[ChevyTech]

The system will not go into closed loop without seeing voltage readings on the oxygen sensor that shows the oxygen sensor is warm enough to use to control the mixture.

My guess is that the system is going into closed loop operation.

Try leaving the oxygen sensor unplugged and see how the truck drives after it warms up. This should set a trouble code for the O2 sensor and turn the service engine soon light on. This will hold the system in open loop and it not modify the mixture but will instead run off the program. This will also test if the ECM can recognize a problem and set a trouble code.

If the oxygen sensor is giving the ECM bad data it could be causing a rich mixture when it goes into closed loop operation.

Air getting in the exhaust system can affect the o2 reading. In open loop operation (cold) the ECM ignores the o2 sensor reading, but in closed loop (warm) the ECM uses the o2 reading to determine if the engine is running lean or rich. If the exhaust system leaks ahead of the o2 sensor, (or just behind it), the pulses in the exhaust will draw in air and cause the o2 sensor to get an incorrect reading. A vehicle with an A.I.R. will have the same problem if the air management valve allows air to be injected in the exhaust manifolds during closed loop operation.
Exhaust flow moves forward and backward in the exhaust system more then many people realize.

You can monitor open or closed loop status without a scan tool on OBD1 vehicles.

You can go into "Field Service" mode by grounding the A&B pins and starting the engine. In this mode, the SES light will indicate whether the system is in Open or Closed loop. In Open Loop the SES light flashes two and one-half times per second. In Closed Loop the SES light flashes once per second. Also in Closed loop the light will stay OFF most of the time if the system is running Lean. It will stay ON most of the time if the system is running Rich. While in the Field service mode new codes cannot be stored and the closed loop timer is bypassed.