1999 Chevy Malibu idle up.

Doese the engine die when you turn the AC on? Has the check engine light come on?

The IAC is responsible for maintaining the idle. If it is good and you are still getting a surging, you may need to have the PCM checked.

It should be around 700 + or - 100 RPMs. As far as the AC, yes it should idle up to compensate for the added load from the compressor. Are you sure the IAC is working properly? Also, have you checked for vacuum leaks and the throttle position sensor?

Here is a good description of it I was able to find. The directions to check it as listed last.

The Throttle Position (TP) sensor is a potentiometer that provides a signal to the PCM which is directly proportional to the throttle plate position. The TP sensor is mounted on the side of the throttle body and is connected to the throttle plate shaft. The TP sensor monitors throttle plate movement and position, and transmits an appropriate electrical signal to the PCM. These signals are used by the PCM to adjust the air/fuel mixture, spark timing and EGR operation according to engine load at idle, part throttle, or full throttle. The TP sensor is not adjustable.

The TP sensor receives a 5 volt reference signal and a ground circuit from the PCM. A return signal circuit is connected to a wiper that runs on a resistor internally on the sensor. The further the throttle is opened, the wiper moves along the resistor, at wide open throttle, the wiper essentially creates a loop between the reference signal and the signal return returning the full or nearly full 5 volt signal back to the PCM. At idle, the signal return should be approximately 0.9 volts.

TESTING

With the engine OFF and the ignition ON, check the voltage at the signal return circuit of the TP sensor by carefully backprobing the connector using a DVOM. Attach the black lead to known good ground, the red lead to terminal B.
Voltage should be between 0.2 and 1.4 volts at idle.
Slowly move the throttle pulley to the Wide Open Throttle (WOT) position and watch the voltage on the DVOM. The voltage should slowly rise to slightly less than 4.8v at Wide Open Throttle (WOT).
If no voltage is present, check the wiring harness for supply voltage (5.0v) and ground (0.3v or less), by referring to your corresponding wiring guide in. If supply voltage and ground are present, but no there is no output voltage from the TP sensor, replace the sensor. If supply voltage and ground do not meet specifications, make necessary repairs to the harness or PCM.

Let me know if it helps.

I can't tell where I'm looking on the engine. Where is that located?