GM Full-Size Trucks 1988-1998 Repair Information

Mechanical Fuel Pump


The carbureted engines all use a mechanical fuel pump located on the right side of the engine. The pump is a diaphragm type pump and is actuated by a rocker arm through a link and pull rod.

Some engines have a special pump with a metering outlet for a vapor return system. Any vapor that is formed is returned to the fuel tank along with hot fuel. This reduces the chance of vapor lock because cool fuel from the tank is always circulated through the fuel pump.

The fuel pump rocker arm is moved back and forth by a rod which rides on an eccentric on the engine camshaft.


See Figures 1, 2 and 3

Observe all applicable safety precautions when working around fuel. Whenever servicing the fuel system, always work in a well ventilated area. Do not allow fuel spray or vapors to come in contact with a spark or open flame. Keep a dry chemical fire extinguisher near the work area. Always keep fuel in a container specifically designed for fuel storage; also, always properly seal fuel containers to avoid the possibility of fire or explosion.

When you connect the fuel pump outlet fitting, always use two wrenches to avoid damaging the pump.

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

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Fig. Fig. 1: Mechanical fuel pump-4.8L engines

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Fig. Fig. 2: Mechanical fuel pump-5.7L engines

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Fig. Fig. 3: Mechanical fuel pump-7.4L engines

  1. You can remove the upper bolt from the right front engine mounting boss (on the front of the block) and insert a long bolt to hold the fuel pump pushrod on the 5.7L engine.
  3. Remove the two pump mounting bolts and lockwashers; remove the pump and its gasket. The 5.7L utilizes a mounting plate between the pump and gasket.
  5. If the rocker arm pushrod is to be removed, unfasten the two adapter bolts and lockwashers and remove the adapter and its gasket.

To install:
  1. Install the fuel pump with a new gasket. Tighten the mounting bolts to 17 ft. lbs. (23 Nm) on the 4.8L engine, 27 ft. lbs. (34 Nm) on the 7.4L engine and the top bolt on the 5.7L engine, and, 3 ft. lbs. (4 Nm) on the 5.7L engine lower bolt. Heavy grease can be used to hold the fuel pump pushrod up when installing the pump. Coat the mating surfaces with sealer.
  3. Connect the fuel lines an check for leaks.



See Figure 4

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Fig. Fig. 4: Pressure testing the mechanical fuel pump

Fuel pumps should always be tested on the vehicle. 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.

  1. Tighten any loose line connections and look for any kinks or restrictions.
  3. Disconnect the fuel line at the carburetor. Disengage 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.
  5. If fuel flows in good volume, check the fuel pump pressure to be sure.
  7. Attach a low pressure gauge to the pressure side of the fuel line. On trucks equipped with a vapor return system, squeeze off the return hose.
  9. Run the engine at idle and note the reading on the gauge. Stop the engine and compare the reading with the specifications listed in the Tune-Up Specifications chart located in Section 1. 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.
  11. Remove the pressure gauge.


  1. Disconnect the fuel line from the carburetor. Run the fuel line into a suitable measuring container.
  3. 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.
  5. If the 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.