GM Metro/Sprint 1985-1993 Repair Guide

Evaporative Emission Control

Print

OPERATION



See Figures 1 through 6



Click image to see an enlarged view

Fig. Fig. 1: Evaporative emission control system operation - fuel injected engine



Click image to see an enlarged view

Fig. Fig. 2: Evaporative emission control system operation - carbureted engine



Click image to see an enlarged view

Fig. Fig. 3: Charcoal canister and valve



Click image to see an enlarged view

Fig. Fig. 4: Bimetal vacuum switching valve



Click image to see an enlarged view

Fig. Fig. 5: 2-way check valve



Click image to see an enlarged view

Fig. Fig. 6: Switch vent solenoid valve testing

The basic evaporative emission control system used on all fuel injected vehicles is the charcoal canister storage method. This method transfers fuel vapor from the fuel tank and, if equipped, the carburetor bowl, to an activated carbon storage device. This canister stores the vapors in activated charcoal when the vehicle is not running. When the engine is started, the fuel vapor is purged from the carbon element by intake air flow and consumed in the normal combustion process.

Carbureted Engines

Carbureted vehicles use a switch vent solenoid operated by the ignition switch and the Electronic Control Module (ECM). The switch vent prevents the fuel vapor in the float chamber of the carburetor from flowing out into the atmosphere.

When the ignition switch is in the OFF position, or when cranking the engine, the vent tube is closed by the solenoid valve and allows the fuel vapor to flow from the float chamber into the vapor storage canister. When the engine is operating, the solenoid receives an electrical signal from the ECM to keep the inner vent tube open. As a result, the vapor passes through the tube into the carburetor and is drawn into the engine.

Fuel Injected Engines

Fuel injected vehicles use a vacuum operated canister purge control valve. The canister purge control valve prevents the vapor in the fuel tank from flowing out into the atmosphere.

Certain conditions must be satisfied for the canister purge to occur. When the engine is running, the coolant temperature is operational and the throttle valve is at any position other than idle, the canister purge control valve will receive ported vacuum. As a result, fuel vapor in the canister is sucked into the intake manifold through the purge control valve and purge line.

When coolant temperature is below normal (cold start), the vacuum signal is interrupted by a bimetal vacuum switching valve and ported vacuum is not applied to the purge control valve. In this condition, the canister is not purged.

A 2-way check valve is also used to keep pressure in the fuel tank constant. When the pressure in the fuel tank becomes positive and reaches its specified value, it opens the valve to let the vapor flow to the charcoal canister. If a vacuum develops in the fuel tank, it opens the valve to let air flow into the tank.

SERVICE



The evaporative emissions system should be inspected at least once a year. Inspect the canister for internal damage or clogging. Clean with compressed air, as necessary. Inspect the fuel tank, cap and evaporative emission hoses for damage or leaks. Replace hoses as necessary.

Canister Purge Control Valve
  1. Using a small length of vacuum hose connected to the tank tube, blow into the canister. Air should escape the air tube at the bottom of the canister.
  2.  


CAUTION
The fuel vapor in the canister may be harmful. Care should be taken not to inhale the vapor.

  1. Using a small length of vacuum hose connected to the purge tube, blow into the canister. No air should pass through the valve.
  2.  
  3. Apply 20 in. Hg of vacuum to the vacuum signal line tube of the valve, while blowing air into purge tube. Air should come out of the throttle body (TB) tube.
  4.  
  5. If the valve fails to function as specified, replace the canister.
  6.  

Switch Vent Solenoid Valve
  1. Disconnect the canister hose from the carburetor bowl nozzle of the canister, and connect a new hose to the pipe connecting with the solenoid.
  2.  
  3. Blow air into the new hose with the ignition switch in the OFF and ON positions. Air should not pass through the solenoid valve.
  4.  


CAUTION
The fuel vapor in the canister and float chamber may be harmful. Care should be taken not to inhale the vapor.

  1. Start the engine and run at idle speed. Check the solenoid valve as before. Air should not pass through the solenoid.
  2.  
  3. If the valve fails to function as specified, replace the canister.
  4.  

Bi-Metal Vacuum Switching Valve
  1. Remove the vacuum hoses from the VSV.
  2.  
  3. Connect a length of hose to the valve with the engine cold.
  4.  
  5. Blow into the valve, air should not pass through the valve.
  6.  
  7. Start the engine and allow to reach 140°F (60°C).
  8.  
  9. Blow air into the valve, air should pass through the valve.
  10.  
  11. If the valve fails to function as specified, replace it.
  12.  

2-Way Check Valve
  1. Remove the valve and blow hard into the BLACK side of the valve. Air should pass through the valve.
  2.  
  3. Blow lightly into the ORANGE side of the valve, air should pass through the valve.
  4.  
  5. If the valve fails to function as specified, replace it.
  6.  

REMOVAL & INSTALLATION



Charcoal Canister
  1. Label and disconnect all vacuum lines from the canister.
  2.  
  3. As required, disconnect the electrical connector from the canister.
  4.  
  5. Loosen the canister mounting bracket and remove the canister.
  6.  
  7. Inspect and replace vacuum lines as necessary.
  8.  
  9. Installation is the reverse of removal.
  10.  

Bi-Metal Vacuum Switching Valve
  1. Label and disconnect all vacuum lines from the valve.
  2.  
  3. Remove the valve attaching screw, as required.
  4.  
  5. Remove the valve.
  6.  
  7. Inspect and replace vacuum lines as necessary.
  8.  
  9. Installation is the reverse of removal.
  10.  

2-Way Check Valve

The valve may be located either in the engine compartment or near the fuel tank at the rear of the vehicle.

  1. Raise and support the vehicle safely, as required.
  2.  
  3. Locate the check valve.
  4.  

The valve must be replaced in the same position. Take note which way the valve is removed.

  1. Label and disconnect the hoses from the valve.
  2.  
  3. Remove the valve.
  4.  
  5. Installation is the reverse of removal.
  6.  

 
label.common.footer.alt.autozoneLogo