Checked out the diagram of the IAC in the photos you attached. I doesn't look like it is a solenoid type of actuator. I think it is more of a stepper motor. There are two separate coils (remember 6 wires...). It seems that computer would control the motor to shorten/lengthen the extension of the rod. The idle speed is controlled by how far the rod is extended. While I was trrying to understand how this system operates, I stumbled onto a description of problems with IACs. One of them says - "Air leaks in either the stepper housing or pipes will cause elevated idle RPM." Seems to me like the situation you are having. The problem is that you got the same malfunction with both new and old IAC. So, not quite sure if this fully applies to the current situation.
Couple other things came up: vacuum leaks and faulty TPS.
Is your Check Engine Light ON? You said it you think your Montero has OBD1 system in it. On those older systems you can't pull codes by plugging OBDII scanner, but you should be able to ground one of the pins on the data connector and the CEL light will flash a two digit code for whatever code is stored in the memory. Please check that out first. If you got an issue with some other sensor, or system, the high idle could be a byproduct of it, instead of a primary issue.

 

Yep, the IAC is some type of stepper motor instead of a plain solenoid.....so it can vary how far the IAC rod goes back and forth. The further the rod extends, the more it shuts off the bypass air to the intake manifold, and the slower the idle.

Air Leaks.....yep, could be a small leak someplace, especially with all those vacuum hoses. Guess I should crank her up, and while she idles at 1200RPM, I could wiggle/move all the vacuum lines and look for RPM change.

Engine Light is not on, no "check engine" light, so it seems that the computer is not sending a signal to alert of any problems.
OBD1 or OBD2......Im still not sure. I read online somplace that the 94 should be an OBD2, but the reader I have does not access anything. The plug under the dash does fit the OBD2 reader plug, so does that mean that it is OBD2........or did OBD1 have the same type of plug?

Not sure what a CEL light is. Is that a light in the dash that will flash that I can count the flashes and find out on a chart what it means?

Boy, alot of head scratching for something which I will bet is a simple fix.

 

I've been reading this thread for awhile but don't recall any mention of checking the plenum or intake manifold gaskets, my apologies if you did and I missed it. I know mine has vibrated loose a bolt or two in various areas until I started putting a dab of loctite on the threads.

 

Naeos......Thanks for reading all my posts and making a suggestion. You could be right, as I never tried to remove the large intake plenum. I just removed the throttle body and related components, and left the plenum in place. I wonder if I could spray WD40 all around the plenum as the engine is running to see if that makes any RPM change.....if so, that would mean there is a leak and you would be 100% right.

How much of a job was it to remove the plenum, after the throttle body is off?

 

It should be easy enough to remove plenum. You have to disconnect TB from it if you are removing it anyway. I think starter fluid may work better as a detecting agent for vacuum leaks.

 

Hunter......OK. I just did not want any fires going on by using starting fluid......guess I could try the WD40 first, and if no idle change, then pump up the volatility with the starter fluid.

On another note, I was studying the Shop Manual a bit and came to the section "Special Tools". It shows a Scan Tool (MUT-II) and a part number MB991502, and says "Checking of the diagnosis code". Do you think this "MUT-II" is the same as "OBDII", and any OBDII scan tool would work? Which makes me wonder why the OBDII scanner I tried failed to pull codes on either of my 94 Monties. I did a google search on the Mitsubishi MUT-II scan tool, but could not find anything. I would like to find one that works on the 94 Monties.......any suggestions?

 

MB991502 is an OBDII scantool on steroids. It is capable of reading codes, activating individual relays, components and re-flashing. PCM. Last time I looked into buying it, it was over $1k. The fact that your Montero is "refusing" to communicate with a standard OBDII scanner, makes me wonder if there is some issue with the PCM. I have no experience troubleshooting those, but it doesn't mean I will not look in to the "how to" for your brnefit
Do you have access to another scanner in order to eliminatevscanner malfunction?

 

Hunter......Thats interesting, so this MB991502/MUT II Scan Tool is an "OBDII".......and thus my 94 should allow any OBDII scanner to work. I think I mentioned in an earlier post that I plugged the OBDii scan tool into both of my 94 Montero SRs, and got the same results (said something about not being able to access the data and to call technician). So, maybe both Monteros are fine, and I am just using a bad "el cheapo" scan tool. Since it is borrowed from my brother, guess I need to go ahead and buy a new one. I found a Mitsubishi one being sold by "TUI-21" (whoever that is), and sent an email to them to verify that it will work on my 94 Monty. That scanner is:
Its only $77. Do you think this is a good choice, or can suggest a better one?

Memoscan MITSUBISHI Professional OBD2 Scanner Tool M608

http://obd2.tui21.com/items/car-diagnostic-tool/memoscan-mitsubishi-professional-obd2-scanner-tool-m608.html

But another question maybe you can answer: I have no Check Engine light on, but with this Idle problem, shouldnt the computer be sending a signal to turn on the Check Engine Light?

 

I guess first we have to figure out if '94 is OBDI or OBDII. Take a look at this site:
https://www.mitsubi****echinfo.com/epacarb/default.jsp
It's says - "assess to all '95 and later data". That makes me wonder if '94 was a changeover year. On the other hand, is possible that '94 is OBDII, since some manufacturers went there sooner, while 95/96 was when all vehicles were required to use this system.

As far as the scantool you listed, that is not It. Sorry man, but that's not even close. It is just a cheapo chinezium code reader.

I think there is one for sale here:
MB991502 | MULTI USE TESTER (Mitsubishi) | 1654.10 € | Mitsubishi

Unfortunately, there is not much for description, but the price is in the right ballpark. It is listed as about 1600 Euros...

I think. MB991502 is the same tool that is referenced in the FSM for my Montero. Could it be that it is just some "magic" stuff that can do both OBD I/II? I don't know.
The fact that your chech engine light is not on is surprizing. Does the light come ON when you first turn on ignition? This is a chech procedure to see if bulb is not out or disconnected.

 

Hunter.....That link did not work.....I guess because of the 4 "****" in the words. I tried changing to ...mitsubi****echinfo...., but still did not work.

Do you think the OBD 1 and OBD2 used the same size plug? I will have to do more digging regarding if it is OBD1 or 2, but thought that the Shop Manual code reader part number was for OBD2 since it called it "MUT-II".

The scantool that I found appeared to be made by Mitsubishi, so I thought it would be good quality.....maybe like you said, it is just a Shinnezium cheapo. You were right earlier regarding the price of the "true blue" one....way over $1000. (not in my wee little budget). Guess I could drive the rig to Advance Auto and ask the guy to plug in his code reader......surely he has both OBD 1 and OBD 2 code readers, and can tell me which type it is, and maybe retrieve some defective codes..

Check Engine Light....Yep, it comes on for a few seconds and then goes off.......so it has a good bulb.