Testing Your Outboard's Low-Pressure Fuel Pump: A Guide

Testing a low-pressure fuel pump on an outboard motor is a straightforward process that can be done in under 30 minutes with minimal costs. Outboard fuel pumps rarely fail, and a process of elimination is often required to narrow down the issue. To test the pump, first, turn off the main electrical cutoff switch and remove the outboard motor's upper case and cowl cover. Then, remove the spark plug wire boot and attach a compression gauge hose to the spark plug hole. With the throttle position wide open, pull the rope start so the engine turns over at least four or five times. Refer to the owner's manual for the correct psi reading, and if the compression falls below 30 psi, the fuel pump will not function properly due to low pulse pressure.

Check the fuel tank vent for clogs

Checking the fuel tank vent for clogs is an important step in maintaining your boat's fuel system. A clogged fuel tank vent can lead to problems such as difficulty filling the tank, reduced fuel flow, fuel leakage, and engine stalling or misfiring. Here are some detailed steps to check the fuel tank vent for clogs:

Firstly, locate the fuel tank vent on your boat. It is usually found near the fuel tank and may have a screen or filter to catch dirt and debris. Remove any covers or caps that may be blocking access to the vent.

Next, inspect the vent for any signs of clogging. Look for dirt, debris, salt crystals, or corrosion that may be blocking the vent. If the vent appears dirty or clogged, you will need to clean it. Use a small brush or compressed air to remove any debris or dirt from the vent. Ensure that you blow or brush away any debris that could restrict airflow.

Once the vent is clean, it's time to test its functionality. Place your hand or a piece of paper over the vent and have someone else try to fill the fuel tank. If the vent is working properly, you should feel air being pushed out of the vent, and the paper should move. If there is no airflow, the vent may be clogged or blocked.

If the vent is still not functioning properly, you may need to check the vent hose. Ensure that the hose is not kinked, twisted, or blocked in any way. Check for any signs of damage or deterioration. If the hose is damaged, it will need to be replaced.

Finally, reattach any covers or caps that you removed and secure them tightly. Ensure that the vent is clear of any debris or obstructions before operating your boat. It is important to regularly check your fuel tank vent to prevent clogs and maintain the proper functioning of your boat's fuel system.

By following these steps, you can ensure that your fuel tank vent is clear of clogs and functioning properly. A properly maintained fuel tank vent will help your boat's engine run smoothly and efficiently.

Check for adequate compression

Checking the compression on an outboard engine is a simple test that can reveal a lot about the engine's health. It involves measuring the pressure built up by the motion of the piston inside the cylinder, given in pounds per square inch (PSI). A good compression reading indicates that the piston rings are working properly and that the cylinders are in good condition.

• Before beginning the test, it is recommended to warm up the engine to its normal operating temperature. This is because a warm engine can get oil into the cylinders, sealing the rings and valves better, which will give you the most accurate compression test results. However, this is a controversial topic, and some people prefer to perform the test on a cold engine.
• The next step is to remove the spark plugs. Number the leads so you know where they go back. Check the spark plugs for damage. The electrodes should be gapped properly, and their colour should be a medium tan to brown after a high-speed run or dark brown to black after idling at slow speeds. If they are lighter in colour, it could indicate that the engine is running too lean, meaning there is not enough fuel.
• Screw your compression gauge into the first cylinder. Ensure that the plug leads are clear of the flywheel and any moving parts.
• Crank the engine with a starter or pull cord through at least four or five flywheel revolutions, or until the compression gauge needle stops advancing. The engine won't start as the plugs have been removed, but some fuel may be injected into the cylinder. Be cautious as smoking or sparks could ignite these gases.
• Note the reading for that cylinder. A proper PSI will vary based on the engine. Generally, a low PSI (under 30) indicates a compression issue preventing the fuel pump from working. For a four-stroke engine, good results should be between 180-210 PSI, while a two-stroke engine should be around 110-130 PSI.
• Repeat the process for each cylinder and note the results. The compression shouldn't vary by more than 10-15 PSI per cylinder. If your results show low or erratic readings, you may need to remove the engine cylinder head(s) to inspect the piston/cylinder condition or consult a mechanic.

Test the fuel pump's valves

To test the valves of a low-pressure fuel pump on an outboard, you will first need to remove the pump from the engine. There are two valves to consider: the inlet and the outlet. Each pump has at least one of each, but some pumps may have two outlets.

The inlet valve accepts the flow of fuel and is often marked with an arrow pointing inward towards the centre of the pump. The outlet valve moves the fuel to the rest of the system and may be indicated by an arrow pointing outward.

To test the functionality of these valves, you will need a pressure gauge. Follow these steps:

• Attach the pressure gauge hose to the outlet.
• Seal the inlet valve firmly with your thumb.
• Pump the pressure gauge a few times.

If the pressure releases, it indicates that the inlet valve is faulty and is allowing pressure to escape in both directions. If the pressure does not release, and your fuel pump has a diaphragm, you will need to test that next.

The diaphragm is a rubber part that pushes the fuel through the pump and is generally found in mechanical fuel pumps. You can usually see part of the diaphragm through a small hole on the back of the pump.

To test the diaphragm, you will need two 10-24 bolts, four washers, and a hose clamp. Here are the steps:

• Clamp and tighten the pump using the bolts and washers. The bolts should go through the holes used to attach the pump to the engine, with washers on either end. This setup will maximise pressure.
• Attach the fuel line to the inlet.
• Attach another fuel line to the outlet and clamp it off tightly.
• Prime the bulb and watch the hole at the back of the pump. If fuel is leaking through the hole, your diaphragm is damaged.

If your diaphragm is damaged, you can either repair it using a fuel pump repair kit or replace the fuel pump altogether.

Test the fuel pump's diaphragm

To test the fuel pump's diaphragm, you will need two 10-24 bolts, four washers, and a hose clamp.

First, clamp and tighten the pump using the bolts and washers. The bolts will go through the holes used to attach the fuel pump to the engine, with washers on either end. This step aims to maximise pressure.

Next, attach the fuel line to the inlet, and attach another fuel line to the outlet, clamping it off tightly.

Now, prime the bulb and keep an eye on the hole in the back of the pump. If you can see fuel visibly leaking through the hole, your diaphragm is bad.

A damaged diaphragm can cause engine issues, such as stalling, sputtering, or even cause the engine to stop running altogether.

Check the sump screen and fuel primer bulb

Checking the sump screen and fuel primer bulb is a crucial step in troubleshooting a low-pressure fuel pump on an outboard motor. Here's a detailed guide to help you through the process:

Check the Sump Screen

The sump screen, also known as the sump pickup screen, plays a vital role in ensuring the fuel pump receives enough fuel from the tank. Over time, this screen can become clogged with debris or contaminants, restricting the flow of fuel. To check the sump screen:

• Ensure there is sufficient fuel in the tank: Before inspecting the sump screen, confirm that the fuel tank has enough fuel. This step is crucial because a low fuel level may give the false impression that the sump screen is clogged.
• Locate the sump screen: Refer to your outboard motor's manual to identify the location of the sump screen. It is usually found within the fuel tank or near the fuel pump.
• Inspect for clogs: Once you have located the sump screen, carefully remove it and visually inspect it for any signs of clogging or debris buildup. If the screen appears clogged, proceed to clean it.
• Clean the sump screen: To clean the sump screen, you can use a mild detergent and a soft-bristled brush to gently remove any debris or buildup. Ensure you rinse and dry the screen thoroughly before reinstalling it.

Inspect the Fuel Primer Bulb

The fuel primer bulb is a crucial component in the fuel system of an outboard motor. It helps prime the fuel pump by creating a vacuum and drawing fuel from the tank to the engine. To check the fuel primer bulb:

• Inspect for cracks, leaks, and deformities: Carefully examine the fuel primer bulb for any signs of cracks, leaks, or deformities. Ensure that the bulb is intact and there are no signs of fuel leakage.
• Check for proper priming: Squeeze the primer bulb several times to ensure it is functioning correctly. It should feel firm and resilient. If it feels too soft or spongy, it may indicate a leak or damage.
• Check for fuel flow: With the engine off, squeeze the primer bulb multiple times to draw fuel from the tank. Observe if fuel is flowing through the bulb and reaching the fuel pump. If there is no fuel flow, it may indicate a blockage in the fuel line or a faulty primer bulb.
• Replace if necessary: If you notice any cracks, leaks, or deformities, it is essential to replace the fuel primer bulb with a new one. This will ensure the fuel system functions optimally and reduce the risk of fuel leaks.

Remember, when working on your outboard motor's fuel system, always refer to the owner's manual for specific instructions and safety precautions. Additionally, if you are uncomfortable performing these checks or repairs, seek the assistance of a qualified marine mechanic.

Frequently asked questions