Man about to start working on lawn mower.

How To Expertly Diagnose And Fix A Bad Lawn Mower Fuel Filter

13 minutes

If your lawn mower is running poorly, one possible problem is the fuel filter going bad. Knowing how to fix a bad lawn mower fuel filter will save you time and money. Here’s how.

Diagnosing a bad lawn mower fuel filter requires looking for signs of poor engine performance, difficulty starting the lawn mower, and reduced fuel efficiency. Fixing requires accessing and replacing the fuel filter, then testing the lawn mower for further issues.

My Experience With Diagnosing And Fixing Bad Lawn Mower Fuel Filters

Growing up in the 1960s and 1970s, it was my job as a youngster to mow the lawn around our house every few weeks. This was preferable to going to my grandpa’s house, as he only had an old rotary lawn mower requiring brute strength to push and make it cut.

My first real encounter with fuel problems, specifically fuel filters, came one summer day when I was about 8 or 9.

It stopped as I pushed my dad’s lawn mower across our lawn.

Checking the fuel tank revealed the problem – no fuel. I went to the shed and proceeded to fill the fuel tank. A few minutes later, I pulled the starter cord, only to be enveloped by a cloud of white smoke that quickly became black as the lawn mower coughed and spat until it just stopped.

No amount of pulling on the starter cord started it again, and I gave up.

My old man came home, inspected the lawn mower, and after spanking my backside, explained the problem.

He’d decanted (pouring liquid from one container into another so as to leave sediment at the bottom of the pouring container) oil into an old fuel can, which I had mistaken for gasoline. Needless to say, my old man wasn’t too happy about having to dismantle the lawn mower and clean out the fuel tank, hoses, and carburetor before putting it all back together again.

I did, however, learn a great deal about fuel, fuel filters, and why keeping the fuel lines clean was so important. What I learned over 40 years ago has helped me keep my lawn mowers and other garden equipment in good condition and saved me a small fortune in repair costs.

I’ve replaced fuel filters on just about every piece of equipment I own and will continue to do so for the foreseeable future.

Understanding The Lawn Mower Fuel Filter

Closeup of someone inspecting a fuel filter.

Before going to diagnose and fix a bad lawn mower fuel filter, it’s essential to understand them. The fuel filter is connected to the fuel line from the fuel tank or built into the bottom.

Let’s start with mounted fuel filters.

Fuel Filters Mounted On The Fuel Tank

Fuel filters mounted on the fuel tank are metal screens you can inspect when the fuel tank is empty by looking into the tank with a flashlight. These filters seldom clog. If they do, you’ll have to disconnect the fuel tank from the lawn mower, which is best left to a qualified mechanic.

Inline Fuel Filters

You can find inline fuel filters between the fuel tank and the carburetor or injectors. It is simple to trace the black, rubberized fuel line from the gasoline tank, following it towards the white or see-through round plastic fuel filter. 

The fuel tank is usually a black container with a large filler cap on top. Depending on the make and model, the fuel line is either located on the bottom or the top of the tank. You can remove the fuel filler and look inside the tank to find the fuel feeder hose indicating a fuel line at the top. If there is just a round metal filter at the bottom of the tank, then the fuel line will exit the fuel tank underneath.

Gasoline flows through the inlet port on the fuel filter. This is normally indicated by an arrow pointing in the direction of flow.

Once the gasoline enters the fuel filter, it encounters the filter element, which allows fuel to pass through its tiny holes. The size of these holes prevents contaminants (dirt, rust particles, and even water particles) from leaving the filter.

The clean fuel passes through the filter and exits through the other side. It travels along the fuel line connecting it to the carburetor or fuel injectors.

Usually sealed, lawn mower fuel filters need replacing when they become too dirty to do their job correctly. It is best to use manufacturer-recommended filters. This avoids the problem of fitting a substandard filter that may not do its job correctly.

Fuel Filters Inside The Fuel Line

Some lawn mowers have fuel filters that are inside the fuel line. These are normally inside the fuel pipe where it exits the fuel tank. You must check your manual to see if your lawn mower has an internal fuel filter. 

Internal fuel filters resemble a straw with a bulge on the end. To replace them, you’ll need to remove the fuel line from the fuel tank and then withdraw the fuel filter from the line attached to the fuel tank.

Other Things To Know About Fuel Filters

Fuel filters are inexpensive items that can last a year before needing replacement. It pays to give them a look over once every few months to see if they are dirty brown inside. If the filter appears discolored (not clear) or dark brown, and your lawn mower is no longer running smoothly, it is safe to assume you need to replace it.

If you can see fuel inside the fuel filter and there is no discoloration (the filter is still clear), then you can leave it in place.

Also, with the advent of eco-fuels and additives, gasoline can go bad quite quickly.

In fuels with ethanol, you should use gasoline within a few months of purchase as it degrades afterward. As the fuel degrades, the ethanol separates from the fuel, reducing its octane and attracting water.

This makes your lawn mower run poorly and reduces its efficiency.

A clogged fuel filter restricts the flow of gasoline to the engine, making it hard to start and run. So, it’s essential to regularly check it and replace it if needed.

How To Diagnose A Bad Lawn Mower Fuel Filter: Step-By-Step Guide

Closeup of someone working on a lawn mower.

A bad fuel filter is a common problem encountered by repair shops. Often a lawn mower will come in for repair after the owner cannot start or keep the lawn mower running.

Here’s what to do.

1 – Pay Attention To The Signs Of A Bad Fuel Filter

There are many possible signs of a bad lawn mower fuel filter (although keep in mind these signs could also be showing other issues).

Let’s start with the engine stalling or power loss.

Engine Stalling or Power Loss

If the engine stalls after starting the lawn mower and you have fuel in the tank and a spark at the spark plug, you should suspect the fuel filter.

A clogged fuel filter will create intermittent problems that get worse over time. However, if you can’t start your lawn mower at all, check the fuel system first.

As the fuel filter clogs, it slowly starves the engine of fuel, making it hard to start and difficult to keep running. If you don’t use your lawn mower for a few months, the contaminants in the filter material will harden and make it impossible for the fuel to flow smoothly.

If the engine doesn’t produce sufficient power, the blades will not spin at the right speed, and the lawn mower can bog down. This may cause damage to the blades and increase wear and tear on the engine bearings.

Difficulty Starting The Lawn Mower

A clogged fuel filter makes the lawn mower challenging to start, as the engine needs more fuel when it is cold.

If the filter is blocked, increasing the choke won’t have any effect, as the fuel cannot reach the carburetor or fuel injectors. Enough fuel won’t reach the cold combustion chamber, so the mixture won’t ignite easily.

The engine will not start if the mixture is too lean, requiring extra fuel to start a cold engine.

Poor Engine Performance And Misfires

When a block fuel filter restricts fuel flower, the engine will starve of fuel. A float bowl inside the carburetor emptying faster than the fuel flowing into it can replace the used fuel is typically the cause.

This causes the engine to slow down and speed up, also called “hunting.” Pops and bangs usually accompany this. The engine will have a rough, uneven idle and struggle to run.

It will also misfire, and in most cases, small amounts of whitish smoke will come out of the exhaust as excess fuel is expelled as the carburetor struggles to keep a constant flow of fuel.

Reduced Fuel Efficiency

Besides the uneven running, the engine will also use more fuel to keep running. You’ll also end up with a very uneven cut requiring you to go over the lawn again.

All this uses more fuel and is a waste of resources.

The engine requires a very precise mixture of fuel and air to run correctly. If the fuel flow is interrupted by a blocked fuel filter, the engine will not be able to develop enough power to maintain an even cycle, as there will be too much air and too little fuel in the combustion chamber.

3 – Gather Your Tools And Supplies

You’ll need the following supplies for the below jobs:

3 – Follow Proper Safety Precautions

Always work on your lawn mower on a stable, level surface. Chock the wheels to prevent it from rolling. Ensure you’re in a well-ventilated space and are wearing all safety gear.

4 – Visually Inspect Your Fuel Line And Fuel Filter

Check for any breaks or damage to the fuel hose, especially in areas around and under the metal clips that hold the hose in place. Follow the fuel hose from the fuel tank to the fuel filter and then onto the carburetor.

If you see signs of wear, you can remove the hose clips using a pair of pliers to loosen the clip and check whether the rubber hose is cracked or torn. It is best to replace broken hoses to avoid fuel leaking, which will affect how the engine runs.

Then look at the filter to see if it is damaged or shows signs of contamination inside. This is usually evident by a brownish-red build-up inside the filter.

5 – Do A Fuel Flow Test

To conduct a flow test to check the filter’s functionality, disconnect the fuel lines from the fuel filter by loosening the clips holding the hose onto the filter inlet and outlet ports. These are the short pipe pieces extending from each side of the filter. Place a container underneath the end of the fuel line that comes from the fuel tank.

The fuel should run out of the hose quickly in gravity-fed fuel systems. This is the case with most push lawn mowers. Check your manual to see if your lawn mower has a fuel pump. If not, then the fuel flows to the carburetor under the force of gravity.

Ride-on lawn mowers usually have a fuel pump located in the fuel tank.

Switch on the ignition and observe whether the fuel comes out of the fuel hose, which indicates the fuel pump is working. If it is, then the fuel filter is likely bad and needs replacing. If no fuel comes out, you can look to the fuel pump as the source of fuel flow problems.

6 – Use Professional Diagnosing Tools

You can use a fuel pressure gauge to check whether the fuel system is developing enough pressure to push the gasoline through the filter to the carburetor.

You connect the gauge to the fuel hose on the outlet side of the fuel filter, then try to start the lawn mower.

Do this a few times to get an average reading and to see whether the fuel system is developing enough pressure. The fuel pressure will be higher in lawn mowers that use fuel injection instead of a carburetor. If you cannot see a carburetor on top or to the side of your engine, then you most likely have a fuel injector motor with a high-pressure fuel pump. You can confirm this by consulting your manual.

You can expect a reading between 2.5 psi (pressure per square inch) and 4 psi. This indicates the fuel pump is working correctly and flowing properly through the fuel filter. If insufficient fuel reaches the filter, the pressure will be lower.

You can then change the fuel filter to see if it is the culprit. If not, look to the fuel pump as a possible cause of fuel starvation in the engine. 

How To Fix A Bad Lawn Mower Fuel Filter: Step-By-Step Guide

Closeup of someone performing maintenance on a lawn mower.

The steps to fix a bad lawn mower fuel filter is quick and easy, so long as the fuel hoses are not damaged. To fix a bad lawn mower fuel filter, you’ll need to replace it.

1 – Gather All Necessary Tools And Parts

You’ll need all the same tools as you do for diagnosing a bad lawn mower fuel filter, plus:

  • Replacement fuel filter (check your manual to see which one you need)

2 – Take The Proper Safety Precautions

Follow all the same safety precautions for this job as you did when diagnosing a bad lawn mower fuel filter.

3 -Take The Following Steps

Before removing the fuel filter, turn off the fuel shutoff valve or clamp the fuel line with hose clamp pliers. If your lawn mower has one, you can find the fuel shutoff valve between the filter and the fuel tank.

Then take a pair of flat-nose pliers and squeeze the tabs on each hose clamp together, moving the clamp back along the fuel line.

Pull the fuel hose off the fuel filter, taking note of the filter’s orientation and the direction of the flow arrow on the filter, if it has one.

Some fuel will spill out of the filter and fuel line, so have a clean cloth handy to catch the drips.

Push the fuel hose back onto the new filter, ensuring the hose clamps are still on the hose. Squeeze the tabs on the hose clamps together and position the clamps close to the edge of the filter in the same position they were in before.

Check to see if there are any leaks by starting the lawn mower. When the lawn mower is running, inspect the fuel filter to see if any fuel is dripping from the area around the hose clamps. If there are no drips and the lawn mower is running smoothly, the fuel filter is doing its job correctly.

Tips To Maintain A Healthy Lawn Mower Fuel Filter

You should replace your fuel filter at least once per year.

Avoid using old fuel as well, ensuring you only fill the fuel tank with clean fuel no older than three months. If you store your lawn mower over the winter, replace the fuel filter before using the lawn mower again, as the fuel inside will have degraded and probably clogged the filter. 

You can avoid the fuel degrading by adding a fuel stabilizer (and there are plenty of great options for this). But it is best to use all the fuel in the tank before storing the lawn mower for a few months with an empty fuel system.

Final Thoughts On How To Diagnose And Fix A Bad Lawn Mower Fuel Filter

After more than 30 years of working on lawn mowers, I’ve found that replacing the fuel filter is one of the easiest ways to ensure a lawn mower runs properly.

You don’t need to be a mechanic, and it requires no specialized tools. By preventing contaminants from reaching the carburetor, you’ll save yourself a lot of trouble and expense.

So, check your lawn mower fuel filter regularly and replace it if the filter becomes discolored or if you see any debris inside.

John Mulder
Lawn Mowers Enthusiast
John has years of experience maintaining and servicing small engine equipment dating back to 1982. While not out flying or sailing, you’ll find him tinkering with lawn mowers, weed whackers, chainsaws, and ATVs. He loves looking after his landscaped gardens, and perfect lawn, and tending his fruit trees.