Replacing Fuel Pump In 40 Hp Mercury Outboard: Step-By-Step Guide

If you're experiencing issues with your Mercury outboard fuel pump, you might be dealing with a sputtering or misfiring engine, rapid fuel consumption, or a rise in temperature. These are common symptoms of a malfunctioning fuel pump, which can cause major headaches and ruin your day out on the water. Luckily, there are plenty of replacement fuel pumps available for Mercury outboards, and you can even find tutorials online that teach you how to replace the pump yourself.

Troubleshooting a bad fuel pump

• Turn off the main electrical cutoff switch and remove the outboard motor upper case and cowl cover. Take the spark plug wire boot off the plug tip and move the wire out of the way.
• Remove the spark plug using a socket wrench.
• Screw a compression gauge hose into the spark plug hole and twist the throttle position wide open. Pull the rope start or turn the starter over several times.
• Read the compression gauge in PSI. If the compression falls under 30 PSI, this indicates that there is a compression issue causing the fuel pump to malfunction.
• If the compression seems okay, reinstall the spark plug and perform the same test on the other cylinders.
• Check that there is enough fuel in the tank and inspect the sump pickup screen for clogs. Blow air through the tank vent to ensure it is not blocked. Inspect the fuel primer bulb for leaks and cracks.
• Inspect all fuel connections and hoses for leaks or kinks. Loosen the fuel filter hose clamps and remove the filter. Blow through both ends to check for obstructions and reinstall if it is clear.
• Loosen the clamp on the pulse hose, which connects the intake manifold and fuel pump. Have a container ready to catch any fuel discharge. With an electric start, get a helper to crank the engine several times with the ignition cutoff activated and the plug wire removed. If there is a lack of fuel discharge, the diaphragm or fuel pump check valve may be faulty.
• Connect a fuel pressure gauge to the pulse hose and crank the engine. Normal fuel pumps will output between five and seven PSI. Readings below this range indicate a faulty diaphragm or pump check valve.
• If necessary, replace the fuel pump or opt for a rebuild kit.

Testing the fuel pump

Next, screw a compression gauge hose into the spark plug hole. With one hand, twist the throttle position wide open, and with the other, pull the rope start so the engine turns over at least four or five times. Check the compression reading in PSI (pounds per square inch) on the gauge. If you have an electric start motor, ask an assistant to crank the engine while you push the throttle wide open.

Refer to your owner's manual to determine the correct PSI for your engine. If the compression falls below 30 PSI, your fuel pump will not function correctly due to low pulse pressure. In this case, you should address the compression issue first. If the compression seems okay, reinstall the spark plug and perform the same test on the other cylinders.

Now, check if there is enough fuel in the tank for the sump to pick up. Ensure that the sump pickup screen is unclogged and that the tank vent is clear by blowing through it to ensure airflow. Inspect the fuel primer bulb for any cracks, leaks, or deformities.

Next, inspect the fuel line hoses and connections from the tank to the fuel pump for any kinks or leaks. There should be none. Loosen the in-line fuel filter hose clamps and remove the filter. Blow through both ends of the filter to check for obstructions, then reinstall the filter and tighten the clamps.

Loosen the clamp on the pulse hose, which connects the fuel pump to the intake manifold. Place a container underneath the hose and pull the starter rope several times. If you have an electric start engine, ask an assistant to crank the engine several times with the ignition cutoff activated and the plug wire removed. If there is no fuel discharge, you may have a faulty fuel pump check valve or diaphragm.

Finally, connect a fuel pressure gauge to the pulse hose and crank the engine. A normal fuel pump will produce between five and seven PSI of pressure. Readings below this range indicate a faulty fuel pump check valve or diaphragm. In this case, you will need to replace the fuel pump or purchase a rebuild kit.

Checking the one-way valves

Step 1: Remove the Fuel Pump

First, you will need to remove the fuel pump from the engine. This will make it easier to access the one-way valves and perform the necessary tests.

Step 2: Identify the Inlet and Outlet Valves

The one-way valves are crucial for directing fuel flow. The inlet valve allows fuel to enter the pump, usually indicated by an arrow pointing inward. The outlet valve moves fuel to the rest of the system and may be marked with an arrow pointing outward. Each pump has at least one inlet and one outlet, but some may have two outlets.

Step 3: Attach the Pressure Gauge

To test the functionality of the valves, you will need a pressure gauge. Attach the hose of the pressure gauge to the outlet valve. Ensure that the connection is secure.

Step 4: Seal the Inlet Valve

Using your thumb, firmly seal the inlet valve. This step is important to create the necessary pressure and isolate the valve's function.

Step 5: Pump the Pressure Gauge

With the inlet valve sealed, pump the pressure gauge a few times. Observe whether the pressure holds or releases.

Step 6: Interpret the Results

If the pressure holds, it indicates that the inlet valve is functioning correctly, preventing fuel from flowing backward. However, if the pressure releases, it is a clear sign that the inlet valve is faulty and allowing fuel to escape in the wrong direction. In this case, you will need to replace the inlet valve to ensure proper fuel delivery.

It is important to perform this test on both the inlet and outlet valves to ensure comprehensive checking of the one-way valves. Remember to seal the other valve while testing each one.

By following these steps, you can effectively check the one-way valves on your Mercury outboard fuel pump and identify any issues that may be causing insufficient fuel flow.

Testing the diaphragm

For the two-hose type fuel pump, leave the hoses attached and remove the screws that attach the pump to the powerhead. Reinsert the screws and add a nut to each, clamping the pump securely. With the fuel line attached to the engine, pump the primer bulb while observing the pressure/vacuum operating hole at the back of the pump. If fuel leaks from this hole, the diaphragm is faulty and will need to be rebuilt or replaced.

For the three-hose type fuel pump, remove the pressure/vacuum hose that leads from the pump to the crankcase, leaving the other hoses attached. With the fuel line attached to the engine, pump the primer bulb while observing the pump pressure/vacuum fitting. If fuel leaks from this fitting, the diaphragm is faulty and will need attention.

In both cases, if the carburetor is flooding, clamp the hose shut that leads to the carburetor. Additionally, note that repair kits may not be available for all fuel pumps, in which case replacement is necessary.

To further test the diaphragm, remove the fuel pump from the engine. You will need two 10-24 bolts, four washers, and a hose clamp. Clamp and tighten the pump using the bolts and washers, maximizing pressure. Attach the fuel line to the inlet and another fuel line to the outlet, clamping it tightly. Prime the bulb and observe the hole at the back of the pump. If fuel is leaking through this hole, the diaphragm is bad and requires replacement or repair.

Preventing fuel pump problems

Before you find yourself needing to change your fuel pump, there are some preventative measures you can take to keep your Mercury outboard's fuel pump in good condition.

First, ensure that you are performing the required engine maintenance at the recommended intervals. Refer to the manufacturer's maintenance checklist for your specific model, which will include all the parts that need to be checked for cracks, leaks, or other signs of damage and wear.

Yamaha, for example, recommends that outboard fuel pumps should be replaced every 3 years. Even if this is not specified by Mercury, it is good practice to replace the fuel pump at regular intervals if possible.

Another way to support the longevity of your outboard motor is to use fuel stabilizers. Fuel stabilizers will keep fuel from going bad, remove carbon deposits, and protect the system during storage.

Additionally, fuel additives are necessary when using ethanol gasoline, which is less than ideal for marine environments. Ethanol gasoline is more likely to be exposed to water droplets, which bind to the ethanol and form a sludge-like material that sinks to the bottom of the fuel tank. This can lead to clogs throughout the fuel system.

By preventing this with fuel stabilizers, you can help keep your engine healthy for longer.

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