GM S10/S15/Sonoma Pick-Ups 1982-1993 Repair Manual

Early Fuel Evaporation (EFE) System

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OPERATION



Carbureted engines covered by this manual should be equipped with an EFE system. The purpose of the system is to promote the vaporization of fuel droplets as they enter the intake manifold during cold engine operation. A heater grid is used to provide the heat necessary to promote vaporization of the air/fuel mixture which then ensures complete combustion providing the maximum power and minimum emissions output for the fuel used. In this way, the system provides quick fuel evaporation and more uniform fuel distribution during cold starts. The system also reduces the length of carburetor choking time, thus reducing exhaust emissions.

The system consists of a ceramic heater grid (located between the carburetor and the intake manifold) and a temperature switch (all non-ECM models) and/or a relay (ECM models and 1985 non-ECM models) which activates the heater during cold operation. The relay, usually located under the right fender, is operated by the ECM on most engines. Some 1985 non-ECM controlled models utilize the relay in the ignition circuit.

As the coolant temperature increases, the temperature switch (non-ECM models) or the relay (ECM models) turns Off the current to the ceramic heater, allowing the engine to operate normally.

Operational checks should be made at normal maintenance intervals.

TESTING



See Figures 1, 2, 3 and 4



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Fig. Fig. 1: EFE system schematic-pre-1985 non-ECM controlled



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Fig. Fig. 2: EFE system schematic-1985 non-ECM controlled



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Fig. Fig. 3: EFE system check-non-ECM controlled



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Fig. Fig. 4: EFE system check-ECM controlled

If trouble is suspected with the EFE heater grid system, follow the trouble tree charts in the accompanying figures in order to locate the problem. In following these charts you will need some basic electrical test equipment, a volt/ohmmeter and a test light. For more information on electrical system troubleshooting and test equipment, please refer to Chapter 6 of this manual.

REMOVAL & INSTALLATION



EFE Heater

See Figures 5, 6, 7, 8 and 9



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Fig. Fig. 5: Disengage the EFE wiring connector



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Fig. Fig. 6: Carefully lift the EFE from the intake manifold



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Fig. Fig. 7: When cleaning the gasket mounting surface, cover the opening to keep debris out of the engine



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Fig. Fig. 8: EFE heater unit installation-2.0L engine



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Fig. Fig. 9: EFE heater unit installation-2.8L engine

The EFE heater unit is located directly under the carburetor and is electrically operated.

  1. Disconnect the negative battery terminal, then remove the air cleaner assembly.
  2.  
  3. From the carburetor, tag and disconnect the vacuum hoses, electrical wiring and fuel hoses. Disengage the EFE heater electrical connector from the wiring harness.
  4.  
  5. Remove the carburetor-to-intake manifold retainers, then carefully remove the carburetor from the manifold.
  6.  

Be careful once the carburetor is removed from the manifold not to allow anything (bolts, debris) to fall into the opening or the manifold may have to be removed.

  1. Lift the EFE heater unit from the intake manifold.
  2.  

To install:
  1. Using a putty knife, clean the gasket mounting surfaces carefully.
  2.  

During installation, replace all gaskets to assure proper seal and prevent vacuum leaks.

  1. Position the EFE heater unit to the intake manifold.
  2.  
  3. Install the carburetor to the intake, then tighten the carburetor-to-intake manifold retainers to 13 ft. lbs. (18 Nm).
  4.  
  5. Engage the EFE wiring connector to the harness.
  6.  
  7. Connect the fuel hoses, electrical wiring and vacuum hoses as noted during removal.
  8.  
  9. Install the air cleaner assembly and connect the negative battery terminal.
  10.  

Temperature Switch (Non-ECM Models)

See Figures 10, 11, 12 and 13

The temperature or heater switch is located near the thermostat housing (2.8L carbureted engine), on the bottom rear-side of the intake manifold (2.0L carbureted engine) or on the top right-side of the engine (1.9L carbureted engine).

  1. Position a drain pan under the radiator, then open the drain cock and drain the coolant to a level below the heater switch.
  2.  



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Fig. Fig. 10: EFE heater switch installation-2.0L engine



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Fig. Fig. 11: Disengaging the EFE switch connector on a 2.8L engine



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Fig. Fig. 12: EFE heater switch installation-2.0L engine



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Fig. Fig. 13: EFE heater and switch-1.9L engine

  1. Disengage the wiring harness connector from the EFE heater switch.
  2.  
  3. Remove the EFE heater switch (turn it counterclockwise) from the intake manifold.
  4.  

To install:

If replacing the temperature switch, refer to the calibration number stamped on the base.

  1. Coat the threads of the EFE heater switch with a soft setting sealant, then thread it into the intake and tighten to 10 ft. lbs. (14 Nm).
  2.  

When applying sealant to the EFE heater switch threads, be careful not to coat the sensor and/or the switch.

  1. Reconnect the wiring harness to the EFE heater switch.
  2.  
  3. Properly refill the cooling system. Coolant can be reused unless it is several years old or appears contaminated.
  4.  

Relay (ECM Models and 1985 Non-ECM Models)

See Figures 14 and 15



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Fig. Fig. 14: Common EFE heater switch relay mounting for the 2.8L engine



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Fig. Fig. 15: Common EFE heater switch relay used on later model carbureted engines

The heater switch relay is usually located on the left-fender.

  1. Disconnect the negative battery cable.
  2.  
  3. Disengage the wiring harness connector(s) from the heater switch relay.
  4.  
  5. Remove the heater switch relay-to-bracket screw, then remove the relay from the truck.
  6.  
  7. To install, reverse the removal procedures.
  8.  

 
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