This is my opinion of how the idle circuit works based on personal experience and the Haynes Manual. I'm sure the factory manual gives a better explanation, but I don't have that yet.

The idle circuit in the 2nd generation cars is fairly complex, perhaps more so than it needs to be. The biggest mistake I made early on was to assume that the computer controlled everything. In reality the only idle related components that the computer controls are the BAC valve and the air bypass valve (Haynes calls this AWS - accelerated warmup solenoid). The main sensor controlling idle is the water themo sensor. Devices controlled directly from water temperature are thermo wax device and water themo valve.

The following list explaines the sequence from initial cold startup to fully warm.

• If engine is started in neutral, the air bypass solenid (on top of intake runners, next to intercooler) is opened to provide accelerated warmp at 3000 rpm. The main purpose of this is to quickly warm up the cats. This lasts less than a minute.
• The wax pellet device on the back (close to the firewall) of the throttle body is sensing coolant temperature. When cold, the rod on the termo wax device is fully retracted allowing the fast idle cam to hold the primary throttle plates open such that idle is at 1500 rpm. As the coolant warms up, the thermo wax expands causing the rod to extend and kick down the fast idle cam allowing idle to fall to 700rpm. Both the opening temp and the fast idle speed can be adjusted with set screws.
• The water themo valve which screws into the thermo wax device is also sensing coolant temp. When cold, this valve supplies vacuum to a device that holds the secondary throttle plates closed until the coolant temp reaches 140 degrees F. If your car never seems to warmup and generate full power, or takes a long time to warm up, check to make sure the coolant flow is not blocked to the themo wax device.
• The water thermo sensor is monitored by the ECU to determine A/F ratio.
• Once the engine is warm, the BAC valve compensates for the load created by the air conditioning system. The air supply valve compensates for the load from the power steering system.

• ASV - air supply valve - actuated by the power steering relay to supply extra air whenever the power steering is active.
• AWS - accelerated warmup solenoid - air bypass valve - actuated by computer to provide 3000 rpm warmup to quickly warm up cats
• thermo wax device - mechanical device containing wax that melts at a specified coolant temperature and extends a rod controlling 1500 rpm warmup idle
• water thermo valve - temperature controlled valve that switches vacuum to a device that holds the secondary throttle plates closed when the coolant temperature is cold. This prevent excessive boost before the engine is warm.
• water themo sensor - input to the ECU to determin how rich to run the A/F mixture. This sensor should have a resistance of 2.45Kohms at 68F and 320ohms at 176F. This sensor screws into the back of the coolant filler neck, under the alternator.
• intake air sensor - input to the ECU, intake air temperature helps determine air density so MAF values can be calibrated.  It should have a resistance of 11.85Kohms at 122F and 3.5Kohms at 185F
• The BAC valve is controlled by the computer which varies the duty cycle of the drive signal. This valve comes into play once the engine is warmed up to compensate for the load created by the air conditioning system. The main set screw that controls warm engine idle speed is located on the BAC valve.

TPS

By far the simplest method of setting the TPS for me (being an EE) is using a volt meter to dial in 0.95 volts at the connector. There is no guess work involved, just turn the adjust screw until the meter reads 0.95 volts.

• Warm up car
• Remove intercooler
• Turn key to ON position
• Leaving all connectors plugged in, find the three prong connector that comes directly from the TPS
• Insert you probes into the back of the connector and find the two prongs that are close to 1 volt (the others should be around 4 volts).
• Turn the adjusting screw until the voltage is 0.95 volts.