Mitsubishi Pick-ups and Montero 1983-1995 Repair Guide

Mechanical Fuel Pump



See Figures 1 and 2

Mechanical fuel pumps are found on all non-diesel Pick-up trucks through 1989 and all 4-cylinder Monteros. The mechanical fuel pump is mounted on the side of the head, centered between the runners of the intake manifold. The intake manifold does not require removal to remove the pump, but access can be tricky. A selection of socket extensions, swivels and open end wrenches can be helpful.

A pushrod or lever arm runs from a special camshaft lobe to the pump rocker arm, transferring the motion to the pump diaphragm. As the diaphragm moves, fuel is alternately drawn into the pump and then sent to the carburetor.

Gasoline in either liquid or vapor state is EXTREMELY explosive. Take great care to contain spillage. Work in an open or well-ventilated area. Do not connect or disconnect electrical connectors while fuel hoses are removed or loosened. Observe no smoking/no open flame rules during repairs. Have a dry-chemical fire extinguisher (type B-C) within arm's reach at all times and know how to use it.

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Fig. Fig. 1: Mechanical fuel pump assembly-2.0L engines

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Fig. Fig. 2: Mechanical fuel pump assembly-2.6L engines

  1. Disconnect the negative battery cable. Depending on the vehicle, it may be necessary to remove the air cleaner assembly.
  3. Set the engine to TDC/compression for No. 1 cylinder. This is generally done by turning the crankshaft pulley bolt clockwise (never counterclockwise) until the timing marks on the case align. Double check the positioning by removing the distributor cap and making sure the rotor points to the No. 1 terminal.
  5. Label or diagram the fuel lines running to the pump. Disconnect the three fuel lines by using a pair of pliers to shift clamps away from the nipples on the pump and then pulling the lines off with a twisting motion.
  7. Remove the two mounting bolts from the head. Remove the pump, spacer, and two gaskets from the head. As you pull the pump off the head, catch the pushrod which is located just behind the pump. If the pump is the lever type, it will come off with the pump.
  9. Clean the gasket surfaces of the insulator, pump and cylinder head. Insert the two bolts through the pump's mounting base. Slide a new gasket, the insulator, and a second new gasket into position over the two bolts. Turn the pump so its mounting surface faces the cylinder head.
  11. Apply a light coat of clean engine oil to the pushrod. Place the pump pushrod against the cupped surface of the operating lever and angle it upward in the position it was in during removal. Hold the pushrod at that angle as you insert it into the bore in the head. Once the pushrod is in the bore in the cylinder head, you can release your fingers and move the pump toward the head along the line of the pushrod.If the pump is the lever-arm type, make certain the arm is installed above the camshaft. The cam will move the lever as it turns.
  13. Start the two bolts into the bores in the head and tighten them finger tight. Tighten the mounting bolts alternately and evenly to 12 ft. lbs. (16 Nm).
  15. Inspect the hoses for cracks (even hairline cracks can leak) and replace if necessary. Reconnect the fuel hoses. Make sure the hoses are installed all the way onto the nipples.
  17. Work the clamps into position. Make sure the clamps are located well past the bulged portion of the nipples but do not sit at the extreme ends of the hoses.
  19. Replace the distributor cap.

If the fuel pump was replaced because of a ruptured diaphragm, there is a strong possibility of fuel contamination in the engine oil. Changing the oil and oil filter is strongly recommended after the fuel pump is replaced.

  1. Install the air cleaner assembly if it was removed.
  3. Connect the negative battery cable. Start the engine and check for leaks. Because the fuel lines have been partially emptied, the engine may crank for a longer than normal period before starting.


On Vehicle Testing
  1. Disconnect the inlet line (coming from the filter) at the pump. Connect a vacuum gauge to the pump nipple. Remove the coil-to-distributor high tension cable at the coil.
  3. Have an assistant crank the engine with the key as you watch the gauge. A vacuum should be produced in a regular cycle as the pump turns over. There should be no blowback of pressure (abrupt drop in vacuum or even positive pressure for a short time). If there is blowback of pressure, the inlet valve on the pump is leaking and the unit must be replaced.
  5. The vacuum shown with each pump stroke should be strong and constant. If the vacuum is low (or none at all), the diaphragm is leaking and the pump must be replaced.

Off Vehicle Testing
  1. Inspect the small breather hole or tube (above the diaphragm) which vents the pump's upper chamber. Leakage of fuel or oil here confirms that the diaphragm or oil seal is leaking.
  3. Inspect the end of the pushrod and the contact surface on the pump operating lever. Replace the pushrod or pump if there is obvious wear. If the camshaft end of the pushrod is badly worn, remove the valve cover and inspect the camshaft eccentric (which operates the fuel pump) for excessive wear.
  5. If the pump has a lever arm, check the contact pad (face) of the lever for wear or scoring. Inspect the camshaft if heavy scoring is present. Check the motion of the arm and the tension of the spring.