2.5L ('98-'01) Idles down roughly.

[Housem970]

My 99 Ranger 2.5L starts up and sometimes idles poorly but most of the time it idles just fine after initial startup. However almost always it will start missing while idling down and then continue for a bit at idle then smooth back out. It doesn't seem to matter if I punch the throttle and let off quickly or do a smooth ramp up and back down slowly. Sometimes when I shut it off (like while getting gas) then turn it back on it will run really good and it feels like it's making way more power than normal mid way through the rpm range. It feels like hitting the power band on an old 2 stroke Honda dirt bike. However when it starts running like this it will randomly "run away" when I push in the clutch sometimes it will catch itself and idle down, but sometimes it just keeps revving higher. It only does this after a hot start just running it after the initial cold start it will not behave this way no matter how long I run it until I shut it off and restart it.
I have done a bleed down test.
Plugs, wires, and coil packs.
Cleaned old IAC then later replaced it.
Throttle position sensor.
Two different MAF sensors.
Timing belt, water pump, and idler pulley.
Looked at the camshaft lobes, the valves, and lifters.
EGR deleted
New upstream 02 sensor.
Sprayed every visible component of the vacuum system with starting fluid with the truck running.
It has no check engine codes other than for the EGR.

 

[franklin2]

Have you cleaned the throttle body? I know Ford and others say do not clean it, but what do you do when it has brown sticky crap all over it?

Once you clean it really good, try it. If it is still acting up, get the engine fully warmed up, and with it idling, unplug the IAC valve. What does the engine do? The idle should drop way down, till it sounds like it might quit but it doesn't. if it quits right away, the throttle blade stop screw needs to opened a little bit. If it keeps running fast, the idle screw needs to be screwed back a little bit.

I would mark the screw where it is now before you turned it any. The IAC needs to work in it's designed range of movement. A portion of the idle air goes by the throttle blade, and the rest goes by the IAC. The portions of air through each need to be correct for it to control the idle speed under the different conditions the engine sees.