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It's called the "auxiliary air valve" in the manual and it's part of the throttle body, located below the throttle valve (butterfly). It's a thermostat-like coolant-operated device that allows extra air to bypass the throttle when the motor's cold. It is sometimes called the "cold idle air bypass". When the hot coolant hits it, the valve is supposed to close the air passage, reducing idle speed.
The main air bypass - that controls the hot idle - is above the throttle plate, and is controlled by the idle set screw - the large screw on the top of the throttle body. It allows air to bypass the throttle plate at all times. It is sometimes referred to as the "hot idle air bypass".
There is no "setting" on the aav: it is supposed to be open when the motor is cold, and closed when the motor is hot. It can get gummed up and get stuck open or closed, and also the thermostatic device can fail, though it normally lasts very many years and miles. If coolant is low, that can also cause the aav to fail to operate.
I think on the 22re the aav can be removed and replaced with one from a junkyard part, but I don't think you can buy the aav by itself from the dealer. On the 3vze, the aav is an integral part of the throttle body and so if it's bad, the whole throttle body must be replaced.
There is a simple test to see if it's working. When the motor is cold, start it, and immediately screw the idle set screw all the way in (count the turns). If the aav is open and unclogged, as it should be, the idle speed will go down but the motor should still run. Back out the idle screw to it's original position and wait for the motor to get hot. Then turn the screw all the way in again. The motor should stall, or nearly stall. If the motor doesn't stall, or if there isn't a difference in idle speed with the screw all the way in hot vs. cold, then the aav is not working.
If the problem is that the aav is clogged with gunk, you can often get it working again by cleaning it. Get some O2 sensor- and catalyst- safe throttle body cleaner, a clean rag and toothbrush, and clean the throttle body, giving several good shots into the holes in front of the throttle plate, above and below. Those are your air bypasses. Clean the back of the throttle plate with the toothbrush. Be very careful not to get any of the cleaner on the black plastic throttle position sensor, as solvents will kill it.
Once the throttle body is clean, set the hot idle speed to 750 rpm (22re) or 800 rpm (3vze) with the idle set screw.
There's one additional air bypass, and that's the A/C idle up VSV (vacuum solenoid valve). It provides an additional air bypass when the A/C compressor is compressing.
It's a two part device: the a/c vsv, and the actuator. On the 22re the vsv is on top of the valve cover and the actuator is attached to the engine side of the plenum between the "E" and the "F". It has a big hex-shaped plastic screw which is used to set the amount of a/c idle increase. (On the 3vze they're both on the passenger inner fender.) The way the thing works is that the vsv applies vacuum to the actuator when the a/c compressor is on, which causes the actuator to open and allow additional air to enter the plenum, increasing the idle speed.
89 4x4 X-Cab V6
WARNING: I'm not a professional tech - use any advice at your own risk. Working on vehicles is risky: you can kill or maim yourself or damage your truck if you don't know what you are doing. It's your responsibility to get qualified help if you lack auto repair training/experience.