GM Full-Size Trucks 1988-1998 Repair Information

Evaporative Emission Control System

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OPERATION



The Evaporative Emission Control System (EECS) is designed to prevent fuel tank vapors from being emitted into the atmosphere. Gasoline vapors are absorbed and stored by a fuel vapor charcoal canister. The charcoal canister absorbs the gasoline vapors and stores them until certain engine conditions are met and the vapors can be purged and burned by the engine.

The charcoal canister purge cycle is controlled either by a thermostatic vacuum switch or by a timed vacuum source. The thermostatic switch is installed in the coolant passage and prevents canister purge when engine operating temperature is below 115°F (46°C). The timed vacuum source uses a manifold vacuum-controlled diaphragm to control canister purge. When the engine is running, full manifold vacuum is applied to the top tube of the purge valve which lifts the valve diaphragm and opens the valve.

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
 
A disconnected, misrouted, kinked or damaged vapor pipe or canister hoses
 
A damaged air cleaner or improperly seated air cleaner gasket
 

TESTING



Vapor Canister

See Figure 1

  1. Apply a length of hose to the lower tube of the purge valve assembly and attempt to blow air through it. There should be little or no air should passing into the canister.
  2.  

If the canister is equipped with a constant purge hole, a small amount of air will pass into the canister.

  1. Using a hand-held vacuum pump, apply a vacuum of 15 in. Hg (51 kPa) to the control vacuum (upper) tube. If the vacuum does not hold for at least 20 seconds, the diaphragm is leaking. Replace the canister.
  2.  
  3. If the diaphragm holds vacuum, attempt to blow air through the hose connected to the PCV tube while vacuum is still being applied. An increase of air should be observed. If no increase is noted, the canister must be replaced.
  4.  



Click image to see an enlarged view

Fig. Fig. 1: Cross-section of the vapor canister

Fuel Tank Pressure Control Valve
  1. Attach a length of hose to the tank side of the valve assembly and try to blow air through it. Little or no air should pass into the canister.
  2.  
  3. Using a hand-held vacuum pump, apply vacuum equivalent to 15 in. Hg (51 kPa) to the control vacuum tube. If the diaphragm does not hold vacuum, the diaphragm is leaking. Replace the valve.
  4.  
  5. If the diaphragm holds vacuum, attempt to blow air through the hose connected to the valve while vacuum is still being applied. Air should pass. If no air is noted, the valve must be replaced.
  6.  

Thermostatic Vacuum Switch
  1. With engine temperature below 100°F (38°C), apply vacuum to the manifold side of the switch. The switch should hold vacuum.
  2.  
  3. Start and continue to run the engine until the engine temperature increases above 122°F (50°C). The vacuum should drop off.
  4.  
  5. Replace the switch if it fails either test.
  6.  

REMOVAL & INSTALLATION



Vapor Canister
  1. Tag and disconnect the hoses from the canister.
  2.  
  3. If equipped, unplug the electrical connector from the canister.
  4.  
  5. Remove the vapor canister retaining nut.
  6.  
  7. Remove the canister from the vehicle.
  8.  

To install:
  1. Install the canister. If necessary refer to the vehicle emission control label, located in the engine compartment for proper routing of the vacuum hoses.
  2.  
  3. If equipped, attach the canister electrical connection.
  4.  

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

To install:
  1. Install the thermostatic vacuum switch. Make sure to apply sealer to the switch threads.
  2.  
  3. Connect the vacuum hoses.
  4.  
  5. Refill the cooling system.
  6.  

Canister Purge Solenoid
except 1996-98 models

See Figure 2

  1. Disconnect the negative battery cable.
  2.  
  3. Disconnect the electrical connectors and if equipped, the hoses from the solenoid.
  4.  
  5. Pull the solenoid away from the bracket and remove the assembly.
  6.  



Click image to see an enlarged view

Fig. Fig. 2: The canister purge solenoid is usually mounted on a bracket-except 1996-98 models

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.  

1996-98 models

See Figure 3

  1. Disconnect the negative battery cable.
  2.  
  3. Unplug the solenoid electrical connection.
  4.  
  5. Unfasten the solenoid retaining bolts.
  6.  
  7. Remove the solenoid from its mounting on the intake manifold.
  8.  

To install:
  1. Place the solenoid in position on the intake manifold and install its retaining bolts.
  2.  
  3. Attach the solenoid electrical connection.
  4.  
  5. Connect the negative battery cable.
  6.  



Click image to see an enlarged view

Fig. Fig. 3: The canister purge solenoid is located on the intake manifold-1996-98 models

EVAP Vacuum Switch

See Figure 4

  1. Disconnect the negative battery cable.
  2.  
  3. Unplug the vacuum switch electrical connection.
  4.  
  5. Tag and disconnect the hoses from the switch.
  6.  
  7. Remove the switch.
  8.  
  9. Installation is the reverse of removal.
  10.  



Click image to see an enlarged view

Fig. Fig. 4: View of the EVAP vacuum switch used on 1996-98 models

 
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