GM Full-Size Trucks 1980-1987 Repair Guide

Early Fuel Evaporation

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GENERAL INFORMATION



See Figures 1 and 2

The EFE system consists of an EFE valve at the flange of the exhaust manifold, an actuator, and a thermal vacuum switch (TVS). The TVS is located in the coolant outlet housing and controls vacuum application to the EFE actuator valve at the exhaust manifold.

When the engine is cold, manifold vacuum is applied to the EFE actuator, which closes the EFE valve. When the EFE valve is closed, hot exhaust gases are routed to the base of the carburetor. The hot exhaust gases help warm the intake air temperature which improves fuel evaporation. This helps reduce the length of time that the choke is engaged. Once the engine coolant temperatures reach a set limit, the thermal vacuum switch closes to prevent vacuum from reaching the EFE actuator. Without vacuum applied, the EFE valve opens which reroutes exhaust out the exhaust system.

If the EFE system does not operate when the engine is cold, stalling and stumbling may be experienced until the engine warms up. If the EFE valve were to get stuck in the closed position, overheating and lack of power after the engine has warmed may be experienced.



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Fig. Fig. 1: EFE valve installation



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Fig. Fig. 2: Thermal Vacuum Switch (TVS)

TESTING



Thermal Vacuum Switch
  1. Allow the switch to cool below the calibration temperature which is stamped on the valve.
  2.  
  3. Heat the thermal vacuum switch in some water above the calibration temperature.
  4.  
  5. Check if the valve closes (holds vacuum) at the calibration temperature by connecting a hand vacuum pump to the output port of the switch. 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.
  6.  

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

EFE Valve and Actuator
  1. Disconnect vacuum hose from EFE actuator.
  2.  


WARNING
The EFE valve actuator and exhaust system components will be extremely hot and could severely burn you if the engine was recently run. Do not touch the EFE valve actuator or any exhaust components until they have cooled.

  1. Check that the EFE valve actuator arm can be moved. Lubricate the valve if the valve will not move freely.
  2.  
  3. Start the engine and check for vacuum at the hose when the engine is cold. If no vacuum is detected check the hose and test the thermal vacuum switch.
  4.  
  5. Check if the EFE valve actuator closes (holds vacuum) by connecting a hand vacuum pump to the vacuum port of the valve. Apply 51 kPa (15 in. Hg) of vacuum. If the valve does not close, replace the EFE actuator.
  6.  

REMOVAL & INSTALLATION



EFE Valve and Actuator
  1. Disconnect the vacuum hose from the valve.
  2.  
  3. Remove the exhaust pipe to manifold nuts and tension springs.
  4.  
  5. Lower crossover exhaust pipe and seal. Complete removal of the pipe is not always required.
  6.  
  7. Remove the EFE valve and actuator.
  8.  

To install:
  1. Install EFE valve actuator with new seals and gaskets.
  2.  
  3. Install crossover exhaust pipe seal.
  4.  
  5. Reattach exhaust crossover pipe. Tighten exhaust pipe to manifold nuts to 15 ft. lbs. (20 Nm).
  6.  
  7. Connect vacuum hose to EFE actuator valve.
  8.  

Thermal Vacuum Switch

See Figure 2

  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 thermal vacuum switch.
  6.  

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

 
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