Signs of a Bad Idle Air Control Valve
An idle air control valve is, in a very real way, the predecessor to the drive-by-wire throttle systems commonly employed today. It performs much the same function as any electronic throttle: to control the amount of air going into the engine so the computer can match it with the right amount of fuel. The only difference between these two systems is that an IAC is smaller and only works at idle.
Common Problem
Because an IAC valve controls air going into the engine only when the throttle is shut or almost shut, these are the only two times you're likely to notice it malfunctioning. The most common type of failure results from clogging of the IAC valve with carbon, dirt and debris, which restricts the amount of air going through and slows the valve's response time.
Classic Symptoms
Classic symptoms of a clogged IAC are a rough or unsteady idle, and stalling if the valve passage gets too clogged or the valve fails outright. Another sign is engine rpm that drops very low -- possibly to the point of stalling -- when you lift your foot off the gas pedal to slow down. This may be accompanied by an immediate, jerky deceleration as soon as you lift your foot off the pedal, and a backfire through the tailpipe as the engine loads up with excess fuel.
Less Common
Less common are valves that get stuck in the open position; in this case, you'll probably only see stalling at idle and hesitation during acceleration. Modern cars will almost always trigger a check-engine light following any kind of IAC failure, and may display codes specifically diagnosing that.
