Riding mowers and tractor-mowers experience fuel pump issues from time to time. These problems usually cause the lawn mower to stall or prevent it from operating on full RPMs. If you suspect your lawn mower fuel pump might be broken or is struggling to pump fuel, there are 5 quick tests you can do to diagnose it. In this article, I’ll show you how to test your lawn mower fuel pump to determine if you need to install a replacement pump. Let’s get started.
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How Do You Diagnose A Broken Fuel Pump?
To diagnose a faulty lawn mower fuel pump, you’ll need to check the pump’s outflow and see how the fuel flows from the pump outlet while the engine is running. If your mower’s fuel pump doesn’t produce a constant flow of fuel during this test, then you’ll have diagnosed an issue with the pump.
Why Is My Lawn Mower Not Pumping Fuel?
Riding mowers or tractor mowers use an impulse fuel pump powered by an OHV or an overhead valve. The positive pressure from the OHV pushes fuel to the carburetor, and the negative pressure pulls the fuel from the fuel tank. For the pumping process to work correctly, the OHV needs to create the +/- pressure, the path from the fuel tank to the carburetor must be unobstructed, and the fuel pump has to work properly.
Causes of a Lawn Mower Fuel Pump Not Working
Several issues can stop a lawn mower fuel pump from pumping fuel. It’s not always due to a defective fuel pump. So, while diagnosing a fuel pump that is not pumping fuel, you’ll need to look out for the following issues.
- Blocked Fuel Lines
- Clogged Fuel Filter
- Damaged OHV Line
- Faulty Fuel Pump
How Do You Test a Lawn Mower Fuel Pump?
Here are 5 tests you can do to test the fuel pump on your lawn mower.
- Test the Fuel Flow from the Fuel Tank to the Fuel Filter
- Test the Fuel Flow from the Fuel Tank to the Fuel Pump
- Test the Pressure from the OHV
- Test the Fuel Flow from the Fuel Tank to the Fuel Pump Outlet
- Test the Fuel Flow from the Fuel Tank to the Carburetor
During your diagnosis, if the only test that fails is the outflow from the fuel pump test, then you’ll have diagnosed that the fuel pump is faulty and needs to be replaced. If you find a blockage or crack in the fuel lines, check out my articles on how to clean a fuel line and how to replace a lawn mower fuel line.
Testing a Lawn Mower Fuel Pump (5 Easy Tests)
Now let’s go over how to test a lawn mower fuel pump. Here are my five complete guides to test each part of your lawn mower’s fuel system to diagnose any potential fault with your mower fuel pump.
What You’ll Need to Diagnose a Lawn Mower Fuel Pump
Test 1 – Test the Fuel Flow from the Fuel Tank to the Fuel Filter
The first test is to check that the fuel coming from the fuel tank isn’t restricted. To test this part of the fuel system, you’ll need to disconnect the fuel line that connects the fuel tank to the fuel filter.
- First, you need to stop the flow of fuel. So, clamp the fuel line by using a fuel line clamp or switch off the fuel cut-off valve located on the bottom of the fuel tank.
- Then, use a pair of pliers to remove the clip securing the fuel line to the intake of the fuel filter, then disconnect the fuel line from the fuel filter.
- Next, place the disconnected end of the fuel line into a fuel-safe container.
- Remove the fuel line clamp or open the fuel cut-off valve.
- Inspect the flow of fuel and ensure that there is a continuous steady flow of fuel.
Results – A good flow of fuel means that your lawn mower has passed TEST 1. However, if you find the fuel flow is slow or inconsistent, then there is an issue with the fuel line and fuel tank and not the fuel pump.
Test 2 – Test the Fuel Flow from the Fuel Tank to the Fuel Pump
This next test is to check how the fuel flows from the fuel tank through the fuel filter. Here are the steps you can follow.
- First, stop the fuel flow by clamping the fuel line with a fuel line clamp or switching off the fuel cut-off valve on the bottom of the fuel tank.
- Then, using a pair of pliers, remove the clip securing the fuel line to the fuel pump intake, then disconnect the fuel line from the intake.
- Next, place the disconnected end of the fuel line into a fuel-safe container.
- Remove the fuel line clamp or open the fuel cut-off valve.
- Inspect the flow of fuel and ensure that there is a continuous steady flow of fuel.
Results – Again, you should see a good continuous flow of fuel coming from the fuel line. If the fuel is significantly slower than TEST 1, you likely have a clogged fuel filter that needs to be replaced. Additionally, the fuel line could be clogged, causing a reduction if fuel flow. If you see a good flow of fuel, then you want to move on to TEST 3.
Test 3 – Test the Pressure from the OHV
The next test is to check that you have +/- pressure coming from the OHV while your mower is running. If you cannot start your lawn mower, then you’ll just have to carry out a visual inspection. Check for any holes or splits in the rubber line between the fuel pump and the OHV.
- Remove the clip securing the rubber line to the fuel pump OHV inlet.
- Start your lawn mower’s engine.
- Place your finger over the end of the removed rubber line and feel for the air pressure pulsing as the lawn mower is running.
Results – If you can feel the air pressure pulsing against your finger and there aren’t any leaks, then you have diagnosed that your OHV line and pressure are ok. If you don’t feel any pulsing air or you locate hose damage, then this will likely be the reason your fuel pump is not working.
Test 4 – Test the Fuel Flow from the Fuel Tank to the Fuel Pump Outlet
Test 4 checks whether or not your lawn mower’s fuel pump can actually pump. If you have successfully completed TESTS 1-3, there shouldn’t be anything preventing your lawn mower fuel pump from working. So, for this test, you’ll need to run your lawn mower with the fuel line disconnected. Typically, your lawn mower should run for a minute as there should be enough fuel in the carburetor fuel cup. So, as long as you are quick, you should be able to check the fuel pump. Let’s go over how to test a lawn mower fuel pump.
- Using a pair of pliers, remove the clip securing the fuel line to the fuel pump outflow.
- Remove the fuel line from the outflow of the fuel pump.
- Connect a separate temporary line to the fuel pump outflow and place the other end into a fuel-safe container.
- Start the lawn mower.
- Review the flow of fuel coming from the temporary fuel line.
Results – If your fuel pumped passed TESTS 1-3, then the fuel flow coming from the temporary fuel line should be strong and continuous. If the flow isn’t strong and consistent, then your lawn mower’s fuel pump is faulty and will need to be replaced. More than likely, the internal rubber diaphragm is damaged. Unfortunately, most fuel pumps are sealed, preventing any form of servicing, and you’ll need to install a replacement lawn mower fuel pump. Check out my article on how to replace a lawn mower fuel pump.
Test 5 – Test the Fuel Flow from the Fuel Tank to the Carburetor
The final test is to check the flow of fuel through the last section of the fuel line between the fuel pump and the carburetor. This test looks for any blockages in the fuel line which could lead to your fuel pump not being able to pump fuel. Here are the steps for this test.
- Disconnect the fuel line from the carburetor.
- Place the disconnected end of the fuel line in a fuel-safe container.
- Start your lawn mower’s engine.
- Observe the flow of fuel into the fuel container.
Results – If the fuel pump has passed TESTS 1-4, and the fuel flow is poor, then there is a problem in the fuel line between the fuel pump and the carburetor. If your lawn mower has a good flow of fuel from the final test, then the problem with your lawn mower is not within the fuel system. If your lawn mower has a problem with fuel reaching the engine, I recommend continuing your diagnosis by looking at the carburetor. Your carburetor might be dirty or have a stuck fuel float. Check out my article on how to clean a carburetor jet.
