The best way to check if your going into closed loop is to have a scanner. You can then monitor your coolant temp and your O2 voltage. However if you don't have one, use your DVOM on MILLIVOLTS and tap into the O2 sensor signal line. It's located in the exhaust manifold or Y-pipe. There maybe 2, either way each one would be UPSTREAM the cat converter. The signal wire should produce 0-1.0 v. Select your MV scale and monitor this with the engine running. Should flucuate above and below 450 mv about 1 1/2 times per second. You may need to come off idle slightly as the O2 may cool and not give you an accurate reading. Also make sure your grounds are good, tight, and clean. Again using your volt meter disconnect the negative battery cable and use your DVOM in parellel. You should have LESS than .50V, with everything off. One more thing, there maybe a flash programming update for the ECM. Call your local Chevy dealer and give them the VIN. They will be able to tell you if there are any reprogramming updates.
OK, things are starting to make sense. The fact there is no fluctuation COLD doesn't concern me as this is a normal thing untill engine temp is at normal operating temp. However after 30 minutes, there should be a big fluctuation above and below 450 mv. You are correct in saying it's NOT going into closed loop. Now lets see if it's a fuel control issue or a temp issue or something else. With your DVOM connected to the O2 sensor and engine running, pull off a vacuum line. There should be a change in voltage almost instantly. Voltage should rise to aprox 650 or higher. Then reconnect it, it should drop back down. Since it's TBI, spray some carb cleaner down the TBI unit while running and the DVOM should change once again. If these things are happening, the O2 is ABLE to work and respond to changes and therefore should be able to control air fuel ratio. If this doesn't happen, make sure the mating surface where the O2 threads into the exhaust is clean. This is a ground. How many wires is the O2? You can also connect the DVOM on VOLT scale and read your voltage at the coolant temp sensor. Voltage will be high on cold engine-aprox 4-5 volts and lower as engine warms. Let me know