[Kevin45]

The O2 sensor is in your exhaust pipe or exhaust manifold. You may also want to look at your temperature sending unit and/or thermostat. If it is sending a wrong temp to the computer the computer may be wanting to compensate for something that does not need compensated for. Same as when a knock sensor kicks it it will tell the computer to change the timing.
With your truck being a '91 it will have an O2 sensor unless someone has removed it to save a couple of bucks. But without it the computer will still act as there is one and compensating for it being bad. Look at your driver side exhaust manifold and if not there it will be in your exhaust pipe before the catalytic converter.

Kevin