Idle Speed Control

At idle the throttle is almost closed, and small changes in air make a big difference. Older engines used a separate idle air control valve; modern engines with electronic throttle control the idle with the throttle plate itself. The control unit sets a target idle that is higher when cold and adjusts it for loads such as the air conditioning, the alternator and power steering. Older engines used a separate idle air control valve; most modern engines control idle with the electronic throttle body itself.

The control unit compares actual engine speed with the target and opens or closes the throttle or idle valve slightly, often also adjusting ignition timing for quick corrections. The control unit compares actual engine speed with a target that changes with temperature and load, and adjusts air, ignition timing and sometimes injection to hold it.

Dirty throttle bodies, vacuum leaks and lost adaptations cause high, low or hunting idle and stalling, with codes like P0505 to P0507. After cleaning or battery work, an idle relearn may be needed. Unstable idle, stalling or a high idle often come from a dirty throttle body, vacuum leaks or lost adaptation after battery work. Some cars need a relearn procedure after cleaning.