My 88 Chevy truck CK/1500 with 5.7 TBI is setting codes 21 and 22. Runs fine most of the time but will sometimes idle high and then low, check engine light comes on and when correct idle returns light goes off.
The code 21 is TPS (throttle position sensor) voltage is low.
The code 22 is the TPS voltage is high.
The Throttle Position Sensor is probably sticking and needs to be changed.