It's the IAC (what Haulin was talking about but misspelled
). You need to get some throttle body cleaner and spray it at the hole inside the throttle body. It's a square hole on the front half in front of the throttle plate. It allows air past the throttle plate at idle, and the IAC (Idle Air Control valve) opens and closes depending on how the car needs to idle.
The problem comes in when the PCV system is active. The PCV system recirculates any excess oil and gas vapors into the Intake
system to burn them, instead of releasing them into the atmosphere. These fumes have to go through the IAC at idle, and will condense into liquid form after a while. Unfortunately, it's more of a sludge than a liquid, and this sludge gums up the IAC, limiting its ability to open and close (it's only operated by a magnet), so your idle can become irratic after a while.
The vapors also condense on the throttle plate and inside the Intake
manifold, so spraying them clean will improve airflow and performance a bit. When the vapors enter the combustion chamber, they collect on the valves and piston tops and create carbon deposits. You can use this stuff to clean out most of the Intake
system, and help the fuel and oil system as well:
Though, Haulin was wrong about adjusting the idle. The butterfly valve has nothing to do with the idle speed. It's just there to reduce low RPM engine noise. It opens between 3000-4000 RPM to let more air into the engine, and there is no adjustment for it (it's controlled by the engine's vacuum). The only way you can adjust the idle on the Celica is to get the Apexi PowerFC. The idle is controlled entirely by the ECU, so even tightening the throttle cable to increase it will be cancelled out by the ECU closing up the IAC to compensate. It's not instantaneous, but the ECU will catch on after a little while and reset it back to OEM specs.