Checking Fuel Pressure: Outboard Motor Maintenance 101

Checking the fuel pressure on an outboard motor is an important part of boat maintenance. Low fuel pressure can lead to poor performance or even engine failure, and can be caused by a number of issues such as vapor lock, vacuum lock, or restrictions in the fuel system. To test the fuel pressure, you will need to follow a series of steps, including checking fuel filters, hoses, and connections for any blockages, leaks, or damage, and using a fuel pressure gauge to measure the pressure. The process may vary depending on the specific make and model of your outboard motor, so it is important to refer to the owner's manual for specific instructions.

Check for fuel starvation problems

To check for fuel starvation problems, you can perform the following steps:

• Check for basic symptoms of fuel starvation: low fuel pressure, poor performance, or engine failure.
• Inspect the fuel filter and replace it if blocked, as this can cause hard starting, stalling, misfire, or poor performance.
• Pressurize the fuel system with the priming bulb and check for leaks in the fuel system fittings and hoses. Replace any leaking or damaged components.
• Use a test tank with fuel lines that are at least as large as those on your boat to run the motor through its full range of speed. If the motor runs fine on the test tank, pressurize your boat's fuel tank and start the motor again. If the problem persists, clear your fuel tank vent or replace the tank.
• Ensure that your fuel tank is free of water and debris. Clean or replace the tank if necessary.
• Check the fuel pickup tube for any blockages or cracks, especially if you suspect a crack in the pickup tube.
• Inspect all fuel hoses and connections for kinks, leaks, or other damage. Replace any faulty components.
• Verify that the sump pickup screen is unclogged and that the tank vent is clear by blowing through it to ensure airflow.
• Check the fuel primer bulb for any cracks, leaks, or deformations.
• Refer to your owner's manual to determine the correct psi for your engine. Use a compression gauge to test the compression and identify any issues with low pulse pressure.
• Inspect the pulse hose for fuel discharge. Lack of fuel discharge may indicate a faulty fuel pump check valve or diaphragm.
• Utilize a clear fuel bowl or fuel lines to identify air leaks in the system.
• Ensure all fittings and connections are tight and secure, including any "dead ends" in the system.

Test the fuel pump

To test the fuel pump, you will need to turn off the main electrical cutoff switch that governs the motor ignition. Remove the outboard motor's upper case by unclasping the snaps and taking off the cowl cover. Pull the spark plug wire boot from the spark plug and direct the plug wire away from the spark plug hole opening. You can do this by wedging or taping it to another component.

Now, use a plug socket and wrench to loosen and remove the spark plug. Screw a compression gauge hose into the spark plug hole, then twist the throttle position wide open with one hand and pull the rope start with the other 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 over while pushing the throttle wide open.

Refer to your owner's manual for the correct psi your engine needs. If the compression falls below 30 psi, your fuel pump will not function properly due to low pulse pressure. You will need to repair the compression problem first. Once that's done, unscrew the compression gauge hose and reinstall the spark plug, tightening it with a socket. Put the plug wire back on the plug tip.

Next, check that you have enough gas in the tank for the sump to pick up. The sump pickup screen, as well as the tank vent, should be unclogged. Make sure the fuel primer bulb is in good condition, with no cracks, leaks, or deformations.

Inspect the fuel line hoses and connections from the tank to the fuel pump location for any kinks or leaks. There should not be any. Loosen the in-line fuel filter hose clamps with a screwdriver and pull out the filter. Blow through both ends of the filter to ensure there are no obstructions, then reinstall the fuel filter and tighten the clamps with a screwdriver.

Now, loosen the clamp on the pulse hose that connects to the fuel pump and goes to the intake manifold. Place a coffee can under the hose and pull the starter rope several times. If your outboard motor has an electric start function, ask an assistant to crank the engine over several times, remembering to keep the ignition cutoff activated and the plug wire removed. If there is no fuel discharge, this indicates a faulty fuel pump check valve or diaphragm.

Finally, hook up a fuel pressure gauge to the pulse hose coming out of the fuel pump. Crank the engine over using the pull rope or electric starter and read the gauge. A normal fuel pump will generate approximately five to seven psi of pressure. Any reading below this indicates a defective fuel pump check valve or diaphragm. In this case, you will need to replace the fuel pump or purchase a rebuild kit.

Check for leaks

Checking for leaks is an important step in testing your boat's fuel system. Here is a detailed guide on how to do it:

Step 1: Check the Fuel System for Air Leaks

This is the easiest test. You will need a clear vinyl line and a couple of barbs. No fuel gauge is needed at this point. Start the boat engine and look for bubbles in the clear line. A small bubble here and there is normal, but if you see a steady stream of bubbles, you have an air leak. Turn off the engine and inspect all clamps, fuel fittings, nipples, and barbs. Ensure that all metal connectors and threads are tight, clean, and properly sealed. Re-test the engine after making any necessary adjustments.

Step 2: Vacuum Test the Fuel System

This test is to check for any restrictions in the fuel system that could affect engine performance. It is best performed with the engine under load, running in gear on the water. Ensure all fuel line connections are secure and leak-free. Install a vacuum and fuel pressure gauge, along with a clear, fuel-resistant vinyl hose, into the fuel line at the filter fuel pump inlet. Start the engine and run it at full throttle for at least two minutes. Observe the vacuum gauge reading and check the clear hose for air or vapor bubbles.

Gauge Reading Interpretation:

• If the gauge reading does not exceed 4 inches of mercury (Hg) and there are no bubbles, your fuel system is likely functioning normally.
• If you see bubbles, check for issues such as loose, cracked, or corroded fittings, a faulty filter seal, or a defective pickup tube in the fuel tank.
• A gauge reading of 0-4 inches of mercury is normal, but if it's consistently close to 4, you may have a restriction in the fuel system.

Step 3: Testing the Fuel Pump for Vacuum

This test checks the fuel pump's ability to create a vacuum and draw fuel. Connect the test rig between the fuel pump and the fuel filter. Start the boat engine and let it warm up for a minute or two. While the engine is running, pinch the fuel line shut between the fuel tank and the compound gauge using pliers. The engine should continue to run for a bit, and the gauge should show that the fuel pump is trying to pull fuel through the restriction.

If the fuel pump cannot generate enough vacuum, it may need to be rebuilt or replaced. A healthy fuel pump should be able to maintain a certain vacuum level:

• For outboard fuel pumps: 3 to 5 inches of vacuum at idle or high idle over 10 seconds.
• For sterndrive mechanical or suction-type electrical fuel pumps: 6 to 9 inches of vacuum over 10 seconds.

By carefully performing these tests and checking for leaks, restrictions, and proper vacuum levels, you can ensure that your boat's fuel system is functioning optimally and that your engine receives the fuel it needs for smooth performance.

Use a test tank

To check the fuel pressure on an outboard motor, you can use a test tank. This method involves filling a portable tank with fresh gasoline and connecting it to the motor. Here are the steps to follow:

• Ensure that the fuel lines on the test tank are at least as large as those on your boat. Using smaller fuel lines can cause fuel starvation problems and other issues that will complicate troubleshooting.
• Connect the test tank to the motor and run it through its full range of speeds.
• If the motor runs fine on the test tank, open the top of your boat's fuel tank and pressurize the fuel system.
• Start the motor again. If the problem persists, clear your fuel tank vent with a small piece of wire or consider replacing the tank.
• If the motor still doesn't run properly, replace the fuel in the tank.

By using a test tank, you can isolate the issue to either the fuel tank or another component of the fuel system, and take the appropriate steps to resolve the problem.

Check the fuel filter

Checking the fuel filter is an important part of testing the fuel pressure on an outboard motor. A blocked low-pressure fuel filter can cause hard starting, stalling, misfire, or poor performance. Here's a detailed guide on how to check the fuel filter:

Step 1: Loosen the in-line fuel filter hose clamps with a screwdriver. This will allow you to access the fuel filter.

Step 2: Carefully pull out the fuel filter from its housing. Be gentle to avoid damaging any nearby components.

Step 3: Inspect the fuel filter for any signs of debris, clogging, or damage. Look for any obstructions that may be blocking the flow of fuel.

Step 4: If the fuel filter appears dirty or clogged, replace it with a new one. Make sure to use a compatible fuel filter designed for your specific outboard motor.

Step 5: Reinstall the new fuel filter securely, ensuring that all hose clamps are tightened properly.

Step 6: Prime the fuel system by turning the key to the "on" position several times. This will help build up pressure and ensure that fuel is flowing through the new filter.

Step 7: Start the engine and observe its performance. If the engine runs smoothly without any issues, your fuel filter check and replacement were successful.

It is important to consult the owner's manual or a marine technician for specific instructions regarding your particular outboard motor. Fuel systems can vary, and it is crucial to take the necessary safety precautions when working with fuel.

Frequently asked questions