Check vacuum, also try unplugging the IAC valve to see if the idle changes at all. If you have a vacuum leak an easy way to find it is to spray Carb cleaner on or around all the lines to see if the idle changes. I doubt its a distributor cap.

 

Tried the new dist cap, didn't change anything.
Unplugged the IAC while it was idling and it shut it off.
Sucked on the EGR vacuum while it was idling and it shut it off as well.
Sprayed some carb cleaner to check for leaks, didn't change the idle.

It seems to idle smoother, try and rev it up and it misses and shakes like a sob. Hold it at 1400 rpm and the needle will go up and down continously between 1400 and 2000rpm. Tried driving it up the road and you have to floor it just to get it to move.

 

Check your distributor shaft for worn bushings. A good way to check is hook up a timing light and rev it up - the timing should advance smoothly - if it jumps around or gets intermittent, that's probably what it is.

 

Well, we're in the process now of taking the valve cover off. My dad is thinking there may be a bad/dirty injector, since only one spark plug got dirty and the rest were clean. Pinpointed it to cylinder 5 where the plug was at. I'll check the timing and bushings as soon as I can, though. Thank you for the continued help everyone. I'll update after we check all of that.

 

Took a while to get back on here, not sure if anyone is still interested in this problem lol. I ended up replacing crankcase breather filter, fuel filter, replaced the fuel pressure regulator, replaced ignition coil, replaced PCV valve, replaced ALL of the fuel injectors, and checked the timing.

Didn't help... Took the catalytic converters off earlier to see if maybe they were blocked, and that did not help either. If I try to accelerate, it will hesitiate a bit, then zoom up, and drop to very low RPMS like it is going to die.

Seems impossible to fix...... I made a short, small video to kind of show how it idles and accelerates.

http://www.youtube.com/watch?v=8TW1VrOGQQ0

 

Bump.... I got the new catalytic converter on, as well as double checked the vacuum pressure, it somewhat hovers around 16. Yet it's still having the same problem I showed in the video. Strong "miss" or "hesitation" on accelerating..

Any more advice is much appreciated, I've tried everything that everyone mentioned thus far, so I'm doing my best! thank you.

 

psalmthirteen,

I was wondering if you ever got the problem fixed. I am experiencing a similar situation with my '90 F150.

 

iIv been having the same issues described above and was wondering if anyone has check the Thermistor system (has to do with the Smog pump / air pump)? I have not yet checked mine and its about all I haven't done, like most others. Iv seen this mentioned in other Forums before, but have never seen anyone solve the issue by checking or replacing subcomponents of the thermistor system. If you did solve the issue please post back. thanks

 

I'm willing to put money on it being the valve right behind the intake manifold, that is difficult to reach. I'm thinking due to it being close to the fire wall behind the manifold, it gets overlooked..