The plastic cover IS part of the pot. Screws loosened, the pot can be rotated, so scribe an alignment mark before this step. It should slip straight out off the pivot shaft of the airflow meter, while the wiper stays captive to the shaft. Find previous posts to see pics of what the pot looks like.

Cleaning the pot is unlikely to be successful, but a worn pot displays definite symptoms. Idle speed varying erratically is typical. A rough idle with minimally varying idle speed is not a symptom.

Resistance measurements are useless, as this pot is used as a voltage divider. It incorporates a large resistance in series with the wiper. A valid test would involve a test harness to apply a voltage across the outer terminals while monitoring the voltage at the center pin. It would be done with the pot mounted, engine off, and manually actuating the airflow meter plate. Simpler is to go by symptom and visual inspection.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7