GM S10/S15/Sonoma Pick-Ups 1982-1993 Repair Manual

Evaporative Canister


See Figure 1

This system is designed to limit gasoline vapor, which normally escapes from the fuel tank and the intake manifold, from discharging into the atmosphere. Vapor absorption is accomplished through the use of the charcoal canister. The canister absorbs fuel vapors and stores them until they can be removed and burned in the combustion process. Removal of the vapors from the canister to the engine is accomplished through: a canister mounted purge valve, the throttle valve position, a Thermostatic Vacuum Switch (TVS) or a computer controlled canister purge solenoid.

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Fig. Fig. 1: Example of a common evaporative canister and hose assembly-6 cylinder engine

In addition to the canister, the fuel tank requires a non-vented gas cap. This cap does not allow fuel vapor to discharge into the atmosphere. All fuel vapor travels through a vent line (inserted high into the domed fuel tank) directly to the canister.


Every 30,000 miles or 24 months, check all fuel, vapor lines and hoses for proper hookup, routing and condition. If equipped, check that the bowl vent and purge valves work properly. Remove the canister and check for cracks or damage and replace, if necessary.


  1. Apply a short length of hose to the lower tube of the purge valve and attempt to blow through it. Little or no air should pass into the canister.
  3. With a hand vacuum pump, apply a vacuum of 15 in. Hg to the control valve tube (usually upper tube). If the diaphragm does not hold vacuum for at least 20 seconds, the diaphragm is leaking and the canister must be replaced.
  5. If the diaphragm holds vacuum, again try to blow through the hose connected to the lower tube while vacuum is still being applied. An increased flow of air should be observed. If not, the canister must be replaced.


  1. Label and disconnect the charcoal canister vent hoses.
  3. Remove the canister-to-bracket bolt(s).
  5. Lift the canister from the bracket.
  7. To install, reverse the removal procedures. It is essential that the vent hoses be properly connected as noted during removal.