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How to Diagnose & Replace a Mechanical Fuel Pump

by Larry Carley copyright 2019 AA1Car.com

Mechanical fuel pumps are used on older engines that have carburetors (though some may have a low pressure electric fuel pump mounted in or near the gas tank). The pump siphons fuel from the gas tank and pushes it to the carburetor when the engine is cranking or running.

Mechanical fuel pumps use a lever that rides on the camshaft to pump a rubber diaphragm inside the pump up and down. This creates suction that pulls fuel into the pump, and then pushes it along. A pair of one-way valves inside the pump only allow the gas to move in one direction (toward the engine).

The output pressure of a mechanical fuel pump is typically quite low: only 4 to 10 psi. But little pressure is needed to keep a carburetor supplied with fuel.

Fuel Pump Problems

A leak in the diaphragm or one-way valve inside a mechanical fuel pump will cause a loss of fuel pressure and starve the carburetor for fuel. This may cause the engine to run lean, misfire, hesitate or stall. If the pump fails entirely, no fuel will be delivered to the carburetor and the engine will not start or run.

Fuel leaks are another common problem, usually due to cracks or holes in the rubber diaphragm, or loose inlet or outlet fittings.

Mechanical Fuel Pump Checks:

A mechanical pump can be checked any one of four ways:

Remove the air cleaner, look into the throat of the carburetor and pump the throttle linkage. You should see fuel squirt into the carburetor if the pump is working. If you do not see any fuel squirting into the carburetor, the fuel pump has probably failed (or the fuel line or fuel filter are blocked, or the tank is out of gas).

Visually inspect the pump. If you see any fuel dripping from the pump, the diaphragm inside has failed and the pump needs to be replaced.

WARNING: A leaky fuel pump is dangerous because the fuel may ignite and start a fire!

Another way to check the pump is to disconnect the fuel line at the carburetor and place the end of the line into a container. Crank the engine to see if the pump is pushing any fuel through the line. Strong steady spurts of fuel mean the pump is working. No fuel or a weak stream means a bad pump, a plugged fuel filter, fuel line blockage or no fuel in the tank.

WARNING: Do not smoke near gasoline, and do not allow any sparks near the carburetor or open fuel line as this may ignite the fuel causing a fire! Do not spill gasoline on a hot engine. Wait until the engine has cooled to work on the fuel system. Also, avoid skin contact with gasoline and do not breathe the vapors.

You should also check fuel pump pressure. Connect a fuel pressure gauge to the pump outlet, or tee a gauge into the fuel line at the carburetor. Crank the engine and note the pressure reading on the gauge. If there is no pressure, or if pressure is less than specifications, the pump is bad and needs to be replaced.

Fuel Pump Replacement Options

If your vehicle needs a new fuel pump, we would highly recommend buying a quality NAME BRAND pump rather than some no-name "discount" pump. You want a fuel pump that will last and perform same as new so buy a product that is manufactured with quality materials and is backed by a warranty.

If you are restoring an older muscle car with a large displacement engine or highly modified engine, you might consider replacing the stock fuel pump with a high volume mechanical or electric fuel pump. The more horsepower an engine makes, the more fuel it consumes. A pump that is capable of flowing more fuel will reduce the risk of fuel starvation at high RPM and full throttle.

Fuel line pressure is not a big concern with carburetors because all you need is a reliable 3 to 4 PSI pushing fuel to the carb. An inline fuel pressure regulator can be installed between the pump and carburetor to monitor and limit peak fuel pressure. You do not want too much pressure in the line because it can force open the fuel bowl needle valve inside the carburetor and flood the engine.

Many drag racers use TWO fuel pumps to make sure the engine is never starved for fuel. An electric "pusher" pump is mounted in or near the gas tank to push fuel forward to the mechanical fuel pump on the engine. Mechanical pumps use vacuum to siphon fuel through the line from the tank. During hard acceleration, fuel in the tank may slosh away from the pickup allowing air to be sucked into the line. A mechanical pump does not generate enough suction to keep pulling fuel from the tank if it loses its prime, so the electric pusher pump keeps pushing fuel forward so the mechanical pump never starves.

High output performance engines may also require larger diameter fuel lines in addition to a higher volume mechanical and/or electric fuel pump, especially if an engine is equipped with dual carburetors, a supercharger or turbocharger.

How to Replace a Mechanical Fuel Pump

If you need to replace the fuel pump, disconnect the fuel inlet and outline lines from the pump. Plug the inlet fuel line so fuel does not leak out (the line is lower than the fuel tank and will dribble fuel if it is not plugged).

CAUTION: NEVER SMOKE while working on the fuel system. Also make sure there are no nearby sources of ignition or sparks that might ignite fuel vapors.

1. Unbolt the fuel pump from the engine. There are usually two bolts. Once the bolts have been removed the pump should come off.

2. Remove the old gasket from the pump mounting surface on the engine, and clean the mounting surface. Do not allow gasket residue to fall inside the engine opening.

3. Install a new gasket on the pump. Apply gasket sealer to both sides of the gasket for a leak-free seal. Apply a dab of grease on the tip of the pump lever where it contacts the camshaft, then mount the pump on the engine. Make sure the pump lever is positioned correctly and aligned with the lobe on the camshaft when you install it, otherwise the pump lever may be damaged when the engine is cranked.

4. Carefully inspect the fuel lines that go to and from the pump for cracks, any signs of leakage or deteroriation and replace as needed. If rubber fuel line hoses are more than 10 years old, replace them regardless of their appearance. You don't want old rotten hoses on your engine that may leak or burst and cause a fire

5. The fuel filter should also be replaced when changing a fuel pump. If the old filter is plugged it may be necessary to drain and steam clean the gas tank, or to replace the gas tank with a new one if the old tank is rusty inside.

6. Reconnect the fuel inlet and outlet lines, being careful not to over-tighten or damage the fittings. Some people wrap teflon plumbing tape around the threads on the pump fittings to seal the threads, but others say using tape should not be necessary and creates a risk of plastic shards getting into the fuel line to the carb. Another option is to apply a small dab of gasoline-proof gasket sealer to the threads on the fittings to prevent leaks. The choice is yours.

7. After all the lines have been connected, start the engine and check all the connections and hoses for leaks. If did everything correctly, there should be no leaks and no problems. You're good to go!




fuel pump More Fuel System Articles:

How To Diagnose & Repair Carburetor Problems

Electric Fuel Pumps

Bad Gasoline Can Cause Performance Problems

More Problems Caused by Bad Gas

Fuel Filters


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