GM Full-Size Trucks 1980-1987 Repair Guide

Evaporative Emission Control System

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GENERAL INFORMATION



See Figures 1 and 2

The Evaporative Emission Control System (EECS) is designed to prevent fuel tank vapors from being emitted into the atmosphere. The primary component in the system is the charcoal canister. The canister absorbs and stores gasoline vapors from the carburetor fuel bowl and the fuel tank for burning at a later time. On vehicles equipped with an auxiliary fuel tank, an additional canister is installed to increase the vapor collection capacity. When engine coolant temperature reaches 49ºC (120ºF) and the engine is off idle, the system will allow the vapors to be purged into the intake manifold and burned by the engine.

Three valves are used in the system; the canister purge control valve which prevents vapors from escaping from the canister when the system is not being purged, the vapor vent control valve which prevents venting of the fuel bowl during engine operation and the Thermostatic Vacuum Switch (TVS) which prevents purging before the engine warms. The purging cycle is controlled by the Thermostatic Vacuum Switch (TVS), which is installed in the coolant passage opens and closes depending on the temperature of the coolant. When engine operating temperature is below 115ºF (46ºC) the thermostatic vacuum switch is open and prevents vacuum from being applied to the canister purge control valve. When coolant temperature reaches 49ºC (120ºF) the thermostatic vacuum switch closes to allow vacuum to be applied to the canister purge control valve. The applied vacuum lifts the canister purge valve diaphragm to open the valve and pull the collected fuel vapors into the intake.

A vent located in the fuel tank allows fuel vapors to flow to the charcoal canister. A tank pressure control valve, used on high altitude applications, prevents canister purge when the engine is not running. The fuel tank cap does not normally vent to the atmosphere but is designed to provide both vacuum and pressure relief.

Poor engine idle, stalling and poor driveability can be caused by a damaged canister or split, damaged or improperly connected hoses.

Evidence of fuel loss or fuel vapor odor can be caused by a liquid fuel leak; a cracked or damaged vapor canister; disconnected, misrouted, kinked or damaged vapor pipe or canister hoses; or a damaged air cleaner or improperly seated air cleaner gasket.

California Models

On California models equipped with the Computer Command System the thermostatic vacuum switch is replaced with a electric solenoid valve that is controlled by the Engine Control Module (ECM). The Engine Control Module monitors coolant temperature, vehicle speed, and throttle position and then determines when to turn on the purge solenoid. The Engine Control Module (ECM) operates the valve by applying a ground to the solenoid.



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Fig. Fig. 1: Evaporative Emission Control System (EECS)



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Fig. Fig. 2: Vapor canister assembly

TESTING



Canister Purge Control Valve
  1. Connect a short length of hose to the vapor purge port on the canister purge valve and attempt to blow through the valve. Little or no air should pass through the valve. If air does pass through the valve, replace the canister.
  2.  
  3. Connect a hand vacuum pump to the purge valve vacuum port on the canister purge valve. Apply 51 kPa (15 in. Hg) to the purge valve diaphragm. If the diaphragm cannot hold vacuum for at least 20 seconds, the valve should be replaced.
  4.  
  5. If the diaphragm does hold vacuum, try to blow again into the vapor purge port. An increased flow of air should be observed, if not replace the canister.
  6.  

Vapor Vent Control Valve
  1. Connect a short length of hose to the vapor vent control valve's fuel bowl port and attempt to blow through the valve. Air should pass through the valve freely. If not replace the canister.
  2.  
  3. Connect a hand vacuum pump to the vapor vent control valve vacuum port. Apply 51 kPa (15 in. Hg) to the valve diaphragm. If the diaphragm cannot hold vacuum for at least 20 seconds, the valve should be replaced.
  4.  
  5. If the diaphragm does hold vacuum, try to blow again into the valve's fuel bowl port. Little or no air should pass through the valve, if not replace the canister.
  6.  

Thermostatic Vacuum Switch
  1. Allow the switch to cool below the calibration temperature which is stamped on the valve.
  2.  
  3. Connect a hand vacuum pump to the output port of the TVS switch and apply 51 kPa (15 in. Hg) of vacuum. A leakage of up to 7 kPa (2 in. Hg) of vacuum in two minutes is allowable and does not indicate a defective switch.
  4.  
  5. Heat the TVS switch in some water above the calibration temperature.
  6.  

Do not heat the TVS switch directly with an open flame.

  1. Check if the valve opens (indicated by a loss of vacuum) at the calibration temperature.
  2.  

Canister Purge Solenoid
  1. Pull off the vacuum line to the top of the valve with the engine running. There should be vacuum in the line. Replace the line. No air should be escaping with the engine running at a steady idle.
  2.  

  1. Disconnect the harness connector from the solenoid and then turn the ignition on. Connect a voltmeter or test light between terminal A on the harness connector and ground.
    1. If 10 volts or greater is measured or the test light lights, the solenoid is getting power.
    2.  
    3. If less than 10 volts is measured or the test light does not light, the solenoid is not getting power due to an open or short to ground in the wiring.
    4.  

  2.  
  3. Measure the resistance across the solenoid by connecting an ohmmeter between the two terminals on the solenoid. Make sure that the solenoid is disconnected or a false reading may occur. The resistance of the solenoid should equal about 20 ohms.
  4.  

  1. If the resistance is less than 20 ohms, the solenoid is shorted. Replace the solenoid and the Engine Control Module (ECM).
  2.  
  3. If the resistance is more than 20 ohms, the solenoid is open. Replace the solenoid.
  4.  

REMOVAL & INSTALLATION



Canister
  1. Tag and disconnect the hoses from the canister.
  2.  
  3. Remove the vapor canister retaining nut.
  4.  
  5. Remove the canister from the vehicle.
  6.  

To install:
  1. Install the canister.
  2.  
  3. Tighten the vapor canister retaining nut.
  4.  
  5. Reconnect the hoses to the canister. Refer to the vehicle emission control label, located in the engine compartment for proper routing of the vacuum hoses.
  6.  

Thermostatic Vacuum Switch
  1. Drain the cooling system to below the switch level.
  2.  
  3. Disconnect and label the vacuum hoses from the switch.
  4.  
  5. Remove the thermostatic vacuum switch.
  6.  

To install:
  1. Apply thread sealant on the replacement switch.
  2.  
  3. Tighten the thermostatic vacuum switch to 120 inch lbs. (14 Nm).
  4.  
  5. Connect the vacuum hoses.
  6.  
  7. Refill the cooling system.
  8.  

Canister Purge Solenoid
  1. Disconnect the negative battery cable.
  2.  
  3. Disconnect the electrical connectors and hoses from the solenoid.
  4.  
  5. Pull the solenoid away from the bracket and remove the assembly.
  6.  

To install:
  1. Install the solenoid to the bracket by sliding it into place.
  2.  
  3. Connect the electrical connectors and the hoses.
  4.  
  5. Connect the negative battery cable.
  6.  

 
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