I just bought a 2000 ranger 4.0 4x4 manual, and let me start out by saying it has been great so far. My issue however is that the idle refuses to go down at all when I'm moving, and only when I'm at a complete stop does it slowly creep down. My IAC valve looks brand new and on the inside there is no buildup. If i unplug it, my idle goes down and works fantastic. What's the issue?

 

When you unplug the iac valve your motor should almost stall out or stall completely. If it doesnt you have a vacuum leak causing the high idle. Start checking lines for cracks.

 

Welcome to the forum

After engine warms up and you unplug the IAC Valve the RPMs should drop to 500 or engine may even stall, either is good

It shouldn't "work perfect"

Someone may have messed with anti-diesel screw on the throttle linkage
It "looks like" an idle screw but everyone knows fuel injected engines can't use an idle screw right?

So you need to adjust that screw, warned up engine, IAC Valve unplugged
Turn screw counter clockwise and RPMs should go down
Keep turning until engine is barely running, under 600rpms.
This also adjust TPS(throttle position sensor)

Plug IAC valve back in, RPMs should go high again, thats OK

Now to get computer to relearn IAC and TPS you need to unhook the battery.
Unhook the positive or negative cable from the battery, whichever is easier
Then touch that unhooked cable to the other cable, this will drain the capacitors in computer erasing its Learned memory
Also erases radio presets and clock, lol.

If you don't want to touch cables together, then leave battery unhooked for at least 5 min, capacitors will drain on their own.

Hook battery back up and start engine
Idle should vary/wander a bit as computer relearns IAC settings, that will happen of the next few drive cycles.

Target idle warned up V6 with manual trans should be about 700rpm
My 1994 is 625, but different emissions standards

On 1996 and up vehicles with manual transmissions the emissions requires the RPMs to stay a 1,000 or so until vehicle speed is under 5mph
Engines under 1,000rpms is a big polluter, so computer holds RPMs high until speed drops down.
RPMs should also stay high when shifting, for a few seconds, this is another emissions thing, RPMs will drop but only after say 5 seconds with clutch pedal down.

 

I may have not been too clear, my apologies. With the IAC unplugged, the engine will idle at about 500 rpms when warm. If it's cold, it does stall out. By perfect, I basically meant it drove way better than it does with the IAC plugged in, at least when it's warm. Also, is there a diagram I can look up that shows all the vacuum lines?

 

There should be a label in engine bay with vacuum diagram, thats as good as they get, lol.

So what is the idle RPMs after warmup, IAC Valve plugged in?

And are the idle RPMs higher when engine is cold, should be?

 

 

Actually sounds normal but a bit off.

Pop by a Ford Dealer with VIN in hand and see if there is a software update for the compurter
If I remember correctly the 4cyl and 3.0l around 2000-2002 had high idle issue and there was a software upgrade

 

FWIW, my truck stays at about 1,200 rpm until a stop. Perfectly normal, it's not even using fuel most of the time at that point. It wont drop to 750 unless I come to a complete stop.

 

Yes, the 1,200rpm no load idle would have lower emissions than 750rpm idle, and its ALL ABOUT emissions since the late 1960's

 

 

No won't hurt anything.
Cold starts will be an issue, no high idle so will stall as weather gets colder

Check engine light(CEL) will come on eventually, letting you know IAC Valve is not responding to computer control

MAF just reads air flow so can cause bogging or lean/rich codes but wouldn't effect idle RPMs if IAC Valve is working
A vacuum leak can cause higher RPMs but unplugging IAC Valve would show that because RPMs wouldn't go down, they would stay high because of the vacuum leak

You are fighting a computer program to lower emissions.
You can send out these computers to have them programmed to your liking, call around to local "speed shops" and ask if there is anybody local who can program your 2000 Ford PCM(computer).
I don't know of any, but they do advertise on ebay