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Under electronic control it accomplishes the same thing as your right foot used to do on your old carbureted Chevy or Ford - continuously adjusts the air going into the engine to keep it idling. It bypasses air around the (closed) throttle valve, and it only operates when the throttle position switch signals that the throttle is completely closed. Properly functioning, they are unnoticeable, and the engine idles steadily under all conditions. Gummed up, it typically stalls the engine after a cold start, or it almost dies, then revs up and down a bit trying to find the right speed.
A secondary and little-known function is to prevent manifold vacuum from getting too high under closed-throttle-high-engine-speed conditions, such as going down a hill with your foot off the gas. Not sure why this is necessary on an EFI engine, but some emissions-controlled carbureted engines had little spring loaded valves incorporated into the throttle plates that would open under high vacuum to accomplish the same thing. This prevented excess fuel being drawn in with a resulting emissions increase.
Non-Volvo Trivia: most of us owners of later MGB's with these throttle plates install pre-emissions plates or solder the valves shut, as they are a total PITA in terms of ever getting the engine to idle right. Probably create more pollution than they ever prevented.
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Bob (81-244GL B21F, 83-244DL B23F, 94-944 B230FD plus grocery-getter Dodge minivan, MGB, and numerous old motorcycles)
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