Testing Yamaha's Low-Pressure Fuel Pump: A Step-By-Step Guide

Testing a Yamaha outboard low-pressure fuel pump can be done in several ways. One method is to squeeze the fuel primer bulb while the motor is running. If the motor runs roughly at high speeds and smooths out when you squeeze the bulb, the fuel pump may need replacing. Another test is to operate the motor at a full range of speeds without shutting it down. If the motor stumbles, particularly at high speeds, it could be an issue with the fuel pump or fuel tank vent. A further test is to squeeze the primer bulb several times quickly to force-feed the motor. If the problem disappears while pumping but returns when you stop, the fuel pump likely needs replacing. Additionally, it is possible to test the fuel pump while it is off the motor. This can be done by connecting a hose to the pump and seeing if it can pump fuel. Alternatively, the pump can be left on the engine, and the fuel lines can be disconnected and replaced with longer hoses.

Squeeze the fuel primer bulb while the motor is running

If your Yamaha outboard motor begins to sputter and cough, particularly at high speeds, you may have a problem with your fuel pump. To test this, squeeze the fuel primer bulb while the motor is running. If the motor runs roughly at high speeds, but smooths out when you squeeze the bulb, you should replace the fuel filters, clean out the fuel lines, and repeat the test. If the results are the same, replace the fuel pump. If not, continue your boating activity.

If the problem disappears as you pump but returns after you stop, replace the pump. If the problem doesn't disappear, or if the motor stalls, the problem is somewhere other than in the fuel pump.

If your fuel pump is not the issue, the problem may lie in the fuel tank. Check the fuel tank vent first. A clogged vent will cause fuel shortages to your carburetor at high speeds, as fuel is pulled from an unvented tank, a vacuum develops that your pump can't overcome—a condition called vacuum lock.

If your bulb is collapsing, there is a problem before the bulb, as the fuel pump is trying to draw fuel but it is not reaching the bulb. If your bulb is simply getting a bit soft, this is normal.

If you want to prime your squeeze bulb for optimal operation, you can take a few actions to ensure there’s no excess air in the boat primer bulb and the rest of the system. Try lifting key components and loosening parts. For example, lift the squeeze bulb and engine pump together. By raising both simultaneously, you can often get the fuel to start flowing through the injection pump again.

Operate the motor across the full range of speeds without shutting it down

If you're experiencing issues with your Yamaha outboard motor, particularly if it's stuttering and coughing at high speeds, you can test whether the issue lies with the fuel pump without removing the pump from the motor or returning to shore. To do this, operate your motor across its full range of speeds without shutting it down.

This process will help you determine whether the issue is related to the fuel pump or other components, such as the fuel lines or filters. By running the motor at both high and low speeds for an extended period, you can observe how the motor performs and identify any specific speed ranges where it struggles or falters.

If the motor operates smoothly and consistently across all speeds, the issue is likely not related to the fuel pump. In this case, the problem may lie with the fuel lines or filters. However, if the motor begins to stumble or show signs of distress, especially at high speeds, take note of the tachometer readings and any fluctuations in motor speed.

This comprehensive test will provide valuable insights into the root cause of the problem. Make sure to take detailed notes during the test to help with further diagnostics or discussions with a marine technician. Remember to always follow safety protocols when operating your boat and performing any tests or repairs.

By following these steps and observing the motor's performance across all speeds, you can narrow down the potential causes of the issue and make informed decisions about the next steps in troubleshooting or repairing your Yamaha outboard motor.

Squeeze the primer bulb several times quickly, force-feeding the motor

To test a Yamaha outboard fuel pump, you can perform a quick test while boating, without taking the fuel pump off the motor. Start by squeezing the fuel primer bulb while the motor is running. If the motor runs roughly at high speeds but smooths out when you squeeze the bulb, replace the fuel filters, clean out the fuel lines, and repeat the test. If the results are the same, replace the fuel pump. If the motor operates properly at high and low speeds for several hours, the fuel lines or filters were at fault.

Now, squeeze the primer bulb several times, force-feeding the motor. This action will help determine whether the fuel pump is faulty or not. If the problem disappears as you pump but returns when you stop, the fuel pump needs to be replaced. If the problem persists or the motor stalls, the issue lies elsewhere, such as the fuel tank vent or fuel filters.

The primer bulb, also known as the priming pump, is an essential part of the fuel system for outboard engines. It is used to manually prime the fuel supply, a process that ensures the engine receives an adequate fuel supply to run smoothly. The bulb consists of a flexible rubber bladder with inlet and outlet connectors, facilitating fuel flow from the tank to the engine.

To prime the bulb, you must squeeze it repeatedly until it becomes harder to compress, indicating that the fuel system is full. A correctly primed fuel system will only require a few squeezes. Once the engine is running, the feed pump creates suction, drawing fuel through the bulb valves. The primer bulb is crucial to this process, as the feed pump relies on the fuel supplied through the bulb.

It is important to orient the bulb vertically, with the arrow pointing upwards, to utilise gravity for effective priming. On boats with removable or feeder tanks, the primer bulb is usually placed directly at the tank outlet. For boats with integral tanks, the bulb is typically located about 30 cm from the engine.

Check the fuel tank vent for a clog

If you're experiencing issues with your Yamaha outboard motor, particularly at high speeds, there could be a problem with the fuel pump. However, before you jump to any conclusions, it's important to test and troubleshoot other potential issues, such as a clogged fuel tank vent. Here's a detailed guide on how to check for a clog in your fuel tank vent:

Symptoms of a Clogged Fuel Tank Vent

Before you begin, it's essential to understand the symptoms of a clogged fuel tank vent. This issue can cause poor performance, stalling, and difficulty starting your engine. Additionally, you may experience problems filling up your fuel tank. These symptoms are similar to those of a clogged fuel filter, so it's crucial to check both to identify the root cause.

Locating the Fuel Tank Vent

The fuel tank vent is typically located near the fuel filler neck or another high point on the tank. It is designed to relieve pressure and vacuum in the fuel tank, allowing it to breathe. The vent line should be straight, without any bends or dips, to prevent trapping condensation or fuel.

Checking for a Clog

To check for a clog in the fuel tank vent, carefully follow these steps:

• Find the air vent line and attach a compressor to it. Send air into the line. If the vent is unblocked, you should hear a gurgling sound. If there is no sound, it could be blocked.
• Remove the canister vent line from the gas tank and attempt to blow through it. Due to its short length, you will quickly know if there is a blockage.
• Use compressed air to check for clogs. Many compressed air cans come with long plastic tubing that can be used for this purpose.
• Protect yourself with wasp spray, gloves, and other safety gear. Wasps sometimes build nests in fuel vents, so it's important to be cautious.
• Try inserting a wire or plastic tubing into the vent to check for blockages.
• Drain the fuel tank and let it air dry. Once dry, reconnect the vent hose and fuel line, then try blowing air into the vent again with compressed air.

Other Causes of Fuel Tank Vent Issues

In addition to clogs, there are a few other issues that can affect the fuel tank vent:

• Wasps or other insects building nests in the vent, leading to blockages.
• Kinks or dips in the vent line, which can trap condensation or fuel and affect pressure and vacuum in the tank.
• Corrosion or damage to the vent valve, causing it to stick or fail.

By carefully following these steps and troubleshooting potential issues, you can effectively check for a clog in your fuel tank vent and ensure the proper functioning of your Yamaha outboard motor.

Monitor fuel pressures

Monitoring fuel pressures will give you a lot of information about what is going on with your Yamaha outboard motor. Firstly, you can test your fuel pump theory without taking the pump off the motor and without returning to shore. If your motor is stuttering and coughing, particularly at high speeds, squeeze the fuel primer bulb while the motor is running. If the motor smooths out when you squeeze the bulb, replace the fuel filters, clean out the fuel lines and re-test. If the results are the same, replace the fuel pump. If the motor continues to run rough, the problem is somewhere other than the fuel pump.

If you want to test the fuel pump when it is off the motor, you can do so by following these steps:

• Get about 6 feet of 5/16 or 3/8 hose (whichever fits your pump) and some clamps.
• Hotwire the pump to a battery using proper polarity.
• See if the pump will pump kerosene or diesel fuel.
• Disconnect the fuel lines, IN and OUT, and replace them with longer hoses (3+ footers).
• Intake with a primer bulb.
• Get two clear containers with lids and poke a hole in the lids large enough for the fuel hoses.
• Put fuel in the supply container.
• Turn the key ON (not start). The pump will run for two seconds and then shut off.
• Go to key OFF for 10 seconds or more, then recycle the key ON. The pump will run for two seconds.
• You can also get a mechanic's stethoscope with a long rod and listen to the pump.
• Pull the fuse for the HI-pressure pump so it will not run and cause confusion.
• Tee in a pressure gauge in the fuel line. Expect around 4-6 psi or 8-12 in Hg, whatever your gauge measures.
• You may have to block the exit hose slightly so pressure can build.
• Use a large cotter pin and push the head into the exit hose, then clamp the exit.
• You can also clamp the exit hose end with size grips.

If you are testing the fuel pump while it is on the engine, you should have at least 3 psi past the pumps and no more than 4"hg before them. If you are not getting a steady 3 psi, they are going out.

Frequently asked questions