Pressure-Testing A Mechanical Fuel Pump: A Step-By-Step Guide

Diagnosing a mechanical fuel pump is a straightforward process. Pressure is the first thing to look for. Since carbureted vehicles don't have a test port, the line from the carburetor is usually disconnected, and a hose is attached and placed in a container. The engine is then cranked, and fuel delivery can be observed. If fuel delivery is a problem, it's important to inspect all fuel lines and hoses for leaks or damage.

Check for carburettor faults

Checking for carburettor faults is an important step in diagnosing fuel system issues. Here are some detailed steps to help you identify and address any problems:

Firstly, if your car stops running and has not run out of petrol, check the carburettor for fuel. Disconnect the fuel line to the carburettor, catching any fuel in a plastic bag secured with a rubber band. Then, turn the engine over with the starter. If there is no fuel or only a dribble, there could be a blockage or leak in the fuel line, or the fuel pump may not be functioning.

Another sign of a carburettor fault is when the engine fails to turn over after several attempts, and the spark plug is dry. This indicates a break in the fuel transfer. Additionally, if the engine will not keep running with the choke turned down, there could be a blockage in the jet or passageways in the carb, which will require cleaning or replacement.

If the engine periodically increases and decreases RPMs after being started, this could be another flow issue, and the carburettor should be cleaned and repaired or replaced. Other signs of carburettor issues include reduced engine performance, black smoke from the exhaust, backfiring, overheating, and hard starting.

When checking for carburettor faults, it is also important to consider the fuel itself. Old fuel can cause various problems, and you may be able to identify it by its smell. Ensure your fuel shut-off valve is in the "on" position, and check that your fuel filter and fuel lines are in good condition and adequately distanced from heat sources.

Check for fuel line blockages

Checking for fuel line blockages is an important step in troubleshooting mechanical fuel pump issues. Blockages in the fuel line or filter can cause symptoms similar to those of a failing mechanical fuel pump, so it's crucial to inspect the entire fuel system.

• Remove the air cleaner: Take off the air cleaner and look into the throat of the carburetor. Pump the throttle linkage. If the pump is functioning correctly, fuel will squirt into the carburetor. If no fuel appears, verify that the tank has gas and proceed to the next steps to check for blockages.
• Inspect the fuel line: Check the fuel line for any signs of damage, such as cracks, holes, or rotten spots. A fuel line with a hole can cause the pump to suck in air instead of fuel.
• Detach the fuel line from the carburetor: Place an empty container or a plastic bag secured with a rubber band at the end of the fuel line to catch any fuel. Then, crank the engine and observe the fuel flow. If there is a weak stream or no flow at all, it could indicate a blocked fuel line.
• Check the fuel filter: If the fuel filter is clogged, it can restrict fuel flow and cause similar symptoms to a faulty fuel pump.
• Verify fuel delivery: Disconnect the fuel line from the carburetor and crank the engine. If there is no fuel delivery or only a dribble, it could be due to a blockage in the fuel line.
• Inspect for leaks: Gaskets, fuel pipes, and the seal between the diaphragm and the pump body can all be sources of leaks. Leaks can affect fuel pressure and cause the engine to run poorly.

By following these steps, you can effectively check for fuel line blockages and narrow down the potential causes of mechanical fuel pump issues.

Check for fuel leaks

If your car has stopped running and you haven't run out of petrol, check whether there is petrol in the carburettor. If there is not, there is a fault in the fuel system. Disconnect the fuel line to the carburettor, slipping a plastic bag over the end of the line to catch any fuel, and then secure the bag with a rubber band. Turn the engine over with the starter. If fuel spurts out, there is a carburettor fault. If no fuel, or only a dribble, emerges, there is either a blockage or leak somewhere in the fuel line, or the fuel pump is not working.

A mechanical pump drive rarely fails, but signs of wear may appear on the cam and operating lever, linkages and pins. Diaphragms can also leak. A small leak may only affect the engine at high speeds, but it may also let petrol drip through into the camshaft area and from there to the sump. If this occurs, there is a risk of an explosion. Gaskets and loose fuel pipes may also leak, and so may the seal between the diaphragm and the two halves of the pump body.

Another common place for a fuel leak to develop is in the rubber hose and metal tube that leads from the tank to the fuel pump. The metal tube can become rusted through, causing the fuel to leak out. The rubber hose can also become worn out over time, leading to fuel leaks. If you notice that this component is damaged, replace it with a reinforced rubber fuel hose.

On many automobiles, the fuel pump actuator arm passes through the timing case cover. This allows the constant rotating motion of the camshaft or crankshaft to drive the arm. Where the fuel pump mounts to the timing case cover, a gasket provides a tight seal. Although this gasket is usually reliable, engine vibration can cause the bolts to come loose. When this happens, oil can seep out around the fuel pump. If you notice a leak in this area, replace the seal as soon as possible, as detergents in the engine oil can cause further damage.

Check fuel delivery

Checking fuel delivery is a crucial aspect of troubleshooting a mechanical fuel pump issue. Here are some detailed steps to help you check fuel delivery:

Inspect the Carburetor

If your car stops running despite having enough petrol, check the carburetor for fuel. Disconnect the fuel line from the carburetor and cover the end with a plastic bag secured with a rubber band. Then, turn the engine over with the starter. If fuel spurts out, the carburetor is faulty. If there is no fuel or only a small amount, there could be a blockage or leak in the fuel line or the fuel pump may not be functioning properly.

Check for Fuel Leaks

Fuel leaks are a common issue with mechanical fuel pumps, often caused by cracks or holes in the rubber diaphragm or loose inlet or outlet fittings. Inspect the pump for any signs of fuel dripping, as this indicates a failed diaphragm and the need for a replacement pump.

Disconnect Fuel Line at the Carburetor

To further test fuel delivery, disconnect the fuel line at the carburetor and place the end into a container. Crank the engine to observe if the pump is pushing fuel through the line. Strong, steady spurts of fuel indicate a functioning pump, while no fuel or a weak stream suggests a faulty pump, clogged fuel filter, fuel line blockage, or an empty fuel tank.

Monitor Fuel 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 check the pressure reading. If there is no pressure or the pressure is below specifications, the pump is faulty and requires replacement.

Inspect Fuel Lines and Hoses

If fuel delivery is an issue, carefully examine all fuel lines and hoses for any signs of damage or leaks. Pay close attention to the rubber lines near the fuel tank, as they may be sucking in air due to deterioration or pinhole leaks. Ensure that the metal lines are also in good condition and free from rust or corrosion.

Check the Fuel Tank

Don't overlook the fuel tank itself. Contaminants in the tank are common, especially in older vehicles that have been sitting for a while. The pickup tubes at the base of the sending units can become clogged, and a non-functioning fuel gauge could indicate rust and corrosion in the tank.

Remember to adhere to safety precautions when working with fuel systems, and always address fuel delivery issues with a systematic approach to ensure an accurate diagnosis and effective repair.

Check for fuel pressure

Checking for fuel pressure is a straightforward process. Since carbureted vehicles don't have a test port, you can disconnect the line from the carburetor, attach a hose to it, and place the other end in a container. Then, crank the engine. If fuel is delivered, you will know immediately. If you attach a hose to a gauge, you will see the pressure output of the pump.

If you need to monitor fuel pressure with the engine running, you will have to make an extra fitting between the fuel line and carburetor so you can tee in a pressure gauge. Be careful not to cut or damage the solid line leading to the carburetor.

If you are experiencing fuel delivery problems, inspect all the fuel lines and hoses. There are normally a couple of spots from the tank to the engine where there is a short section of hose instead of a hard line. A pinhole or leak in either the line or one of these hoses will cause the fuel pump to draw in air instead of fuel.

If you are not getting any fuel pressure, it could be due to a ruptured or plugged fuel line before the pump. If there is a fuel filter before the pump, this could be plugged as well. Another possibility is a plugged pickup tube in the tank.

If you are getting inconsistent fuel pressure, it could be due to a faulty diaphragm. If the diaphragm has ruptured, fuel may be leaking into the crankcase instead of going into the carburetor.

When checking for fuel pressure, be sure to take safety precautions. Do not smoke or allow any sparks near the carburetor or open fuel line, as this may ignite the fuel and cause a fire. Wait until the engine has cooled to work on the fuel system and avoid skin contact with gasoline.

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