Sometimes a problem with your lawn mower can be caused by the simplest of things and won’t need you to take out the tool bag and strip the engine in search of the root cause. For example, even though you may be experiencing symptoms like spluttering and stalling, it could be down to a simple problem like a defective gas cap or even just a dirty gas cap. Luckily, with just a few minutes of tinkering with the gas cap, your lawn mower can be up and running again.
Symptoms of a Lawn Mower Gas Cap Not Venting (The Short Answer)
Without proper venting, a vacuum will form in the gas tank, preventing fuel flow. Once sufficient back pressure has been created, the gasoline in the tank will be held back, and the mower will begin to struggle. A gas cap not venting correctly will cause your lawn mower engine to splutter, misfire, and stall, similar to if it was running out of gasoline.
Why Will a Mower’s Gas Cap Stop Venting?
A mower’s gas cap will usually stop venting because it’s blocked. This can be something as simple as grass clipping or dirt caught in the cap. Also, I’ve seen gas caps get damaged and deform the plastic in the cap. Stacking things up on your mower when it’s not in use is an easy way to damage a gas cap.
What’s Actually Going On During Venting
As the lawn mower uses gas, the volume of the gas in the tank decreases and is replaced with air. The vent in the gas cap allows the air to enter the gas tank and replace the space previously taken up by the gasoline.
This is similar to drinking water from a glass bottle. If you don’t let air back in as you drink, water will stop flowing. The knock-on effect for the lawn mower is that it becomes starved of fuel.
Depending on how much fuel you have in the tank and even the size of the gas tank can affect how much of a vacuum is created and how much it will affect your lawn mower. This is why your lawn mower might only run for a few minutes or why it could run for about 15 minutes before symptoms start.
How to Tell If Your Lawn Mower Gas Cap is Not Venting (Telltale Symptoms)
I’m sure you are all too familiar with how your lawn mower acts when it is running out of gas. Problems with your gas cap not venting are just the same. If your engine is spluttering, misfiring, and eventually stalling, these are all symptoms of a clogged gas cap.
In addition to this, your lawn mower starts and runs just fine before the symptoms. It’s not as if there is an ongoing problem with the mower, like having bad gas or a dirty air filter, which create a problem the entire time you’re using the lawn mower.
A final symptom is that the lawn mower will work just fine a short while later. This is because the gas cap vent usually has just enough clearance to release the vacuum. It could take a while, but normally air will find its way back in. So, let’s take a look to see how you can test if your fuel cap is blocked.
Remove the Gas Cap
This is a test you can do if you are on the go and out mowing your lawn. If the symptoms start, then switch off the lawn mower and remove the gas cap. As soon as you remove the gas cap, you will release the vacuum.
Next, top off the gas tank and replace the cap. If it’s a problem with the gas cap venting, then the mower should work fine again until the vacuum builds up. If this is the case for you, then you can move on to the next test.
Inspect the Vent
Gas caps come in all shapes and sizes and use different designs to vent. Small engine lawn mowers usually have a tiny hole in the gas cap that allows air to pass in both directions. If yours does, then you want to check if it is blocked.
If yours doesn’t have this design and appears to be more complicated, then you can use a different test. Let’s take a look at the next test and confirm the venting problem.
Test the Fuel Flow
The final test is to test the venting of the cap and see if you create your own vacuum. To do this, you will need the following tools.
Tools Require to Test Gas Cap Not Venting
- Pliers
- Container to Hold Fuel
During this test, you will be checking to see how the fuel flows from the gas tank once you remove the fuel line. If there is a venting issue, you will see a problem with the flow of the free-flowing fuel. Let’s take a look at the steps you’ll need to follow.
Steps to Test Fuel Flow
First, place a container under the outflow of the fuel tank to catch the draining fuel. Next, use your pliers and remove the clips and fuel line from the fuel tank.
Now, watch the fuel as it flows from the tank into the container. It should flow in a consistent stream until the tank is empty. If you notice that the flow struggles or stops before the tank empties, you could have identified a venting problem.
To confirm this, loosen off the gas cap and continue to watch the flow. If the fuel flow starts up again, then it proves that the venting is a problem.
How to Clean the Lawn Mower Gas Cap Vent (Stupid Simple Solution)
Like the title says, this is a simple solution. If your gas cap does have a breather hole that is blocked, then all you need to do is clear the blockage. All you are going to need to fix the problem is a simple needle or pin. Just use the pin/needle and poke out the blockage.
If you discovered that you have some plastic damage, then you can try using a tiny drill bit to open the hole. A two or three-millimeter drill bit should work just fine.
Whichever method you need to use, just make sure that you don’t make the hole bigger than it needs to be or you will end up needing to replace the cap. For those more complicated gas caps, it would be easiest to just replace the cap.
Lawn Mower Gas Cap is Not Venting Alternatives
I think it would be worth mentioning that the symptoms associated with a gas cap not venting can apply to different situations.
For example, not all lawn mowers use the gas cap to vent the gas tank. You can still look out for the same symptoms and carry out the same test, but you might find that you don’t have a vented gas cap. This can be common with large ride-on and zero-turn lawn mowers since they sometimes use vent pipes and vent valves instead. Either way, the symptoms will be the same, only the fixes will be different.
Great information and much appreciated!
Glad you found it useful, Jim.
Best, Tom.