I have a 97 stx ranger with the 4.0 ohv configuration and the idle is rough and weird and has an abnormal shake between the 1000-1600rpm range and engine light flashes for a misfire when cruising between that range also. Under full throttle it has great power but the second it hits 4500 rpm power plummets to where I notice it and occasionally pops under acceleration if I go over 4500rpm. There is a lean code and a misfire for cylinder one and I listened to all the injectors and they sound completely fine. All the ignition related systems are brand new and listening to the lifters they sound great too. The IAS is unplugged because it likes to levitate the idle rpm at 1500 and very slowly let's it go down to 800rpm so I keep it unplugged since I drive stick. If I cold start it will stall if it's not warm and when I let it idle there is a knocking not a rattle but sounds like it's around the bell housing area. My father seems to think and keeps drilling into me that it has a bad timing chain considering it has 197k miles but from all the diagnosing it keeps leading me to a bad injector because of the low rpm lean condition and the high rpm rich condition but thought I'd seek some other opinions before I tear apart the front end of motor apart....

 

Welcome to the forum

1990 to 2000 4.0l OHV engine never had timing chain issue, the Ranger 2001 to 2004 4.0l SOHC engines did

Double check firing order on coil pack and follow each wire to its spark plug
[3 4]
[2 6]
[1 5]
Front

Coil pack has "matched sets", 1/5, 2/6, and 3/4, the 5 6 4 side can get goofed up, on the coil and on the spark plugs

Plug IAC Valve back in, that's why it is stalling cold
All engines need to be "Choked" when cold, or they will stall, "Choke" means Rich fuel mix with HIGH idle and advance spark

Your warm idle SHOULD stay above 1,000rpms until you are stopped, well under 5MPH, its an emissions thing
1997 Target Idle in computer is 700, so IAC Valve was not the issue

If engine didn't stall when you unplugged the IAC Valve then you have a vacuum leak, you will need to find it
Vacuum leak also causes Lean codes and misfires

With engine warmed up and IAC Valve unplugged, rpms should drop to 500, or engine can stall, either is GOOD, it means no vacuum leaks
There is a anti-diesel screw on the throttle linkage, "looks like" an idle screw, but every one knows you can not use an idle screw on a fuel injected engine, must use an air valve like the IAC
If someone adjusted this screw then that would cause a problem
needs to be set so engine barely runs when warm with IAC Valve unplugged

 

The IAC doesn't let the rpm drop to idle it will hover at 900-1200rpm even if I'm dead stopped and will try to sit at 1500 even if the motor is fully warmed up. It also forces the rpm to hover so when I shift the entire truck lurches forward. When the IAC is unplugged it does not lurches and the rpm doesn't hover when I let off the throttle. There are no vacuum leaks I had it tested and no smoke is visible anywhere. If I were to leave the truck in gear and engine brake starting at 4krpm it's normal but the second it hits 2k it's like it's pretty much just coasting instead of slowing down like it should

 

It's been raining and I do not have a space in my garage and I just retraced my could and plug wires and all is good and they are all wired correctly so there hasn't been a mix up in the wires

 

IAC Valve is operated by the computer, so either the IAC Valve is broken or the computer is those are the only 2 choices

Engine can not operate effectively without a working IAC Valve, or a working computer, lol

You can call a Ford dealer, give them your VIN and see if there is an update to fix an IAC valve operation error, there have been a few of those over the years

Since you are having other issues besides IAC valve I would pull out the computer, check connector for corrosion, and open up the top of computer, see if there are any "hot spots", discolorations
These computer are pretty bullet proof, considering EVERY Ford has one, and failures are rare, but there are failures

There are at least 2 strikes against the computer
One is the high idle with IAC Valve plugged in
Next is that there is NO CODE about the high idle with IAC Valve plugged in

Computer should see RPMs are high and so it would lower voltage to IAC Valve to lower RPMs
If RPMs don't go down enough the computer will set a Code
P0505 Idle Control System Malfunction
P0506 Idle Control System RPM Lower Than Expected
P0507 Idle Control System RPM Higher Than Expected

and thats not happening