[mike635]

Need a little troubleshooting advice with my '81 SC oxygen sensor system. I've searched and read many threads but haven't found a solution to my situation. Sorry for the long info dump but I'm out of ideas...

Symptoms:
- poor gas mileage (last tank was 14 mpg/city)
- slight hunting at idle (warm and cold)

Tests conducted and notes:
- 02 signal not fluctuating - fixed at about 0.8V. Unplugged, the sensor still produces the same output so I assume (at this point) that it's functional.
- Pulled 02 relay - no change to idle or engine running conditions. Relay snaps when plugged/unplugged. Verified proper voltages/grounds are present using Bentley manual diagrams
- Felt frequency valve (engine running) and can't be sure if it's buzzing because the whole engine vibrates right? Inconclusive.

Wiring checks (with ohm meter)
- O2 cable to pin 2 on the lambda computer ok
- Frequency valve wiring between computer (pin 15) and valve connector, and between valve and relay ok
- Frequency valve resistance (2.9 ohms) verified between pin 15 on lambda connector and 87B on the relay

Dwell testing (using an oscilloscope)
- Cold start dwell about 75% (estimate). As engine warms up, dwell settles to 50% with O2 sensor unplugged
- Plug in O2 sensor: dwell jumps to about 80% (again, sensor output is about 0.85V fixed)
- Ground O2 signal wire: dwell gradually (about 10 seconds) decreases and engine starts to die. Assume this means the frequency valve is doing its job?
- Unplug frequency valve - little/no change to idle! This seems strange.
- Tried opening the oil tank cap (to force a lean condition, with frequency valve plugged back in) No change to O2 signal output.

The car starts quickly, pulls pretty smoothly so I don't think anything is really wrong with the fuel pressure (plan to check control and system pressures anyway) but I want the 02 system to function properly.

I have a new 02 sensor but haven't installed it yet, that's probably next but since the current sensor (which has about 10k miles on it) I've heard that 02 sensors can get "lazy" and not respond quickly but mine just seems to put out a constant voltage no matter what.

Any suggestions on what to try next?

Mike