Can a Lawn Mower Get Vapor Lock? All You Need To Know

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When it comes to lawn mowers, there are a lot of things that can go wrong. You might end up with a mower that won’t start or one that’s cutting unevenly. But one problem you might not have heard of is vapor lock.

During warmer months when the temperatures are high, it’s not uncommon for lawn mowers to get vapor lock. Vapor lock occurs when the gas inside a lawn mower starts to evaporate because of the heat. The evaporated particles inside the tank keep the gas from getting to the engine, making it lock up.

If you want to know what causes lawn mowers to get vapor lock, how to diagnose it, and how to start an engine after the lock, keep reading!

What Causes Vapor Lock in a Lawn Mower?

When the gas inside a lawn mower gets too hot, it evaporates and becomes vapor. The extra vapor inside the mower prevents the remaining gasoline from making it to the engine, making it difficult for the lawn mower to start.

It’s worth noting that gasoline is highly volatile, so it vaporizes easily due to its low boiling point components. That’s why gas-powered lawn mowers are more susceptible to vapor lock than other mowers.

Other causes of vapor lock in lawn mowers include:

  • Leaving the gas tank cap off: When you leave your gas tank cap off, it allows heat to enter the tank. Being volatile, fuel gets vaporized even by small amounts of heat. If you live in an area with intense sun, the heat from the sun can also vaporize fuel in the tank. So, always keep your gas tank cap on when you’re not using your mower.
  • Faulty carburetor: A carburetor is essential to your lawn mower as it ensures the correct engine combination of fuel and air. A faulty carburetor can cause a vapor lock. If the carburetor leaks, it can let air into the fuel line. This causes the fuel to vaporize, leading to a vapor lock.
  • Filling the tank when it’s hot outside: If you fill your gas tank when it’s hot outside, the fuel can quickly turn to vapor. This is especially true if the gas station is in direct sunlight. To avoid this, try to fill your tank in the evening when the temperature is cooler. Alternatively, you can fill it in the morning before the sun gets too hot.
  • Using old or contaminated fuel: If you use old or contaminated fuel, it can cause a vapor lock. Older, as well as contaminated, fuel can evaporate faster than fresh fuel. Apart from causing vapor lock, contaminated fuel can lead to engine knock. You’ll most likely need to buy a new engine if it suffers from engine knock. So, always use fresh fuel, not one that’s been sitting in your garage for months.

How Do I Know if Engine Has Vapor Lock?

You can know that the engine has a vapor lock when it stalls, misfires, or has poor acceleration. These symptoms indicate the engine is not getting sufficient fuel to keep the lawn mower functioning properly.

Let’s look at the symptoms of engine vapor lock in detail below:

Stalling and Hard Starting

Just like a fire needs heat, fuel, and oxygen to keep burning, there’s also a few things an engine needs to start and keep running:

  • Precise air-fuel mixture: Too little air or fuel can lead to engine stalling.
  • Spark plugs: They create a spark that ignites the air-fuel mixture in the cylinders.
  • Compression: The pistons need to compress the air-fuel mixture sufficiently so that it ignites and powers the engine.

There must be a balanced ratio of gasoline and air inside the engine for it to run properly. Excess vapor that occurs during vapor lock changes this ratio, which can cause the engine to stop working. 

If your lawn mower’s engine stalls frequently, it’s likely due to a vapor lock.


When an engine misfires, the cylinders’ air-fuel mixture is not appropriately ignited. This leads to incomplete combustion, which is characterized by a knocking sound.

Incomplete combustion is essentially when the engine doesn’t ignite all the way, and it’s one of the results of vapor lock. You’ll notice this when the lawn mower vibrates more than usual.

Poor Acceleration

Modern lawn mowers have high acceleration rates. For instance, the Honda Mean Mower V2 can go up to 100 miles per hour.

If you notice your lawn mower is taking a long time to get to its normal speed, or it isn’t reaching its top speed at all, the engine could be suffering from vapor lock. 

The following are some ways to confirm vapor locking in your engine:

  • Check the fuel line: If it’s full of vapor, that’s a sure sign of vapor lock.
  • Try starting the engine after sitting for some time: If it starts without any problem, the issue was the vapor lock.
  • Remove the spark plug and check if it’s wet: If it is, then there’s too much fuel in the cylinders and it’s being ignited prematurely. This can also cause vapor lock.

How Do I Know if My Lawn Mower Is Locked Up?

You can know that your lawn mower is locked up if it has a stuck blade, produces a rough sound, or the starter rope won’t pull. These signs indicate that your lawn mower’s engine is not getting enough fuel to power it.

Let’s discuss these symptoms in detail below:

  • Stuck blade: If the blade on your lawn mower is stuck, it could be due to an engine vapor lock. When the engine vapor locks, it doesn’t have enough power to turn the blade. As a result, the blade gets stuck. A sure way to diagnose this is by trying to turn the blade by hand. If it turns smoothly, then your lawn mower is locked up.
  • Rough sound: When an engine is locked up, it’ll produce a rough sound. This is because it’s not getting enough fuel to power it. If you hear a distinct grinding sound while riding your mower, it indicates that it’s locked up. On most occasions, the lawn mower dies after the sound.
  • Starter rope won’t pull: If the starter rope on your lawn mower won’t pull, it’s likely due to an engine vapor lock. When the engine vapor locks, it doesn’t have enough power to turn the blade. As a result, the starter rope won’t pull.

How Do You Start an Engine After Vapor Lock?

You can start an engine after vapor lock by waiting for the fuel system to cool and eliminate the vapor. Pressing the acceleration pedal slightly as you crank the engine is also an excellent way to aid in expelling the vapor.

Once the engine starts, press the acceleration pedal to the floor and hold it there until the engine starts running smoothly. This will help remove vapor in the lines and prevent the engine from stalling again.

How To Prevent Vapor Lock

You can prevent vapor lock by:

  • Keeping the fuel system clean: Over time, dirt and debris can build up in the fuel system and clog it. This can cause the fuel to vaporize before it reaches the engine.
  • Regularly checking and cleaning the fuel filter: A clogged fuel filter can restrict fuel flow to the engine, causing it to vaporize.
  • Replacing old or damaged fuel lines: Old or broken fuel lines can leak, allowing air to enter the system. This can cause the fuel to vaporize before it reaches the engine.
  • Adding a fuel stabilizer: Fuel stabilizers help keep the fuel fresh and prevent it from vaporizing.
  • Use a thermal barrier: A thermal barrier, like a heat shield, helps keep the fuel system cool and prevents the fuel from vaporizing.
  • Use the right oil level: The oil level should be between the two holes, as close as possible to the full mark. This helps prevent the accumulation process.

Wrapping Up

Vapor locking is common in lawn mowers, especially in summer. There is no need to worry when this happens, as you can easily diagnose and solve the problem in the comfort of your lawn. You should just wait for the fuel system to cool and release the vapor.

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