Replacing The Fuel Pump Diaphragm On A Mercury 250 Outboard

If you're experiencing issues with your Mercury outboard fuel pump, you may be wondering how to change the fuel pump diaphragm. This is a guide on how to do just that.

First, you'll need to purchase a fuel pump diaphragm kit that's compatible with your Mercury 250 outboard. You can find these kits online or at marine supply stores. Once you have the kit, follow these steps:

- Turn off the main electrical cutoff switch that governs the motor ignition.

- Remove the cowl cover by unclasping the snaps on the outboard motor upper case.

- Disconnect the spark plug wire boot from the tip of the spark plug.

- Attach a compression gauge to the spark plug hole and open the throttle position wide.

- Pull the rope start or use an electric starter to crank the engine over several times.

- Check the compression reading on the gauge; it should be above 30 psi.

- If the compression is adequate, remove the compression gauge and reinstall the spark plug.

- Now, you can proceed to remove the fuel pump. This is usually located near the fuel tank and is connected to the fuel lines.

- Disconnect the fuel lines from the pump and set the pump aside.

- Open the fuel pump kit and remove the new diaphragm.

- Carefully install the new diaphragm into the fuel pump, ensuring it's seated properly.

- Reattach the fuel lines to the fuel pump.

- Prime the fuel pump by squeezing the bulb several times to ensure fuel is flowing.

- Finally, reinstall the cowl cover and turn on the main electrical cutoff switch.

By following these steps, you should be able to successfully change the fuel pump diaphragm on your Mercury 250 outboard.

Checking for fuel leaks

When checking for fuel leaks in your Mercury 250 outboard, there are several signs and symptoms to look out for.

Firstly, use your sense of smell. A faint or strong odour of gasoline or diesel could indicate a fuel leak.

Next, check the bilge, the lowest point in your boat, for any signs of fuel. If there is fuel in the bilge, this confirms a fuel leak.

You can also check for the following symptoms that indicate issues with the fuel line, which may be due to a leak:

• Engine performance issues such as difficulty starting, frequent stalling, dropping RPMs, or the engine not running at all.
• Visible fuel leaks or fuel in the bilge.
• Decay of fuel lines or hoses, which can lead to leaks.

If you detect a fuel leak, it is important to act quickly to reduce any risks. Open all hatches to create airflow and disperse fumes, and turn off the fuel valves to stop the flow of fuel. Do not smoke or use lighters near the leak, and try to contain the spill with absorbent pads or kitchen paper towels.

Testing the fuel pump

Electrical Test:

• Check the Fuel Pump Fuse: Refer to your owner's manual to locate the fuse box. Find the fuse linked to the fuel pump and inspect it for any signs of failure, such as a blown, broken, or burned fuse. If the fuse appears damaged, replace it with a new one of the same amp rating.
• Check Voltage at the Pump: Ensure that the power is reaching the fuel pump by checking the voltage at the pump itself. Consult your vehicle's service manual to identify the correct location and procedure for this step.
• Test for Source Voltage: Determine if the charge from the fuse is reaching the pump by checking for source voltage. If there is no power at the fuel pump, move on to the next step.
• Check the Fuel Pump Relay Circuit: If there is no voltage at the pump, the issue may lie with the fuel pump relay circuit. A faulty relay could be the cause, so consider testing or replacing the relay.
• Perform a Drop Test with a Voltmeter: Connect a voltmeter to the power wire and grounding wire of the fuel pump. If the voltmeter shows a voltage difference of more than one volt, you may have corroded wires or a problem with the circuit.

Fuel Pressure Test:

• Eliminate the Filter: Remove the fuel filter from the engine and drain any excess fuel. Use a rubber hose on the filter inlet and blow through it to check for resistance. If there is minimal resistance and the filter screen is clear, the filter is likely not the cause of the problem.
• Obtain a Fuel Pressure Gauge: You can purchase or borrow a fuel pressure gauge from most auto parts stores or machine shops. This tool will help you measure the fuel pressure accurately.
• Hook up the Pressure Gauge: Locate the fuel pump test point near the fuel injectors and connect the pressure gauge to the test fitting or port. This location may vary depending on your vehicle, so refer to your owner's manual for specific instructions.
• Rev the Engine: With the engine slightly warmed up, ask an assistant to rev the engine while you observe the pressure gauge. Check the pressure at idle speed and the rated speed specified in your pump specifications.
• Interpret the Results: If the needle on the pressure gauge does not move or moves below the specified pressure, it indicates a potential issue with the fuel pump or filter. In this case, replacement of the fuel pump and filter may be necessary.

Checking the fuel filter

Step 1: Remove the Fuel Filter

Locate the fuel filter, which is usually found along the fuel line between the fuel tank and the engine. Using a screwdriver, loosen the hose clamps on both sides of the filter and gently pull it out. Take care not to spill any fuel.

Step 2: Inspect the Fuel Filter

Visually inspect the fuel filter for any signs of damage, such as cracks, leaks, or deformities. Look for any debris, gunk, or water accumulation inside or around the filter element. If the filter appears dirty or clogged, it's likely time to replace it.

Step 3: Blow Through the Filter

Place your mouth on one end of the filter and blow air through it. You can also use compressed air if available. Ensure that air passes through the filter with minimal resistance and exits the other end freely. If air struggles to pass through or if you cannot blow air through at all, the filter is definitely clogged and needs replacement.

Step 4: Reinstall the Fuel Filter

If the filter appears to be in good condition and is not clogged, you can reinstall it. Ensure that the hose connections are secure and tight to prevent fuel leaks.

Step 5: Check for Leaks

Once the fuel filter is reinstalled, check the entire fuel system for any signs of leaks. Ensure that all hoses, connections, and clamps are secure and intact. If there are no leaks, you can proceed.

Step 6: Routine Maintenance

Even if your fuel filter appears to be in good condition, it is recommended to replace it periodically as part of routine maintenance. Refer to your Mercury 250 outboard manual for specific recommendations on fuel filter replacement intervals.

Additionally, keep in mind that fuel filters are relatively inexpensive, and it is always better to be safe than sorry. If you have any doubts about the condition of your fuel filter, it is best to replace it with a new one to ensure optimal engine performance and avoid potential issues down the road.

Testing the fuel pump valves

Step 1: Remove the Fuel Pump

Locate the fuel pump on your Mercury 250 outboard engine. It is usually found near the fuel tank and connected to the fuel lines. Once you have located the pump, disconnect the fuel lines and carefully remove it from the engine.

Step 2: Inspect the One-Way Valves

The fuel pump has two types of valves: the inlet valve and the outlet valve. The inlet valve allows fuel to enter the pump, while the outlet valve directs fuel out of the pump towards the carburetor. Inspect both valves for any signs of damage or debris buildup. Ensure that the valves are clean and free of obstructions.

Step 3: Test the Inlet Valve

To test the inlet valve, you will need a pressure gauge. Attach the pressure gauge hose to the outlet valve of the fuel pump. Use your thumb to firmly seal the inlet valve. Pump the pressure gauge a few times and observe the pressure reading. If the pressure releases or drops, it indicates that the inlet valve is faulty and is allowing fuel to escape.

Step 4: Test the Outlet Valve

To test the outlet valve, you will need to use the pressure gauge again. Attach the pressure gauge hose to the outlet valve and seal the inlet valve with your thumb. Pump the pressure gauge and observe the reading. If the pressure holds steady, it indicates that the outlet valve is functioning correctly.

Step 5: Inspect the Diaphragm (if present)

If your fuel pump has a diaphragm, you can inspect it for any signs of damage. The diaphragm is a rubber component inside the pump that helps push the fuel through. Remove the fuel pump and look for a small hole at the back of the pump, which exposes a portion of the diaphragm. If you notice any leaks, stalling, or fuel bubbling in the water, it indicates a faulty diaphragm.

Step 6: Test the Diaphragm

To test the diaphragm, you will need two 10-24 bolts, four washers, a hose clamp, and a fuel line. Clamp and tighten the fuel pump using the bolts and washers. Attach the fuel line to the inlet and another fuel line to the outlet, securing it with the hose clamp. Prime the bulb and observe the hole at the back of the pump. If you see fuel leaking through the hole, it confirms that the diaphragm is faulty and needs replacement.

By following these steps, you can effectively test the fuel pump valves on your Mercury 250 outboard engine and identify any issues that may be causing problems with fuel delivery or engine performance.

Testing the fuel pump diaphragm

• Check for Fuel in the Carburetor: If your engine stops running, the first thing to do is check if there is fuel in the carburetor. If there is no fuel, you likely have a fault in the fuel system.
• Inspect for Fuel Leaks: Disconnect the fuel line to the carburetor and secure it with a plastic bag and a rubber band. Turn the engine over with the starter. If fuel spurts out, you have a carburetor fault. If there is no fuel or only a small amount, you may have a blockage, leak, or a faulty fuel pump.
• Examine the Mechanical-Pump Drive: The mechanical-pump drive rarely fails, but you should inspect the cam, operating lever, linkages, and pins for any signs of wear or damage.
• Look for Diaphragm Leaks: Diaphragms can develop small leaks, which may affect engine performance at high speeds. Leaks can also allow fuel to drip into the camshaft area and the sump, creating a risk of explosion.
• Check Gaskets and Fuel Pipes: Gaskets, loose fuel pipes, and the seal between the diaphragm and pump body can leak. Ensure all connections are secure and there is no damage.
• Inspect Mountings: Loose mountings can cause severe oil leaks. Ensure the pump is securely mounted and all bolts are tightened.
• Test Pump Operation:

• Seal the outlet pipe(s) with your finger and push the lever up. The lever should move freely, and the diaphragm should remain still if the inlet valve is seating properly.
• Seal the inlet pipe and push the lever again. You should feel resistance, but the diaphragm shouldn't move if the outlet valve is seating correctly.
• Seal off all pipes and push the lever again. You should feel resistance if the diaphragm is intact and properly sealed.
• Seal the vent hole on the underside and push the lever. You should feel resistance if the oil seal between the linkage and diaphragm is intact.

• Use Pressure and Vacuum Gauges: The 'official' way to test the pump is to connect a pressure gauge on one side and a vacuum gauge on the other. This method provides a more accurate assessment of the pump's performance.
• Avoid Using Compressed Air: Do not test the pump by blowing compressed air into it, as you may damage or burst the diaphragm.
• Check for Clogs in the Fuel Line: If the pump appears to be functioning correctly and there are no leaks, the issue may be a clogged fuel line. Use compressed air or a tire pump to clear any potential clogs.
• Clean the Fuel Delivery System: It is recommended to clean the entire fuel delivery system, including filters and lines, to ensure optimal performance.
• Measure Fuel Delivery Rate: If your engine lacks top speed and struggles to start, the pump may not be delivering enough fuel. Disconnect the fuel line at the carburetor and direct it into a measuring container. Let the engine idle for 30 seconds and measure the amount of fuel delivered. For an idling engine, the pump should deliver between 3/8 and 1/2 pint of fuel in 30 seconds.
• Inspect the Rocker Arm: Ensure that the rocker arm is mated correctly to the cam eccentric or the camshaft. This detail is crucial for proper pump operation.
• Check for Gaskets and Oil Leaks: When reinstalling the pump, make sure to use a gasket between the pump and the engine to prevent oil leaks.
• Mark Pump Alignment: Before disassembling the pump for further inspection or repairs, use a file to mark the pump housing and engine block to ensure proper alignment during reassembly.
• Inspect Diaphragm and Valves: The diaphragm and valves are critical components of the pump. Check for any signs of wear, damage, or leaks. If necessary, replace them with a complete rebuild kit or a diaphragm-only service kit.
• Soak Diaphragm in Kerosene: When replacing the diaphragm, allow it to soak in kerosene while you work on other pump repairs. This will lubricate the layers of the diaphragm and improve its performance.

Frequently asked questions