Again I hesitate to be specific, because I do not know your experience with these things, such as abilty to make tests with a Digital Voltmeter.

For now, I suggest that you disconnect the O2 sensor (insulate each exposed wire end) and run "open loop" (do you know that term?).

In Open Loop, the Lambda ECU will (should) operate the Frequency Valve at a fixed, mid-range frequencey of 50%, which should allow satisfactory performance, if all else is correct.

I do not know of a specific FV test, but do know how to "force" or simulate ECU signals that should produce differing FV frequencies (varying fuel mixture values). The results of these forced signals can be determined by measuring the voltage produced by the O2 sensor.

The following is from a GOOGLE search on "Kjet mixture adjust" (as edited by me) ...

>>The frequency valve is an electrically duty-cycled fuel pressure regulator controlled by an on-board computer in response to an oxygen sensor signal. Typical duty cycle on a properly running engine is 45 percent to 55 percent duty. A quick test is to start the engine and test the frequency valve for vibration or noise -- it should vibrate.
Also, unplugging the oxygen sensor will put the system in open loop and fix the frequency valve at a 50 % duty cycle.<<
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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.