any check engine lights..and the only time it seems to run halfway decent is when I run an injector cleaner in the fuel..but then...right back to normal...ok..now..here is the catch...I have changed the plugs..and wires....coil packs....idle air control....fuel filter...etc...all been changed...and still...this kick and bang crap...oh yeah..the gas mileage is fluctuating like crazy...goes from 19-20 mpg..down to 10-12mpg overnight!..any ideas?...please email me @ mrcessna_js@hotmail.com

 

I think I might go spend the 65 bucks to get the injectors professionally cleaned.

Have you cleaned the MAF yet?

 

Have you ever replaced the up stream O2 sensors?

 

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I feel your pain.

FWIW, my '99 4.2 was recently was acting similar (except the mileage never got quite that bad). It has 120,000 miles, and I had changed the plugs at 60,000 and the plugs and wires at 100,000. O2 sensors had been replaced at about 100,000 miles.

At first it would run OK most of the time, but the poor running started happening more and more, until it was running weak all the time. Even then, it would not put out a code. I could not believe it would run that bad and not show any codes. I tried alot of the things you tried, with no luck. I was pulling my hair out trying to diagnose this thing, as nothing seemed to work and there was no code to help sort it out.

It finally put out a code after a 160 mile drive on the Highway. By then the motor would barely idle and was way down on power everywhere. It was a misfire on #6. Replaced the Bosch spark plug (which had no electrode left with only 20,000 miles on it!) with a new Motorcraft plug and it has run great ever since.