Sounds like an O2 sensor, but it should have set a RICH code for the O2(can't remember the #). Your truck probably has the original one anyway. Get a Bosch that will plug right in. Good luck.
1989 S-10 4x4 4.3L/700R-4, 3.08s, 31" Sport Kings
1989 Blazer 4x4 4.3L/700R-4 3.42s,235 Sport Tracks
1985 S-10 4x4 Soon to have 3504BM/TH350/D44/14FF, 35" Boggers