GM Camaro/Firebird 1993-1998 Repair Guide

Evaporative Emission (EVAP) Control System



See Figures 1 and 2

The Evaporative Emission (EVAP) control system used on all vehicles is the charcoal canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon (charcoal) storage device (canister) to hold the vapors when the vehicle is not operating. When the engine is running, the fuel vapor is purged from the carbon element by intake air flow and consumed in the normal combustion process.

Removal of the vapors from the canister is accomplished by a purge valve. In addition to the fuel system vents and canister, the fuel tank requires a non-vented gas cap. The domed fuel tank positions a vent high enough above the fuel to keep the vent pipe in the vapor at all times. The single vent pipe is routed directly to the canister.

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Fig. Fig. 1: Evaporative emissions control system schematic-3.4L L32 engine shown

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Fig. Fig. 2: Evaporative emissions control system schematic-5.7L LT1 engine

These systems commonly use an in-line EVAP pressure control valve as a pressure relief valve. When vapor pressure in the tank exceeds approximately 0.7 psi, the diaphragm valve opens, allowing vapors to vent to the canister. Once the in tank pressure drops below 0.7 psi, the valve closes causing vapors to be held in the tank.

If the EVAP system is not functioning properly, any one of the following conditions may result.

  1. Poor idle, stalling, and poor driveability can be caused by:

    Inoperative purge valve
    Damaged canister
    Hoses split, cracked or not connected to the proper tubes


  1. Evidence of fuel loss or fuel vapor odor can be caused by:

    Liquid fuel leaking from the fuel lines
    Cracked or damaged canister
    Inoperative canister control valve
    Disconnected, misrouted, kinked, deteriorated or damaged vapor hoses or control hoses