Chevrolet Full Size Cars 1979-1989

Early Fuel Evaporation (EFE) System

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Fig. Fig. 1 The vacuum-servo type EFE system is most prevalent on these vehicles



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Fig. Fig. 2 Common vacuum servo EFE system with remote mounted actuator V8 engines

Most 1979 and later carbureted engines are equipped with this system to reduce engine warm-up time, improve driveability, and reduce emissions. On start-up, a vacuum motor acts to close a heat valve in the exhaust manifold which causes exhaust gases to enter the intake manifold heat riser passages and enter the thermostatic air cleaner. Incoming fuel mixture is then heated and more complete fuel evaporation is provided during warm-up.

The system consists of a Thermal Vacuum Switch (TVS), along with an exhaust heat valve and actuator. The Thermal Vacuum Switch is located on the coolant outlet housing on V8s, and on the block on inline 6-cylinder engines. When the engine is cold, the TVS conducts manifold vacuum to the actuator to close the valve. When engine coolant, or oil on the 6-cylinder engine, warms up, vacuum is interrupted and the actuator should open the valve.

On 1981 and later 231 V6 engines, the EFE system is controlled by the ECM.

As of 1981, some 231 V6 engines utilize a heater grid system instead of the vacuum servo system. Although the purpose of heater grid system remains the same (to reduce engine warm-up time, improve driveability and to reduce emissions) the operation is entirely different. The new system is electric and uses a ceramic heater grid located underneath the primary bore of the carburetor as part of the carburetor insulator/gasket. When the engine coolant is below the specified calibration level, electrical current is supplied to the heater through an ECM controlled relay.

CHECKING THE EFE SYSTEM



Vacuum Servo Systems


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Fig. Fig. 3 Common vacuum servo EFE system with heat valve mounted actuator



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Fig. Fig. 4 Vacuum serve EFE system early 231 V6 engine

  1. With the engine cold, have someone start the engine while you observe the vacuum servo exhaust heat valve (on some V8s, the EFE valve actuator arm is covered by a two-piece metal cover, which must be removed for service). The valve should snap to the closed position.
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  3. Watch the valve as the engine warms up. By the time coolant starts circulating through the radiator on most V-type engines or the oil is hot on inline engines, the valve should snap open.
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  5. If the valve does not close upon a cold start, immediately disconnect the hose at the actuator, and check for vacuum by placing your finger over the end of the hose, or with a vacuum gauge. If there is vacuum, immediately disconnect the hose leading to the TVS from the manifold at the TVS end of the hose. If there is vacuum here, but not at the actuator, replace the TVS. If vacuum does not exist at the hose going to the TVS, check that the vacuum hose is free of cracks or breaks and tightly connected at the manifold, and that the manifold port is clear.
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  7. If the valve does not open when the engine coolant or oil warms up, first make sure it is not seized in this position. Then, disconnect the hose at the actuator and check for vacuum by placing your finger over the end of the hose or using a vacuum gauge. If there is vacuum, replace the TVS. If there is no vacuum, replace the actuator.
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Heater Grid Systems


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Fig. Fig. 5 Heater grid relay and power supply schematic

  1. With the engine overnight cold, disengage the connector from the heater grid. Connect a test light or voltmeter between the harness connector terminals. Turn the ignition ON and energize the relay and check to see if power is available to the heater grid. The relay may be energized in 2 ways. The diagnostic test terminal of the underdash ECM connector may be grounded setting the ECM into diagnostic mode, at the end of which the ECM will energize the relay. Also, the relay connector terminal C (ECM ground wire) may be grounded to close the relay and provide power to the grid.
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  3. If no power is available, check the wiring supplying power to the relay. Make sure the fusible link and the ignition fuse are both good.
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  5. If power was present, check the heater grid using an ohmmeter. Take a resistance reading across the grid connector terminals. If the reading is under 3 ohms, the heater is good. If resistance is over 3 ohms, replace the heater.
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REMOVAL & INSTALLATION



Thermo Vacuum Switch (TVS)

The TVS is located on the engine coolant outlet housing for V8 engines or on the block for inline 6-cylinder engines. In order to replace the switch on V8 engines, the coolant must be drained to a level below the outlet housing. No oil need be drained on 6-cylinder engines. Upon installation, be sure to apply sealer to sensor threads on V8 engines. Use no sealer on inline 6-cylinder engines. Note that the valve must be installed until just snug (approximately 120 inch lbs.) and then turned by hand just far enough to line up the fittings for hose connection.

Heater Grid
  1. Disconnect the negative battery cable.
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  3. Remove the air cleaner assembly.
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  5. Tag and disconnect all electrical, vacuum and fuel connections from the carburetor.
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  7. Disconnect the EFE heater electrical connection.
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  9. Remove the carburetor assembly from the intake manifold. For details, refer to the procedure in of this guide.
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  11. Lift the EFE heater from the intake manifold.
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To install:

  1. Install the EFE heater to the intake manifold.
  2.  
  3. Install the carburetor assembly.
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  5. Engage the EGE heater electrical connection.
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  7. Engage all electrical, vacuum and fuel connections to the carburetor as noted during removal.
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  9. Install the air cleaner assembly, then connect the negative battery cable.
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  11. Start the engine and check for any leaks.
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