I've been reading just about every post on "Dies at start", "Bad Idle", "Idle speed" and never quite wrap my head around what's mechanical and what's electronic that can make an engine perform poorly in the driveway or on the road. So far I've figured out:

Mechanical: EGR Valve, Throttle body, Idle speed adjustment screw,
thermal valve on thermostat housing, thermal valve (for EGR vacuum)
under intake manifold

Electronic: MAF, throttle position switch

Not Sure What it is: My 2.6 I-TEC has what's been refered to as an air regulator with a large black tube that goes up to the throttle body and acts as a bypass to the intake manifold. As near as I can tell it actuates at a cold start and closes after a few minutes and it appears to be electric/electronic. I might call it a cold idle air control valve or on another day a fast idle air control valve, but it's real name is probably air regulator. It's got nothing to do with setting the idle on a warm engine because it seems to become inactive after a few minutes, but I have to think that for all of us complaining about engines that die on start-up, it may be something to explore. For those with more know how: Is it a device that sends/ receives a signal to/from the ECM or is this another thermal mechanical device that simply closes after a few minutes of acting as a virtual choke?

I believe you are speaking of the Idle air control valve, or the IAC for short. I will look in my manual for the testing procedure and post it.

That much is correct. It is activated with the ignition and bumps the idle speed and decays over time. I forget the time but it is fairly quick. You are correct about a warm engine, though it does it all the time, so it does affect warm idle as well.



The latter. It is a two-wire device that basically just recieves power and then does it's job. There is no feedback.

Something you left out is the engine coolant temp sensor. If the water is cold (i.e. cold weather) the mixture is richened. This can be used to increase idle speed. I have never seen one fail, though, so I would not suspect it.

Most idle issues on the 2.6L come from the other things you mention (regulator almost never dies) as well as the normal mix of rotted vacuum lines and bad timing. EGR was always my biggest problem after I got vacuum and timing issues sorted out. I removed it and haven't had an idle issue since.