I recently bought a 1989 F150 with a 351W, C6, and 4x4. The truck fires up and idles like a new one. Sometimes it will try to stumble around once you put it into gear, but that is a rarity. The real problem comes in at highway speed. It will skip/misfire around 50-55mph if you try to hold speed. Under heavy acceleration it runs great with normal 351 power. I have felt the concern both when it's warm and cold, although I've noticed it more when it's warmed up. I thought a tune up would solve my problem, but to no avail. Here is a list of what I have done:

-motorcraft plugs gapped to 44
-ford racing 9mm wires
-motorcraft rotor
-new dist cap
-new fuel pressure regulator
-new 195 degree thermostat

This problem seems to be fairly common with these trucks... hopefully somebody can shed some light on it for me!

 

Could be a injector sticking. Couldn't hurt to put some lucas injector cleaner. Also does it jusr skip at a certain speed, or when your gas pedal is in a certain spot?

 

I have ran a double dose of Lucas injector cleaner through it. I think you're going towards a TPS with the gas pedal position idea, but I hope that ain't it cause those ain't cheap! It seems like it does it worst if you're trying to hold your speed with the trans in high gear (3Rd) without it shifting down into second.

 

Throttle position sensor are about $35. Not too bad, but aggravating to replace. If you have to original TPS it wouldn't hurt to replace.

Also, is the fire coming from the coil nice and strong?

 

I replaced the coil tonight after work, made no difference...

 

I did not realize you had two different posts for the same issue. My answer is here: https://www.ford-trucks.com/forums/1...p-misfire.html

 

I believe there are 3 threads.

 

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