Mechanical Fuel Pump

Written by: Mike Ervin

The fuel pump is a single action diaphragm type.  All fuel pumps used on inline six cylinder and V8 engines in trucks are diaphragm type and because of design are serviced by replacement only.  No adjustments or repairs are possible.  The pump is operated by an eccentric on the camshaft on gasoline engines.  On six cylinder engines, the eccentric acts directly on the pump rocker arm.  On V8 engines, a pushrod between the camshaft eccentric and the fuel pump operates the pump rocker arm.  Some trucks have a fuel pump which has a metering outlet for a vapor return system; any vapor which forms is returned to the fuel tank along with hot fuel through a separate line.  This greatly reduces any possibility of vapor lock by keeping cool fuel from the tank constantly circulating through the fuel pump.

WARNING!  When ever working around gasoline, always take necessary measures to contain any fuel spillage.  Before removing the fuel pump make sure the engine is cool to the touch to avoid a fire.  Also it would be a good idea to have a fire extinguisher handy in case something does go wrong.


Fuel pumps should always be tested on the truck.  The larger line between the pump and tank is the suction side of the system and the smaller line, between the pump and carburetor is the pressure side.  A leak in the pressure side would be apparent because of dripping fuel.  A leak in the suction side is usually only apparent because of a reduced volume of fuel delivered to the pressure side.

  • Tighten any loose line connections and look for any kinks or restrictions.
  • Disconnect the fuel line at the carburetor.  Disconnect the distributor-to-coil primary wire.  Place a container at the end of the fuel line and crank the engine a few revolutions.
  • If little or no fuel flows from the line, either the fuel pump is inoperative or the line is plugged.  Blow through the lines with compressed air and try the test again.  Reconnect the line.
  • If fuel flows in good volume, check the fuel pump pressure to be sure.
  • Attach a pressure gauge to pressure side of the fuel line.  On trucks equipped with a vapor return system, squeeze off the return hose.
  • Run the engine at idle and note the reading on the gauge.  Stop the engine and compare the reading with the following:
      6-cylinder inline engines.....4.5 to 6 psi.
      V8 engines.....5 to 9 psi.

    If the pump is operating properly, the pressure will be as specified and will be constant at idle speed.  If pressure varies sporadically or is too high or low, the pump should be replaced.

  • Remove the pressure gauge.  The following flow test can also be performed.
  • Disconnect fuel line from carburetor.  Run fuel line into a suitable measuring container.
  • Run the engine at idle until there is one pint of fuel in the container.  One pint should be pumped in 30 seconds or less.
  • If flow is below minimum, check for a restriction in the line.

The only way to check fuel pump pressure is by connecting an accurate pressure gauge to the fuel line at the carburetor level.  Never replace a fuel pump without performing this simple test.  If the engine seems to be starving out, check the ignition system first.  Also check for a plugged fuel filter or a restricted fuel line before replacing the pump.


  • Disconnect the fuel intake and outlet lines at the pump and plug the pump intake line.
  • On small block V8 engines, remove the upper bolt from the right front mounting boss.  Insert a long bolt (3/8 in.-16 x 2 in.) in this hole to hold the fuel pump pushrod.
  • Remove the two pump mounting bolts and lockwashers; remove the pump and its gasket.
  • If the fuel pump pushrod is to be removed from V8s, remove the two adapter bolts and lockwashers and remove the adapter and its gasket from small blocks and remove the pipe plug and pushrod from 454 cu. in. engines.
  • Install the fuel pump with a new gasket reversing the removal procedure.  Coat the mating surfaces with sealer.
  • Connect the fuel lines and check for leaks.