GM Firebird 1982-1992 Repair Guide

Evaporative Emission (EVAP) Control System


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 solenoid operated bowl vent or vacuum operated purge valve mounted on the canister. 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.

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 help in the tank.

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

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

    Inoperative purge solenoid 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.


If the solenoid valve is open, or is not receiving power, the canister can purge to the intake manifold at the incorrect time. This can allow extra fuel during warm-up, which can cause rough or unstable idle.


EVAP Canister

See Figure 1

The EVAP control system uses a 1500cc charcoal canister to absorb fuel vapors from the fuel tank. Under certain operating conditions, the purge solenoid valve will open, allowing stored vapor to flow into the intake manifold for combustion.

Click image to see an enlarged view

Fig. Fig. 1: Vapor canister assembly-1982-92 models

  1. Tag and disconnect all hoses connected to the charcoal canister.
  3. Loosen the retaining clamps and then lift out the canister.

Some models do not have replaceable filters.

  1. If equipped, grasp the filter in the bottom of the canister with your fingers and pull it out. Replace it with a new one.
  3. Install canister and tighten retaining clamps.
  5. Install all hoses to canister.