This is the old-style Bosch auxiliary air device which is a passive idle speed control valve:


It sits under the inlet manifold and is used to bypass the throttle body for extra air. To check the operation:
1. Remove the device.
2. At room temperature, look through the air hole and you should see around half of the port open. Put it in to a frindge for a while and then the port should be fully open. Finally connect 12V to the connector to heat it up and the port should gradually fully close itself in a minute or two.