Idle speed stabilizer

This device does an amazingly good job of maintaining a constant idle speed no matter what.  The concept is simple, though: It's an electrically controlled rotary valve that opens or closes a slit through which air can pass.  One side connects upstream of the throttle plate and the other connects to the downstream side.  The valve normally lets some air bypass around the throttle plate and idle mixture is calibrated for this normal opening.  If something happens that would cause the idle to speed up (a leak in the intake system, for instance), the valve closes enough to slow it back down.  If something happens to slow the idle down (air conditioner compressor engages, for instance), the valve opens some to allow more air to the engine to speed it back up.  Since the valve is electrically controlled by the ECU, the ECU knows to adjust the fuel metering accordingly as well.

Because the system is calibrated for the valve to be somewhat open normally, clamping the hose off or disconnecting the electrical connector from the valve will cause the normal bypass air to stop and the engine will die.  If you disable the valve, you will at least have to run the idle stop screw in a ways, and you may also have to shim the throttle a bit to keep the engine from dying.  With no  leaks, my engine will just barely remain running if I disconnect the electrical connector from the valve and let the engine come down to idle very carefully.  It will stall if I clamp the hose, unless I compensate with some open throttle.  With a leak of some sort, there's no telling how it will act; it all depends on the size and location of the leak.

To disconnect the connector, push the wire spring all the way down, then pull the connector. 

The valve is not easy to remove.  The intake hose is on the bottom of the throttle body; not easy to get to.  The outlet hose is on the front of the throttle body.  The valve itself (about 5" long, 2" in diameter), hangs horizontally beneath the intake manifold, completely out of sight (you can probably see it with an inspection mirror).  It's got a thick rubber collar around it and the collar has a hole that slides onto a bar about 3/8" in diameter.  Once you get the hoses off, you slide it toward the drivers side (U.S.) to get it off the bar.  The electrical connector is on the end toward the passenger's side (U.S.).  The thermostat replacement procedure has some pictures and text about removing the valve.