GM Lumina/Silhouette/Transport APVs 1990-1999 Repair Guide

Evaporative Emission Controls

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OPERATION



The basic 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 airflow and consumed in the normal combustion process.

Gasoline vapors from the fuel tank flow into the tube labeled tank. These vapors are absorbed into the carbon. The canister is purged by the PCM control when the engine has been running for a specified amount of time. Air is drawn into the canister and mixed with the vapor and then is drawn into the intake manifold.

The fuel tank filler cap is an integral part of the system in that it was designed to seal in fuel vapors. If it is lost or damaged, make sure the replacement is the correct size and fit so a proper seal can be obtained.

See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Shows typical evaporative emission (EVAP) control system schematic. Remember the fuel cap is also part of the System

COMPONENT TESTING



The PCM operates a normally closed solenoid valve that allows vacuum to purge the canister when energized. The PCM supplies a Pulse Width Modulated (PWM) ground to energize the solenoid (purge ON).

See Figure 2

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Fig. Fig. 2: Typical location of the EVAP Purge Solenoid (1) and vacuum switch (2) mounting 3.8L Engine

Preliminary checks



Inspect the canister for cracks or fuel saturation.
 
Inspect the fuel vapor lines from the fuel tank or intake for leaks, disconnected hoses, or sharp bends blocking the vapor flow.
 

See Figure 3

Click image to see an enlarged view

Fig. Fig. 3: Diagnostic flow chart for EVAP Canister Purge

Chart Test Description: Numbers below refer to the circled numbers on the diagnostic chart.

  1. Checks to see if the valve is opened or closed. The solenoid is normally de-energized in this step, so it should be closed.
  2.  
  3. This step completes the functional check. This should normally energize the solenoid and allow vacuum to the gauge (purge "ON")
  4.  

Removal & Installation Canister Purge Valve

  1. To replace the canister purge valve, label and detach the electrical connector and hoses from the valve.
  2.  
  3. Remove the valve assembly from the bracket.
  4.  

See Figure 4

Click image to see an enlarged view

Fig. Fig. 4: Typical Canister Purge Valve Installation

To install:
  1. Mount the valve to the bracket.
  2.  
  3. Connect the electrical connector and hoses to the valve.
  4.  

Fuel Vapor Canister

  1. Label and disconnect the hoses from the canister.
  2.  
  3. Loosen and/or remove the hold down strap.
  4.  

See Figure 5

Click image to see an enlarged view

Fig. Fig. 5: The EVAP canister is usually mounted low along the frame rail.

  1. Remove the canister.
  2.  

To install:
  1. Install the canister.
  2.  
  3. Secure with the hold down strap.
  4.  
  5. Install the hoses. Make sure that the connections are correct.
  6.  

 
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