The idle air control (IAC) valve on a BMW 525i sedan is controlled by the vehicle's powertrain control module (PCM). The PCM opens and closes the IAC valve in order to vary the amount of air allowed into the side of the throttle body. This variation in air raises and lowers the idle according to the PCM’s preference for idle speed. When the IAC valve fails, the vehicle may stall or idle erratically. It's necessary to use a computerized automotive scan tool to properly test the IAC valve.
Place the BMW’s transmission into Park (automatic) or neutral (manual transmission) and apply the parking brake. Leave the engine running.
Connect a scan tool to the BMW’s diagnostic data port, which is under the dashboard just above the pedals on the driver’s side.
Power on the scan tool and choose “Bi-Directional Tests” from the main menu. Select “IAC Valve Actuation” from the tests menu.
Use the scan tool’s buttons to open and close the IAC valve. The idle speed should rise and lower without pressing the accelerator. If the IAC valve does not respond to the scan tool by raising and lowering the engine idle speed, then it is faulty.
