GM Prizm/Nova 1985-1993 Repair Guide

Mechanical Fuel Pump

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REMOVAL & INSTALLATION



See Figure 1

Since the position of the fuel tank is lower than the carburetor, fuel cannot flow to the carburetor under its own power. The mechanical fuel pump is a diaphragm type with built in check valves in the pump chambers. These valves open only in the direction of fuel flow.

The pump is located at the side of the cylinder head intake manifold. Remove the fuel pump as follows:



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Fig. Fig. 1: Location of mechanical fuel pump on engine

  1. With the engine cold and the key removed from the ignition, label and disconnect the fuel hoses from the fuel pump. Plug the lines as soon as they are removed.
  2.  


CAUTION
The fuel system contains gasoline. Wear eye protection and contain spillage. Observe no smoking/no open flame precautions. Have a Class B-C (dry powder) fire extinguisher within arm's reach at all times.

  1. Remove the mounting bolts from the fuel pump.
  2.  
  3. Remove the fuel pump and the heat insulator assembly.
  4.  
  5. Cover the fuel pump mounting surface on the cylinder head to prevent oil leakage.
  6.  

To install:
  1. When reinstalling, always use a new gasket. Place the pump and heat insulator in position and install the 2 bolts.
  2.  
  3. Connect the hoses to the fuel pump.
  4.  
  5. Start the engine and check for leaks.
  6.  

TESTING



See Figures 2 through 6

Before performing any checks on the fuel pump, two conditions must be met. First, the pump must be internally wet. Run a small amount of fuel into the pump so that the check valves will seal properly when tested. Dry valves may not seal and will yield false test results.



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Fig. Fig. 2: Fuel pump components

Hold the pump without blocking either pipe and operate the pump lever, noting the amount of force needed to move it. This is the reference point for all the tests. Do not apply more than this amount of force to the lever during the testing. Excessive force can damage an otherwise usable pump.

  1. To check the inlet valve, block off the outlet and return pipes with your fingers. Operate the lever. There should be an increase in the free-play and the arm should move freely.
  2.  
  3. Check the outlet valve by blocking the inlet port with your finger and operating the lever. The arm should lock when the normal amount of force is applied.
  4.  
  5. The diaphragm is checked by blocking the inlet and outlet pipes. When normal force is applied to the lever, the lever should lock and not move. Any lever motion indicates a ruptured diaphragm. This is a common cause of poor fuel mileage and poor acceleration since the correct amount of fuel is not being delivered to the carburetor.
  6.  

The fuel pump must pass all three of these tests to be considered usable. If the pump fails 1 or more tests, it must be replaced.

  1. Check the oil seal within the pump. Block off the vent hole in the lower part of the pump housing. The lever arm should lock when normal force is applied.
  2.  



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Fig. Fig. 3: Checking the inlet valve



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Fig. Fig. 4: Checking the outlet valve



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Fig. Fig. 5: Checking the fuel pump diaphragm



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Fig. Fig. 6: Checking the fuel pump oil seal

 
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