Idle stabilizer valve functionality
I have owned several cars with Weber carburetors, and even though that's my favorite brand of carburetor, the idle speed of an engine is very difficult to get "just so" in an open-loop system, i.e., without any feedback, since so many factors can affect idle speed.
Last Updated: 03/29/2016
Parts Group: Idle stabilizer valve
On our fuel-injected 1991 Volvo 240, the main airflow is through the throttle body. If the throttle plate, in the throttle body, is as closed as it can be, then the engine is starved of air and will stall, unless there is a bypass that allows air to still flow into the engine. There is indeed such a bypass, and the idle stabilizer valve controls the airflow through this bypass.
If the engine idle speed is too low, the car's eletrical system sends voltage to the positive wire of this electrically controlled valve, which causes it to open and let more air flow into the engine. When the engine speed is too low, the car cuts the power to the valve, and the valve closes. Very soon the engine idle speed is too low again, and the process repeats itself. The only modulation is in the on and off cycling of the voltage being applied, or not. The valve itself is fairly simple in its function. It's either on or it's off. To keep the engine speed within the proper range, this on-and-off cycling tends to occur so quickly that some idle stabilizer valves make a buzzing sound when they're working.
They don't last forever, so if you need a replacement, please consider: In case you're wondering where it's located: Idle stabilizer valve location. Ready to work? Please consider this: Idle stabilizer valve removal and replacement