GMC S10/S15/Sonoma Pick-Ups 1982-1993 Repair Manual
VACUUM DIAGRAMS (See Figures 1 through 85)
is a listing of vacuum diagrams for most of the engine and emissions
package combinations covered by this manual. Because vacuum circuits
will vary based on various engine and vehicle options, always refer
first to the vehicle emission control information label, if present.
Should the label be missing, or should vehicle be equipped with a
different engine from the car's original equipment, refer to the
diagrams below for the same or similar configuration.
Check the link autozone.com (free register), select the year ('91 sample), Brand, Model, Sub-Model, Engine Size and make the next root...1991 GMC TRUCK S15 4WD 4.3L TBI 6CYL REPAIR GUIDE
>> EMISSION CONTROLS
>> VACUUM DIAGRAMS
I really hope helps with this (remember rated this help). Good luck.