The car is a 1980 245DL with a B21F, runs well, poor MPG, odd smell. The Haynes manual does not mention the frequency valve, the Lamda system relay, or show the mixture adjustment screw. Some questions:

1) Is the frequency valve the item with an electrical connector on top and a fuel line in and out sitting directly in front of the boot that sits under the throttle body? If not, where is it? I know it's supposed to buzz ~50% of the time and mine used to, but now I hear it buzzing continuously. What does this indicate -rich mixture, bad valve...? Any tips for diagnosis? The O2 sensor is new, no change. Could this just be due to a bad Lambda relay (I only know of its existence since it was replaced at ~155k miles)?

2) Is the mixture adjustment just for idle, or does it serve as the base from which the Lambda feedback system adjusts the mixture throughout the RPM range? Where is the adjustment screw? I'm assuming this is separate from the CPR adjustment - correct?

Sorry for asking basic questions - Thanks for any help anyone can offer

Zack
1980 245DL M46 273k
1988 745T+ M46 196k