HISTORY OF MY PROBLEM:
Ok I drive a 93 Dak, with the 3.9 and manual transmission. It has a jet performance chip, and magnaflow muffler. Last week, I came home and changed my oil. Let the truck run for about 10, 15 min after and it ran like normal. no problems. I then wash the body, and then applied boot grease to all spark plugs (i had forgot when I replaced plugs a month back). So I then go to start it, and the idle races to like 2k rpms, so I turn it off. Try again and same thing. Then It won't even start. The next morning I go out and it starts, but dies after 5 sec. Then won't start again, or at least won't run for more than a sec. The fuel pump would prime when turning the key on by the way. So the next day I go out and the thing starts and runs, but something is def wrong. It runs like crap. Not terrible, but not 100%. So I check my codes, and I'm getting a o2 code (21), and lean fuel code (51). So I replace the o2 sensor (they could of put the plug in a friendlier position lol) And the truck ran a ton better, but then starts to run crappy again. Racing idle, and a stutter. So I then replace the PCV valve, the crankcase breather, the IAC, and the Throttle position sensor.

WHERE I'M AT NOW:
So my problem now is that when I first start it cold the idle is at around 1100 rpms, and I can hear a slight stutter every sec or so. Then if I turn off the truck and restart it, it'll idle normal (still with the slight stutter though). As long as the truck is warm it seems to start up like normal, but when I start it cold the idle is too high, and I have to turn it off and back on. And the stutter is there all the time. I really can't feel a loss of power but like when I'm going thru town, i can feel it jerking a little bit. I can feel it at higher rpm's too, just not as bad. This is really getting to me, since my problems started on tue (out of the blue) and it's now sunday, and I've spent a bunch of money already it seems, and my truck is not back to normal. Any ideas, or help would be greatly appreciated! Thanks

 

When you replaced the IAC, did you clean the port where it goes into the throttle body?

Replacing the O2 sensor and it running better -- while the engine is cold -- was a fluke. The computer ignores the O2 sensor during open loop (engine cold) operation.
As per the service manual the following are also open-loop operating scenarios: ignition switch On, engine start-up (crank), engine warm-up, acceleration, deceleration, and wide open throttle.
Idle and cruise modes (with engine at operating temperature) are the closed loop modes.

I would recommend cleaning the port for the IAC on the throttle body, and taking a look at the electrical connector for the IAC (corrosion, broken/brittle wires), and at the engine computer for the same.
I don't know why the problem would apparently go away after turning the truck off and restarting it.

You could also try replacing the engine coolant temperature sensor (located by the upper radiator hose; there is two of them, this one is on the drivers side of the hose, and has 2 wires on the electrical connector).

 

Thanks for your reply. I cleaned the port when I replaced the iac. And when I replaced the o2 sensor, the truck did run a lot better. I can't remember what it ran like cold, but I haven't thrown those codes again. The connections to the IAC looked good, no sign for alarm. I will check at the computer in a few for any corrosion and wire trouble. Also my timing chain has made noise ever since I got the truck, and it might be a little worse now. I don't know if this could cause some problems. If the timing chain was wore or something?

edit: just went out and didn't see any wire problems @ the computer, but it took awhile to start. Normally my truck roars to life with the turn of the key, no foot on the pedal needed. I usually don't even get in the truck to start it. And just now the idle was at 1100rpms, and surging. It actually kinda dropped a little as it surged, but not bellow 900 rpms. The only code i'm throwing is 45, and I read that's normal for our trucks. My next step, i guess, is to change the plugs, wires, cap and rotor. I'm running out of ideas...