Hey guys, this is my first post here. I've been looking for a place to chat with people who deal with quirky electrical problems and understand how to properly troubleshoot them, something which is very difficult to find!
I have a 1993 Ford Thunderbird SC with the 3.8 V6, and I use a laptop with software called TunerPro RT for live data monitoring and tuning, along with a device called a QuarterHorse that piggy-backs onto the PCM (which enables the real-time monitoring, uploading tunes, etc.) I can see pretty much any data or parameter that the PCM works with.
Anyway, here's the problem I've been having. When the car is started after sitting for at least a couple of hours, it'll fire up and idle normally. While watching live data in TunerPro, both oxygen sensors start 'switching' as you'd expect, varying voltage from ~0.1v to ~0.9v, up and down. However, once the car is warm enough for the thermostat to open, what happens is the driver side O2 sensor's voltage starts to 'swing' less and less until it just completely freezes at some value, usually either ~0.2v or ~1v. When this happens, the PCM continues to adjust fueling based on this frozen voltage, so if it freezes at ~0.2v, the car will suddenly be running pig rich as the PCM thinks the O2 is telling it that bank is running lean - or, if it freezes at ~1v, the car starts running extremely lean as it thinks the bank is running very rich.
What I've done so far is swap O2 sensors side to side and confirmed the freezing still happens on the driver side, which is why I think it's likely a wiring issue. I also tested for 12v and ground on the two heater circuits at the O2 sensor connector (on the sensor's side) both while the sensor was operating normally and while it was 'frozen.' I also checked resistance between the heater circuits and saw about 7.8 ohms.
Someone did tell me that the O2 sensor element in the exhaust flow could be getting sprayed and cooled by something in the exhaust, like oil or water, to the point that the sensor is no longer hot enough to function, but I've checked and it looks completely normal. Beyond that I'm virtually certain that this is not a "real" mechanical problem going on with the car, purely an electrical gremlin of some kind.
Any help would be greatly appreciated, as I'm very much stumped right now!
