Fuel systems for our Jeeps.

How a pump works

These mechanical fuel pumps are the diaphragm type, driven from an eccentric on the engine camshaft. As the eccentric moves the rocker arm the diaphragm is pulled against the pressure of the diaphragm spring. A vacuum is formed in the fuel chamber and fuel is drawn in through the inlet check valve(s) from the fuel tank. As the eccentric rotates the pressure on the rocker arm is released and the diaphragm spring pushes the diaphragm forcing fuel out to the carburetor through the outlet check valve(s). When the carburetor does not require fuel, the pressure in the fuel chamber holds the diaphragm and diaphragm spring in compression and fuel does not flow. The fuel pump output pressure is proportional to the force of the diaphragm spring.

The vacuum booster section of the fuel pump operates in the same fashion as the fuel pump. Its purpose is to provide a vacuum for windshield wiper operation under higher throttle conditions where the engine manifold vacuum is too low for satisfactory wiper operation. When the engine manifold vacuum is greater than that created by the pump, the stronger manifold vacuum pulls the diaphragm into the air chamber against spring pressure and the pump does not pump.

Helpful information

The CJ-3A, CJV-35/U, and M38 utilize spacers between the fuel pump and the engine block. Over the years these spacers are often removed for various reasons. The CJ-2A style fuel pump is often retrofitted to these vehicles due to its lower cost. This is a fuel pump only, without the vacuum booster section. It does not require a spacer for proper operation. The lack of a spacer on the original CJ-3A, CJV-35/U, and M38 fuel pumps can cause excessive fuel pressure. Fuel pressure can be lowered slightly by adding extra gaskets between the fuel pump and the engine block.

Old stock pumps and rebuild kits. Diaphragms stretch and deteriorate with age. Old diaphragms are not made to withstand today's fuel compositions. It is best to use modern replacement diaphragms.

Venting of the fuel pump. CJ-3A fuel pumps have a vent in the body of the fuel pump, as most fuel pumps do. Since the CJV-35/U was designed to operate underwater, the vent is eliminated as well as the upper oil seal. M38s before serial number 53262 originally had a plug in the body center threaded hole. This is where a vent would normally be located. Apparently, there were problems with this arrangement since many vehicles were retrofitted with various vents here. The service manual also has a modification procedure to be performed on the pumps. M38s, after serial number 53262 had a different underwater ventilation system, and the fuel pump was vented at the center of the body. See M38 Underwater Ventilation System for more information.

Testing

Caution Gasoline and gasoline vapors are highly flammable. A fire could occur if an ignition source is present. Have a dry chemical (Class B) fire extinguisher nearby.

Fuel pump pressure test. Connect a fuel pressure gauge to the fuel pump output line. Run the engine at a fast idle and note the pressure. CJ3A and CJV-35/U fuel pressure should be 2-1/2 to 3-3/4psi and M38 fuel pressure should be 4 to 5-1/4 psi.

Fuel pump capacity test. Connect a hose to the fuel pump output and place in a container. Start the engine and run at idle speed. It should take 20-30 seconds to pump 1 pint of gasoline into the container.

Vacuum pump test. Disconnect the vacuum line going to the intake manifold and plug the manifold side of the line or fitting. With the engine idling, if the windshield wipers operate even at a slow speed, the vacuum pump is satisfactory. With the manifold line still disconnected, attach a vacuum gauge to the line that goes to the wipers. The vacuum should be about 10 in. of vacuum at a fast idle.

Problem and possible cause

Low fuel pressure - Mechanical wear, Stuck or leaking valves, Leaking diaphragm, Fuel supply problem, Dirty filter screen, Suction side air leak

Low fuel volume - See low fuel pressure

High fuel pressure - Tight diaphragm, Diaphragm spring too strong, Frozen link, Missing spacer, The 1956 Utility service manual states "High fuel pressure may be corrected by adding gaskets between the pump body and engine block."


Caution Gasoline and gasoline vapors are highly flammable. A fire could occur if an ignition source is present. Have a dry chemical (Class B) fire extinguisher nearby.

Remove the fuel cap to relieve pressure. Disconnect the fuel and vacuum lines from the pump. Plug the fuel supply line so gasoline will not gravity drain from the tank. Remove the 2 bolts that attach the fuel pump to the engine block. The M38 uses special bolts with and extended hex section to allow easier removal and installation of the fuel pump.

Clean the old gaskets from the pump, spacer, and block.

For easier installation of the fuel pump, rotate the engine crankshaft until the fuel pump camshaft eccentric lobe is on the low point. Look into the fuel pump opening to determine the low point. Alternately the engine can be set at top dead center of cylinder number 1 compression stroke so the lobe is at the low point. Pre-lubricate the fuel pump arm where it will contact the camshaft. Place the bolts in the fuel pump holes. Install 1 new gasket over the bolts. Install the spacer over the bolts. Install the second gasket over the bolts. Insert the fuel pump arm into the block being sure the arm goes on top of the camshaft eccentric lobe. Start the bolts in the block-threaded holes and evenly tighten them. Connect the fuel and vacuum lines.

Part Numbers

The fuel pump part number is usually stamped on the edge of the mounting flange. Some high-production pumps have the part number cast into the body beneath the diaphragm flange.