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Discussion Starter #1
Working on my '92 F150 with the 4.9L engine and have been getting an occasional miss in the motor. Not sure if it is fuel or spark related, but when it happens the truck starts jerking and kicking and bucking. Seem to do it mainly when the engine is under a load, like going up long hills. It has never stalled yet and I just can't figure out what is causing it. It runs fine around town under light throttle and never misses. I checked the codes and everything passes.

I have changed the following parts trying to fix this issue, but nothing has made a difference yet:

Fuel filter
Fuel pressure regulator
Spark plugs
Spark plug wires
Cap and rotor
Coil
Distributor coil pick-up
Ignition module
Battery

I have checked the fuel pressure and it tests fine. I have dual fuel tanks and the problem happens on either tank, so I know it isn't a pump problem. I am at a loss for what is causing this and tired of throwing parts at it in the hopes of fixing it. Figured I would check here for any suggestions before I bite the bullet and take it to a dealer for diagnosis.

Was thinking maybe clogged injectors, but then wouldn't it run bad all the time? Was also thinking maybe a plugged cat convertor, but when on flat ground I can mat the gas pedal and it speeds up just fine with no missing.

Without any codes from the computer, I don't have a clue on what to test next. Open to any and all suggestions.
 

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Discussion Starter #2
Well I must have stumped everybody since I got no responses. Think that I figured out what the problem was though. The running bad all started shortly after I changed the spark plugs a few weeks ago. I have never had bad spark plugs before, so it didn't even cross my mind that they could be the problem. Just for the hell of it, I decided to buy another set of plugs and see if it made any difference. Haven't had a single problem since I put new plugs in. Just strange to me that the bad plugs were only bad when the engine was under a load. Live and learn I guess.
 

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Could be the gas too. In mine, I noticed this when idling in 2nd after my dad had a can of 93 he needed to get rid of. 1 gallon in 20 gallons of 87 made the fuel too good for the I6. So, make sure it isn't a bad batch of 87 or anything higher than 87.
 

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Discussion Starter #6
No problem Gary. It has been fixed for a month and a half now. Just happen to get a bad batch of plugs is all.
 
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