Testing the Volume Air Flow Sensor
I guess since I started this mess of a thread I'd better chime in with something semi-profitable after dealing with these issues for the last two years. Here goes...
Regarding the Volume Airflow Sensor (Mitsu's name for the MAF):
First, it may or may not be the cause of your issue, and unless you put it to the test you're gonna throw a lot of money at the idle gremlins. The sensor is actually a combination of three different sensor readings that combine to tell the PCM how much combustible air is entering the throttle body. There are tests for all three readings.
Testing the Volume Air Flow Sensor itself (see FSM 13Ac-2 thru 17):
You'll need some kind of scanner that reads live data to test this one. It is Item #12 on the universal scanner list. With the engine running and warmed up to normal operating temperature you should get between 17 and 43 Hz at idle, between 64 and 104 Hz at 2,500 rpm, and the Hz should increase smoothly with engine speed.
Testing the Barometric Pressure Sensor (see FSM 13Ac-17 thru 43):
This test will also involve using the scan tool. It is Item #25 on the list. With the ignition on, record the kPa or psi or whatever pressure reading your scanner uses. Standard values should be 101 kPa at 0m, 95 kPa at 600m, 88 kPa at 1,200m (you'll likely need to do the unit conversions). I found my Baro sensor to be quite off. At 184', the exact altitude in my driveway, I was getting 14.2 psi (or 97.9 kPa at 56m)...basically it was telling my PCM that my Monty is sitting at around 600m when it was actually only at 56m...so it was expecting thinner air than it was actually getting.
Testing the Intake Air Temperature Sensor (see FSM 13Aa-17, 13Ac-44 thru 60):
This test has two parts. The first is to again read the live data on the scan tool. It is Item #13 and should match the actual temperature at the sensor (read with a thermometer of some sort). The second part involves reading the resistance between terminals #5 and #6 while raising the temperature with a heat gun of some sort. Standard values should be 2.3-3.0 kiloohms at 68*F, 1.0-1.5 kiloohms at 104*F, 0.56-0.76 kiloohms at 140*F. When I tested my IAT the values were on the edge of the ranges.


Conclusion: well, if you test these and find any or all sensors to be grossly misreporting data to your PCM then you may have a problem. In my case my BARO reading was off, but I doubt that it was a major factor or the only factor in my idle issues (for reasons I'll explain later). But in order to truly understand what kind of problem you have you need to understand how to listen to your 21st century vehicle because this honestly may not be your only or most significant problem. YOU MUST HAVE A SCAN TOOL OF SOME SORT IF YOU PLAN TO DIY YOUR GEN3 MONTERO...period. You do not need the MUT-II or III thingy from Mitsu HQ though. Almost any scan tool that reads live data will help you with 90% of your diy diagnosis work...especially fuel trims.
I broke down and got a decently priced scan tool after realizing there was no way around this...and it was cheaper than throwing a few sensors at the issue with my fingers crossed. I ruled out the EGR, the Throttle Body Actuator, the O2 Sensors, the Coolant Temp Sensor, and a few other expensive variables by diagnosing their health with a scanner (and the FSM). By reading and interpreting fuel trims I learned that I had worse problems that a dirty VAF or loopy Barometric Pressure Sensor. My Long Term Trims were pegged at +25% (no up or down), meaning my engine was running super lean (way too much air reporting to the O2 Sensors). My problem was that air was entering the combustion cycle somewhere after the throttle plate, primarily through a cracked exhaust manifold and dry-rotted vacuum hoses and honestly who knows what else (probably some gaskets too). And sadly, those aren't my only problems so I can't even isolate and whack the moles one by one (also have a bad alternator covered in oil, leaky cam seal, weak battery, a shortage of time & money, poorly installed timing belt by PO's mechanics, mismatched spark plugs by the same hacks, and on and on...smh, what a year it's been.)

My Monty is still sitting in a few dozen pieces (it was a few hundred a week or two ago...yay for progress), so I still haven't figured everything out or fixed all my gremlins. I just know that in my experience there are always more than one gremlin at work at the same time.