Bruce,
Thanks for the reply. Here are some answers to your quesions.


•No real help on your problem yet, but be aware that the ECU controls the ground side of the FV. So power on the + side, while essential, doesn't mean anything in terms of control.
Maybe the ECU is reacting to an excessively rich O2 signal by going excessively LEAN with the FV???

>>>>>>>That could be as when the O2 sensor is unplugged, the frequency valve starts buzzing again and the car begins to run pretty rich as I'm seeing white smoke in the exhaust.




"Upon starting after about 15 seconds, the FV stops buzzing, but I still have 12V at the connector."
• What is the engine doing at this time? I can only guess it's still running. Because with 12V still at the FV, the Fuel relay (and Lambda relay) must still be energized, which mean the Fuel relay is still getting coil pulses from a running engine.
Running badly? Rich with black smoke? Need more clues here.


>>>>>>>Yes, it's still running. The lambda relay is getting power (I verified that it wasn't a relay problem by jumpering the wires on the relay connecter)






How are you measuring O2 Voltage? Disconnected from ECU (open loop)?

>>>>>>Yes, I measured O2 sensor voltage with the sensor disconnected from the ECU. It's staying about .9V at idle and decreases when the engine is revved up. From reading the Haynes manual, I thought it would fluctuate between about .1V and .9V at idle, but it doesn't.
I'll remove the O2 sensor and see what it looks like and clean it up with the propane flame as you suggested.
Remember that I Seafoamed the engine not long ago and have some Seafoam in the gas tank. Would this affect the exhaust enough to cause the O2 sensor to think the engine was running rich?
I'm going to get an O2 sensor tomorrow if all else fails.
The car is definately happier with the sensor diconnected as mentioned above about the rich running.




If not, that might give a clue about mixture, assuming it's not all sooted up already.
See what it reads disconnected, if the ECU is happier that way.
Have you tried another O2 sensor? Or tried cleaning the one you have with a good application of propane flame?

I think we need more background and details. Without knowing what led you to the FV symptom, there are just too many things (for me) to guess at—too much room for off the wall speculation. Like I've done here.
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Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.