I have a 91 4.0 4x4 with 1500-2000 rpm idle. The eng runs smooth, plenty of power. After driving some, idle comes down generally to 13-1500. While shifting, as i depress clutch and come off accelerator to shift, rpm rise to 1800-2000. When in neutral, I apply wot, rpm dont drop immediately, rather it rises then settles to 1500. I have changed all sensors. I have verified connection between sensor and ecu. I have verified no vac leaks. I have tried to choke off air before maf results in stumble and stall. I have verified fuel psi, disconnect a/c with no change. Disconnected all vac lines and plugged. Disconnected iac, removed and plugged port, verified butterfly set screw with tps voltage. I just checked o2 plug. 12v for heater and data wire to ecu, also gnd. I ordered new o2 just to check that off. For all accounts the eng runs great, just has high idle.

 

Welcome to the forum

Good testing

The only RPM control the ECU has is the IAC valve
Warm up the engine
Let it idle
Unplug the 2 wire connector on IAC Valve, this removes ECU control of the idle RPMs
RPMs should drop to about 500-600, barely running, or engine may even stall, either is good, means no air/vacuum leaks

If RPMs do drop then the ECU is setting the high RPMs, so it has an issue, pull it out and open it up, check for damage and leaking capacitors, common issue after 20+ years

If RPMs stay high then ECU has no control of IAC Valve or engine RPMs
Could be stuck open IAC Valve, or vacuum leak
IAC Valve gets 12volts(battery volts) on its red wire with key on
The other wire goes to ECU as the Control wire, pulsed Ground wire
With key on engine off you should see about 11-12volts between the 2 wires, ECU is opening IAC Valve all the way for start up
After start up you should read 6-8volts as ECU closes IAC valve to set idle RPM, higher idle/voltage with cold engine then lower as engine warms up

Motorcraft or Hitachi IAC Valves are the only brands that "play well" with Ford ECUs, Ford used true Solenoid Valves, 3rd party are often solenoid/stepper valves

There is an Anti-Diesel screw on the throttle linkage, looks like an "idle screw" but no such thing on a fuel injected engine because there are no Jets, but with IAC valve unplugged unscrew it and see if idle starts to drop if not reset it where it was
TPS sensor can hold throttle plate open, remove it and see if idle drops

If idle stays high after unplugging the IAC valve and you don't think its the IAC Valve or throttle plate then there is extra air coming in, bypassing the throttle plate, period
So a vacuum leak
Power brake booster and PCV valve hoses are the largest hoses for vacuum
With IAC Valve still unplugged start pulling hoses one at a time and blocking the intake port with a finger
If none of the hoses/devices are leaking air then get a spray bottle of soapy water and start spraying it around the intake areas, lay it on thick, worst case is a cleaner engine, lol

As a LONG SHOT
Under/behind the alternator on the lower intake is the FPR(fuel pressure regulator), it has the Fuel Return hose attached and a Vacuum hose, if the FPR has a small leak fuel can be sucked in to that vacuum hose....................and into the intake, but noticeable drop on MPG

 

Thank you for the quick responds. I have done most if not all. I have changed iac, and esu out. I have used propane with hose to follow the intake edge and all vac hoses. Ive removed iac plug and tested wires and even plugged iac port and driven. I dont believe its a ford brand. I will try soap and see. This problem has existed for years. I think following an injector replacement. I had to remove intake 3time because i was sent a bad injector. So i replaced gasket on last install. Could be just coincidence as that was back 8+ years. Every so often i decide to tackle problem again. I suppose there could be an intake leak even though i can find it. Ill keep at it.

 

How does soapy water reveal an intake leak?
I thought soapy water was for leaking tires as escaping air will make bubbles.

 

Soapy water, well any water, lol, will cause engine to stumble if its sucked in, and safer than combustible sprays, soap is because my grandpa did that way, he always had a spray bottle of soapy water around to clean up stuff

So after warm up and idling
If you unplug the IAC Valve do the RPMs drop?

You said you can block the IAC Valve port, did RPMs drop?

Its very straight forward air = RPMs on gasoline engines
If you remove the air plenum(big air tube) from the upper intake and let engine idle then use a thicker piece of cardboard or ?? and slide it over the opening cutting down air flow into the engine, engine should of course die when enough air has been cut off

Check the Vacuum in intake, 18"-21" in Florida


Extreme long shot, check spark timing, but would expect other running issues at higher RPMs

 

I did unplug iac, no change.
i did remove iac and plug port, no change
i did disconnect intake hose after filter box, but before Maf. Then slowly closed off air until eng stumbled and almost stalled. My experience tells me when there is more air introduced than should, eng runs rough. By cutting off air manually rpm should rise and idle smooths out. Ive used propane in the past to find vac leaks. Same results, eng rpm rise and eng smooths. These techniques have not proven to locate any vac leaks now. If i had a vac leak thru intake or other, i should be experiencing rough idle and possibly too low rpm. Nothing makes sense. Im missing something. Ill try the soap.

 

The computer will add more fuel based on RPMs so not always rough, roughness can come from lower intake vacuum leak fouling up 1 or 2 cylinders air fuel mix

But what is surprising is no codes from the computer
1991 computer will use 2 digit codes, Ford list here: https://www.therangerstation.com/tec...it-ford-codes/

Code 13 for high idle

Also code 41 for long term Lean mix, meaning computer is having to add more fuel(longer injector open time) than it calculated

Unplug MAF sensor and see if computer sets code 26
Unplug ECT sensor and see if computer sets code 21
CEL(check engine light) should come
Its a "sanity" check on computer

You don't need an OBD1 reader, just a paper clip to put computer in test mode then count the CEL flashes