Help, my Renix XJ will not maintain closed loop...

Well, after noticing a dractic decrease in fuel milage, I started doing some investigating. My first obervation was that the o2 sensor was constantly reading 4.98-4.99 volts, I thought it was shorted out causing the engine to run incredibly lean. I replaced the o2 sensor, and still have the same readings. PCM in open loop mode, STFT and LTFT maintain 128 (due to open loop). I have a good ground at the o2 sensor harness, also have good +12 off the o2 heater relay to the sensor. I have good continuity on signal wire between o2 sensor harness and pcm. Signal wire maintains 4.98-4.99 volts in all conditions, cold, warm, load on engine, no load, even 2500 rpms. Now when I add ground to the signal wire (tricking ecm to run rich), I read 0 volts on my MODIS (snap-on scanner), and I notice an engine change so that tells me the wiring is good and pcm is interpereting correct. When I remove ground, momentarily it switches to closed loop, and i watch my short and long term fuel trims climb rapidly, up to 255 (lean limit). Then it defaults back to open loop. I'm at a loss here...and suggestions would be great. Thanks!

You have a scantool that will interface with a Renix?? :worship::worship: How is the CTS?

CTS appears to be functioning properly, i can watch it warm up to about 210, operating temp...

Renix has three different sensors.

Sensor at rear of head is for the gauge. Sensor on driver's side of block is the fuel mixture/coolant temp sensor. Sensor in driver's side of radiator is for the electric fan.

So did you check the right one? I don't own a renix but have seen pics of the coolant temp sensor for Renix's and it's kind of behind the exhaust manifold in a tricky spot. Check it.

It defaults to closed loop when the engine temperature (or sensor), the manifold temperature (or sensor) falls below the minimum operating temp., and at WOT or during deceleration under 1200 RPM's. Your running to full lean and then defaulting to open loop, sounds like the motor going into (through) decel mode. Decel mode shuts off the injectors under 1200 RPM.
The first thing I'd do is check the TPS (ECU side) for a short (or open or dead spot) and then the engine temp sensor and MAT sensor for an open or out of value reading. If the TPS was shorting, a very high idle would likely be a result. Dead spots or open circuit in the TPS often result in noticeable timing problems.
Many of the sensors share a ground at the injector harness bend at the firewall. I had trouble there, a really loose crimp. I'd get variable ground (ohm) readings. The engine temperature sensor tested good, but the sensor ground would raise the resistance when it wiggled at the splice, throwing me in and out of closed loop. My idle would fluctuate, but over extended periods. It almost seemed like every start up it produced a different idle RPM and condition (lean/rich) not exactly randomly, but more like a choice of a few possibilities.
I most always tend to check the harness and connectors before I check any sensor seriously. I've found flaws in the conductivity much more often than I've found sensor failures. I'm still running many of my 20 year old sensors, I've been through the harness a dozen times. Sometimes a fresh sensor will mask poor conductivity, but often isn't the root cause of the problem.
Low vacuum to the MAP also seems to cause major fuel ratio problems. but more often than not, it defaults way rich.
Vacuum leaks or exhaust manifold leaks that venturi tend to run up the idle, though the ECU will try to adjust and run the idle back down again.
You likely have a sensor (way) out of value, while some sensors are more probable than others, poor grounds and shorts in most any sensor on the 5 volt supply circuit would likely cascade.
I once found 12 volts in the five volt supply circuit at the MAP sensor and eventually had to change out the ECU. The motor still ran, but very poorly in open loop.
A stuck open EGR would also likely mess with your fuel ratio, often a loping (very low) idle and sometimes stalls at idle.
Good luck

I don't know if this helps any, but key off, engine off, o2 voltage is 0, key on, engine on, o2 voltage remains zero, key cranking, and engine at idle, o2 voltage is 4.98. I ran a new signal wire from o2 connector straight to pcm terminal, trying to bypass a possible short to power. Still have same symptoms...could ecu have possibly lost a ground, not completing the circuit?

5-90 or EcoMike have a lot better understanding of exactly how the O2 sensor functions than I do. I'm more of a nuts and bolts type.
Do know that the O2 and the knock sensor bypass the ECU to ground, through the same ground wire that grounds the ECU and in effect the sensors.
The knock sensor and the O2 sensor have a splice in the injector harness and then the dual ground splices into the ECU ground after ECU out.
The engine temperature sensor and the manifold temperature sensor need under (around) 500 ohms, for the system to go into closed loop. The higher the resistance in the sensor or the circuit and the colder the message to the ECU.
The O2 sensor, the engine temp sensor and knock sensor harness goes up the front of the motor. If the manifold has been off or the harness rerouted in most any way, the chances of it cooking on the exhaust manifold increase (in 88 or 89 they added a harness clamp in this area).
I've had three O2 senors die on me, one was a faulty sensor (recall), one was old age and the other a fried harness ( though I've changed more than a few unnecessarily).
The MAT sensor gets covered in oil and gunk which can't be good. The stock airbox actually works in tandem with the MAT sensor. The Hot air bypass in the front of the air box is part of the programing that the ECU uses for the MAT values (cold air intakes aren't necessarily good, you'll likely never get peak horsepower in open loop operation).
The MAT and the engine temperature sensor have the same ohm values at the same temperature. A quick test is an ohm check on both, on a cold motor, the resistance values should be nearly the same. An even better test is to do the resistance check at the ECU connector.

My 90 started this same issue just as the temps started dropping around here, unfortunatly it has been in the paint shop and I have not been able to look into it.

Thanks for some great info and some places to start looking!

...If you make the PCM go rich, and the oxygen sensor doesn't indicate a change, then it will go back to open loop. If you have a DRB II, go to KAM and select 02. See if it indicates a fault.

Did it occur to you to just change the oxygen sensor?