If you have a fuel injected car and you are experiencing stalling, hunting or fast idle problems one thing you need to check is the idle control system. These systems vary from car to car but there are basically two types. The first uses a small idle speed control servo motor which is driven by the computer. When the throttle is released and the rpms begin to drop this small motor moves a portion of the throttle linkage (gas pedal linkage) to advance the throttle so that the engine doesn't stall - sort of like you depressing the gas very slightly. As the engine rpm approaches the idle speed the computer measures the rpm and moves this small motor to adjust the speed to a set value stored in the computer's memory. If this motor fails to respond to the computer then the engine will either stall or race. If the motor responds slowly then the engine will hunt for the correct idle speed and will alternately race and approach stalling speed. Check the operation of the motor to see if it is moving the linkage.
The second type of control is the idle air control valve. Its job is to control the idle speed of the engine by allowing a small amount of air to get around a closed throttle plate. The system is similar in that the computer is the controlling factor however here the computer sends signals to a small motor that moves a pintle valve which allows air to get into the engine and thus increase the idle speed. This system gets gunked up and causes the pintle valve to stick in one place or to move slowly. The effect is the same as above, the engine stays at one rpm or it stalls when you let your foot off the gas. Remove the idle control system and soak it in a good strong carburetor cleaner to clean the gunk from the pintle shaft.
One other type of IAC system moves a pintle valve by means of engine vacuum. There is a vacuum diaphragm instead of the small motor which moves a pintle valve. Sometimes the vacuum diaphragm fails and sometimes the vacuum passages get clogged up. Remove the valve and check it out.
Message me at BobHewitt@Misterfixit.com
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