Checking Fuel Pressure On Your Mercury Outboard

Checking the fuel pressure on a Mercury outboard engine is an important part of basic maintenance and can help ensure reliable service. Before checking the fuel pressure, it is essential to understand the basics of fuel lines, including the materials used and the specific application. The fuel line is responsible for carrying fuel from the tank to the engine, and it is crucial to ensure it is securely connected and free of damage or debris. To check the fuel pressure, you can use a fuel pressure gauge, which can help identify issues with the fuel system. By attaching the gauge to the fitting, you can read the pressure and compare it to the recommended specification for your Mercury outboard engine.

Faulty fuel pump symptoms

A faulty fuel pump can cause a range of issues with your Mercury outboard motor. Here are some common symptoms that indicate a problem with the fuel pump:

• Sputtering or misfiring engine: If your engine is sputtering, misfiring, or dying, it could be a sign of a faulty fuel pump. This can cause a bumpy ride or even a complete loss of speed.
• Rapid fuel consumption: If you notice that your fuel tank is emptying faster than usual, it could indicate a problem with the fuel pump.
• Increased temperature: A rise in the temperature gauge could be a sign of a failing fuel pump, although it may also indicate other engine issues.
• Failed starts or inability to idle: If your engine fails to start or struggles to idle, it could be due to a faulty fuel pump not delivering enough fuel to the engine.
• Inconsistent power: Peaks and valleys in engine output may indicate a clogged fuel filter, but it could also be a sign of a faulty fuel pump.
• Whining noise: A damaged fuel pump may make a loud, whining noise, which is different from the low hum of a normal, functioning pump.
• Engine stalling: If the fuel pump is not delivering enough fuel, the engine may stall, especially at higher speeds or when under stress, such as driving up steep inclines or carrying heavy cargo.
• Loss of power: If the engine loses power, it may be due to the fuel pump not providing enough fuel to meet the engine's demands.
• Power surges: If the pump is delivering too much fuel, you may experience speed spikes and drops while driving.
• Lower fuel efficiency: A faulty fuel pump that delivers too much fuel to the engine will cause you to burn through fuel faster, resulting in more frequent trips to the gas station.
• No priming sound: When you turn the ignition on, you should hear the fuel pump priming. If you don't hear this sound, it could indicate a faulty fuel pump.

If you experience any of these symptoms, it is important to troubleshoot and diagnose the issue properly. Checking the fuel pump and other related components can help identify the root cause and determine if a repair or replacement is needed.

How to test fuel pump pressure

Step 1: Check for Basic Symptoms of a Faulty Fuel Pump

Before testing the fuel pump, it is important to be aware of some common symptoms of a faulty fuel pump, which include:

• Engine sputtering or misfiring
• Rapid fuel consumption
• Increased engine temperature
• Inability to hear the fuel pump engaging

Step 2: Check the Entire Fuel System

If you are experiencing any of the above symptoms, it is recommended to check the entire fuel system for cracks, leaks, and blockages. Ensure that fuel can travel easily from the tank to the pump by pumping the primer bulb until it feels hard. The outboard fuel filter should also be full.

Step 3: Remove the Spark Plug

With the engine off and the cowl cover removed, remove the spark plug wire boot and then the spark plug itself.

Step 4: Attach a Compression Gauge

Attach a compression gauge to the spark plug hole. Twist the throttle position wide open and pull the rope start or turn the starter over several times.

Step 5: Check the PSI

A low PSI (under 30) indicates a compression issue that is preventing the fuel pump from working properly. Refer to your engine's owner's manual to determine the correct PSI for your specific engine.

Step 6: Check the Fuel System

Ensure that there is enough fuel in the tank to be picked up by the sump. The sump pickup screen and tank vent should be unclogged. Inspect the fuel primer bulb for any cracks, leaks, or deformations.

Step 7: Inspect the Fuel Line

Check the fuel line hoses and connections for any kinks or leaks. Remove the in-line fuel filter and blow through both ends to ensure there are no obstructions. Reinstall the fuel filter.

Step 8: Check the Pulse Hose

Loosen the clamp on the pulse hose, which connects the fuel pump to the intake manifold. Place a container under the hose and pull the starter rope several times. For an electric start, crank the engine over several times with the ignition cutoff activated and the plug wire removed. Lack of fuel discharge indicates a faulty fuel pump check valve or diaphragm.

Step 9: Use a Fuel Pressure Gauge

Hook up a fuel pressure gauge to the pulse hose. Crank the engine over and read the gauge. A normal fuel pump will have a pressure output of approximately 5-7 PSI. A reading below this indicates a defective fuel pump check valve or diaphragm.

Fuel line installation

The fuel line is a critical component in the overall operation of your outboard engine. It acts as a bridge, connecting the fuel tank to the engine and providing the necessary energy to propel your watercraft. A properly installed fuel line ensures optimal performance and prevents potential issues. Here are the steps to install a fuel line for your Mercury outboard:

Mounting and Securing the Fuel Hose

• Use a fuel hose clamp to secure the hose to the engine or tank. Ensure that it is tightened firmly but not too tightly.
• Route the hose along the side of the hull or up the gunwale, avoiding any areas where it could get pinched or damaged.
• Utilise a fuel hose support or guide to keep the hose in place and prevent it from rubbing against any metal or moving parts.

Connecting the Fuel Line to the Engine and Tank

• Before connecting, ensure that both ends of the fuel line are clean and free of debris.
• Use a high-quality fuel line connector designed specifically for your Mercury outboard.
• Tighten the connection firmly but avoid over-tightening, as it can damage the connector or strip the threads.

Installation Considerations and Tips

• Disconnect the old fuel line before installing the new one to prevent leaks or damage.
• Clean the fittings on the engine and tank to ensure a secure connection.
• Employ the right tools, such as wrenches and pliers, to avoid damaging the fittings or the fuel line.
• Tighten the fittings slowly and evenly to prevent damage or stretching of the new fuel line.

Once you have completed the installation, inspect the fuel line for any signs of damage or leaks. Test the engine to ensure proper function and performance. By following these steps, you can be confident in the correct installation of your Mercury outboard fuel line, maximising the performance and longevity of your watercraft.

Fuel line maintenance

The fuel line is a critical component of your Mercury outboard, connecting the fuel tank to the engine and providing the energy needed to power your watercraft. To ensure optimal performance and prevent issues, regular maintenance is crucial. Here are some detailed instructions to help you maintain your fuel line effectively:

Inspecting the Fuel Line:

• Check the fuel line for any visible signs of damage, such as cracks, cuts, leaks, or wear.
• Pay close attention to the hose, looking for any loose connections or frays.
• Common signs of damage include cracks, leaks around fittings, loose or wobbly connections, and discolouration or brittleness of the hose.
• If you notice any issues, it is best to replace the fuel line to avoid further problems and costly repairs.

Cleaning the Fuel Line and Fittings:

• Use a soft-bristled brush to gently remove any sediment or debris from the fuel line.
• Avoid using harsh chemicals or abrasive cleaners, as they may damage the hose.
• Clean the fittings and connections with a soft cloth.
• Rinse the fuel line thoroughly after cleaning to ensure no residue is left behind.

Preventative Measures:

• Perform regular engine maintenance as per the manufacturer's recommendations.
• Replace fuel pumps as needed, but note that Yamaha recommends replacing outboard fuel pumps every three years.
• Use fuel stabilizers to prevent fuel from deteriorating and to protect the system during storage.
• Use fuel additives when using ethanol gasoline to prevent the formation of sludge and reduce the chance of clogs.

Troubleshooting:

• If you experience engine sputtering, slowing, or rapid fuel consumption, check the entire fuel system for clogs and blockages.
• Listen for the sound of the fuel pump engaging when starting the engine. If it cannot be heard, there may be an issue with the battery or the fuel pump itself.
• Check the fuel primer bulb for cracks, leaks, or deformities.
• Inspect the fuel line hoses and connections for kinks and leaks.
• Use a compression gauge to test the fuel pump. A low PSI (under 30) indicates a compression issue preventing the fuel pump from functioning properly.

Remember to always refer to your Mercury Outboard's user manual for specific maintenance and troubleshooting instructions. With regular maintenance and attention to your fuel system, you can ensure the smooth and efficient operation of your outboard engine.

Fuel pump tests

Before testing your fuel pump, it is important to check the rest of the fuel system for cracks, leaks, and general signs of wear to ensure continuity between the fuel tank and pump. You can check this by pumping the primer bulb until it feels hard. Fuel should travel easily from the tank to the pump. Your outboard fuel filter, located just before the fuel pump, should be full.

With the engine off and the cowl cover removed, remove the spark plug wire boot and remove the plug. Then, attach a compression gauge to the spark plug hole. Twist the throttle position wide open and either pull the rope start or turn the starter over several times.

The proper PSI will depend on the engine, but in general, a low PSI (under 30) means that there is a compression issue that is preventing the fuel pump from working. If you have adequate pressure, this indicates a bad fuel pump.

The two parts of the fuel pump that are most commonly the source of problems are the diaphragm and the one-way valves. To test them, remove the pump from the engine. The one-way valves are how the fuel is delivered to the carburetor, allowing fuel to pass from the tank in only one direction. This allows enough pressure to build to force a continuous flow. If the valves aren’t closing properly, they will allow fuel to pass in both directions, which may be why your outboard is not getting fuel.

There are two valves to consider: the inlet and the outlet. Each pump has at least one of each, but yours may have two outlets. The inlet accepts the flow of fuel and will often be marked with an arrow pointing inward towards the centre of the pump. The outlet moves the fuel to the rest of the system and may be indicated by an arrow pointing outward. To test their functionality, you’ll need a pressure gauge.

Remove the fuel pump and attach the pressure gauge hose to the outlet. Seal the inlet firmly with your thumb and pump the pressure gauge a few times. If the pressure releases, the inlet is allowing pressure to escape in both directions, and the valves are not in proper working condition.

If the pressure does not release, and your fuel pump has a diaphragm, you will need to test this next. The diaphragm is an internal rubber part that pushes the fuel through the pump and is generally found only in mechanical fuel pumps. You can usually see part of the diaphragm through a small hole on the back of the pump.

Some signs of damage include leaking, stalling, or fuel bubbling in the water. To test the diaphragm, you’ll need two 10-24 bolts, four washers, and a hose clamp. 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. The purpose of this is to maximise pressure.

Attach the fuel line to the inlet and attach another fuel line to the outlet, clamping it off tightly. Prime the bulb, and keep an eye on the hole in the back of the pump. If fuel is visibly leaking through it, your diaphragm is bad.

Frequently asked questions