GM Caprice 1990-1993 Repair Guide

Mechanical Fuel Pump


The fuel pump is a single diaphragm type. The mechanical fuel pump used on the 5.0L (VIN Y) engine is of the diaphragm type and because of the design is serviced by replacement only. No adjustments or repairs are possible.

The fuel pump is mounted on the right front of the engine. The fuel pumps is operated by an eccentric on the front of the camshaft sprocket. The spring loaded fuel pump rocker arm is in constant contact with eccentric on the camshaft sprocket.


See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Carbureted engine fuel pump - 5.0L (VIN Y) engine

Do not use a droplight around work area when disconnecting fuel lines. If fuel contacts a droplight, the bulb may explode causing serious personal injury not to mention material damage. Instead a flashlight may be used.

  1. Disconnect the fuel intake and outlet lines at the pump and plug the pump intake line.

When disconnecting the fuel pump outlet fitting, always use 2 wrenches to avoid twisting the line.

  1. Remove the two pump mounting bolts and lockwashers; remove the pump and its gasket.
  3. Install the fuel pump with a new gasket reversing the removal procedure. Coat the mating surfaces with sealer.
  5. Connect the fuel lines and check for leaks.


To determine if the pump is in good condition, tests for both volume and pressure should be performed. The tests are made with the pump installed, and the engine at normal operating temperature and idle speed. Verify the fuel tank has a sufficient quantity of fuel to perform the test. Never replace a fuel pump without first performing these simple tests.

Ensure the fuel filter has been changed at the specified interval. If in doubt, install a new filter first. Always check for broken or deteriorated fuel hoses. If a line has a crack or split, the pump may be operating properly, but the pump will only draw air, not fuel.

Pressure Test
  1. Disconnect the fuel line at the carburetor and connect a fuel pump pressure gauge. Ensure the carburetor float bowl has a sufficient amount of gasoline.
  3. Start the engine and check the pressure with the engine at idle. If the pump has a vapor return hose, squeeze it off so that an accurate reading can be obtained. Pressure should be 5 1 / 2 -6 1 / 2 psi.
  5. If the pressure is incorrect, replace the pump. If it is OK, go on to the volume test.

Volume Test
  1. Disconnect the pressure gauge. Run the fuel line into a graduated container.
  3. Run the engine at idle until a 1 / 2 pint of gasoline has been pumped. A 1 / 2 pint should be delivered in approximately 15 seconds or less. There is normally enough fuel in the carburetor float bowl to perform this test, but refill it if necessary.
  5. If the delivery rate is below the minimum, check the lines for restrictions, cracks or leaks, then replace the pump.