My 84 GTV6 has started running lean at idle and a fluctuating idle speed. I have an Autometer fuel air meter installed and until recently showed good mixture, now it goes full lean after warmup at idle. So far I have checked fuel pressure, 34psi at idle up to 40 when rev engine. Throttle switch is adjusted correctly. O2 sensor replaced with a good one. Recently replaced the small air hoses under the plenum, so I checked for air leaks using propane and a rubber hose to put the propane around all air hoses, no leaks found. Checked idle mixture with voltmeter, between .7 and .8 volts with O2 sensor unplugged (running "S" cams), plug the O2 sensor back up and the mixture goes down to about .3 volts.
What would cause the ECU to lean the mixture that much?? Idle sounds good and even with the O2 sensor unplugged, idle speed "hunts" less than 50 rpm when O2 sensor plugged back up.