Mitsubishi Idle Air Control (IAC) valve ...
As our Mitsubishis age, parts get old and wear out. That is just a fact of life. A common problem that crops up is rough idle and the inability to pass the idle part of an emissions test. Frequently, this is due to a failing Idle Air Control valve (IAC) or Idle Speed Control valve (ISC) as it is also known. While this may not be the 'smokeing gun', it is certinly something to check. Especially on high miliage vehicles.
DSMISC Automotive has an excellent FAQ specifically addressing the 1990's through early 2000's Mitsubishis, such as the Montero, Montero Sport, Pajero, GT3000, Eclipse, and Galant as well as other non-Mitsubishi brand (but Mitsu under the hood) vehicles.
Besides causing your vehicle to fail emission testing, when the IAC valve fails it can damage the Engine Control Unit (ECU), otherwise known as the Engine Control Module (ECM). Ignoring it and tollerating poor idle quality isn't really an option. Replaing an IAC on a Montero or Montero Sport is a 15 minute job requiring only a Phillips head scew driver and a basic automotive supply store metric socket set. Make sure the engine is off, do not use carburetor cleaner to try and clean it, and make sure that when you remove the harness plug from the end of the IAC valve that you don't loose the rubber seal in the plug.
Unfortunately, with every ray of sunshine there must come some rain. IAC valves are expensive (around $375) and up until very recently, there wasn't a 'generic' replacement. That left only two options - scavenging a used one from the junk yard or ponying up the cash at the parts counter of your local Mitsubishi dealership. I refuse to purchase a part like this from a salvage yard. Too many times have I purchased five or six of the same thing to get one that works only to have it give out six months later.
The slighly cheaper option to your local dealership is scavenging the Internet for a 'generic' replacement. Another hard learned lesson is that using cheap knock-off Chinese electronics usually costs more in the long run than the actuall Mitsubishi part would have cost. The added worry of whether or not the new part might fry my ECU if someone decided they could get away with cheesing out on the windings makes me unwilling to take the risk.
Thankfully, some of us have an option. DSMISC Automotive sells original Mitsubishi Idle Air Control valves for many 1990's and early 2000's Mitsubishis. Those of us driving 1997-1999 3.0L 6G72 3.0L and/or 1999,2000 6G74 3.5L Montero Sports or 1990s Monteros can purchase a new Mitsubishi IAC for about 1/3 to 1/2 the cost of the same part from the dealership.
Two final things to note. First, always replace the o-ring gasket when replacing the IAC. O-rings don't last forever and a leaking o-ring can cause poor idle behaviour that exactly mimics a failing or failed IAC.
Lastly, never, ever, EVER spray carburetor/fuel injector cleaner into the throttle body while the IAC is still attached or spray carburetor or any other solvent onto the IAC to 'clean it'. An IAC isn't a sealed unit. If solvents penetrate the IAC they will disolve the insulation on the wire windings inside the IAC and that will cause a short. Besides ruining the IAC, this can (and most likely will) cause damage to the Engine Control Module. Read DSMISC Automotive's FAQ for more information.