Can I Leave Petrol In A String Trimmer?

With winter arriving, it’s time to put your string trimmers and other lawn equipment away until spring. However, you may have leftover petrol in your engine that you’re probably wondering what to do with. Can you leave the petrol in your trimmer? 

Leaving petrol in your string trimmer for longer than a month causes damage to its engine. The fuel degrades and forms sticky deposits that clog the carburetor and tiny passages within the fuel system. It also causes the tank to rust. The end result is catastrophic engine failure and an unusable string trimmer.  

If you’re considering leaving your petrol in the trimmer’s fuel tank or just want more insight into the topic, then keep reading! I’ve covered all the harmful effects of leaving petrol and tips to store leftover petrol properly. 

Why You Must Not Leave Petrol In A String Trimmer

You can only leave petrol in a string trimmer for short periods. Anything longer than a month will cause damage to the engine. 

Petrol is made up of a mixture of chemicals that ignite within the engine and provide power to the trimmer. One of the components in the mixture is ethanol. 

Ethanol is a highly volatile liquid. It attracts water vapor from the air and quickly evaporates, which causes two fundamental problems: 

1. It Clogs The Carburetor 

Ethanol’s evaporation leads to the degradation of the fuel. As a result, the fuel becomes stale and harder than normal. This causes the carburetor of the trimmer to get clogged. As a result, it loses its ability to provide the engine with a mixture of air and fuel.

Without fuel for combustion, the engine can no longer provide the trimmer with power. It stops working until you clean the carburetor and remove all the hardened gunk.

2. It Rusts the Tank

The second issue caused by ethanol’s volatile nature is rusting of the tank. Since it attracts more water, it corrodes the aluminum in the tank and causes rust formation. The rust eventually mixes with the fuel and enters the engine, preventing it from working efficiently.  

The damage caused by ethanol depends upon the tank’s fuel level. The lower the level, the greater the exposure to air. Increased exposure will speed up the evaporation and rusting process and cause increased damage. 

How Long Will it Take Petrol To Cause Damage?

It usually takes about a month for the petrol to start doing any real damage. The longer you leave petrol in the tank, the greater the damage. If left long enough, it may cause significant harm, and you may have to replace the carburetor and other components of the fuel system. 

Sometimes, even shorter storage durations, like a couple of weeks, may cause the petrol to degrade. Although it may not damage and clog the fuel system, the stored petrol may lose its combustibility as its volatile components, such as ethanol, evaporate. The degraded fuel’s ignition temperature increases and strains the engine, causing it to misfire. 

Mentioned below is a table summarizing the damage caused to a spring trimmer by leaving petrol in it. 

Process Mechanism  
Rusting  The ethanol in petrol attracts water from the air and causes rusting of the tank’s aluminum. 
Carburetor Deposits  Fuel degradation causes it to become hard and sticky. Hardened fuel clogs the carburetor and prevents it from functioning. 
Engine Misfires  Old petrol loses its combustibility and causes engine strain leading to misfires. 
Engine Failure All the processes mentioned above prevent the engine from functioning and make the trimmer unusable. 

Two Solutions for Leftover Petrol in Your String Trimmer

You may want to put away your string trimmer in winter or before going on long vacations. Before storing it, you must adequately treat the fuel in it. 

Here are two actions you can take about leftover petrol in the fuel tank. 

1. Add a Fuel Stabilizer 

You can add a fuel stabilizer to the petrol in the tank before storing the trimmer. It will slow down petrol degradation and keep it in pristine condition for long periods. 

After adding the stabilizer, it is essential to let the engine run for a minute. The stabilized fuel must run through the entire fuel system and the carburetor to prevent damage. 

If you do not run the engine, some unstabilized fuel may remain in the carburetor and cause oxidation, gunk formation, and rusting. 

Some experts advise against using fuel stabilizers for storage as they don’t think it is sufficient to keep the fuel system safe from damage. However, good-quality stabilizers are likely to work and prevent fuel degradation. 

Here are three of the best fuel stabilizers available in the market. 

1. Sta-Bil Storage Fuel Stabilizer

Sta-Bil Storage Fuel Stabilizer (Amazon link) claims to keep fuel in premium condition for up to 2 years! It uses an extremely purified petroleum distillate, but the exact formula is kept hidden from the public. 

It is compatible with all gas engines and prevents varnish buildup. However, it is usually slightly heavier on the pocket than other stabilizers on the list. 

2. ISO-HEET Water Remover And Premium Fuel Line Antifreeze

ISO-HEET (Amazon link) claims to absorb five times the water absorbed by regular gas dryers. It prevents petrol from breaking down and keeps it usable for longer durations of time. 

Moreover, this fuel stabilizer also prevents corrosion and rust. It is mainly made of isopropyl alcohol. 

3. Star Tron Enzyme Fuel Treatment 

The Star Tron Enzyme Fuel Treatment (Amazon link) is highly concentrated, and only eight ounces of it can treat 48 gallons of fuel! It claims to keep petrol stable for two years. 

The stabilizer is made up of a combination of enzymes that prevent phase separation. It disperses water throughout the fuel, preventing it from collecting at the bottom and causing rust and corrosion. 

2. Remove the Fuel 

Most experts suggest removing the fuel from the tank as the best option for preventing engine damage. To ensure your engine and fuel system are safe, you should remove all the petrol in your trimmer. 

You can store the fuel from the tank in a sealed plastic container that is approved for fuel storage. You don’t have to overthink it, basically, any gas can will do.

If you don’t already have one, a great option to consider is the Quick-Flow Spout Midwest Can Gasoline Can (Amazon link). It can safely store leftover petrol without leaks and exposure to air. 

I also recommend adding a fuel stabilizer to the container. It prevents the fuel from oxidizing and degrading.

Then, when you are ready to use your trimmer again, you can simply use the fuel stored in the container. A good container and fuel stabilizer will keep the fuel in premium condition.

However, removing petrol from the tank will not get rid of all of it. A little may get left behind in the carburetor or fuel system. Pulling the primer bulb several times after draining the tank would be best. 

You should hear the sound of the engine trying to start but be unable to do so as the tank is empty. This will flush the trimmer of any excess fuel in the carburetor.


How Long Can You Leave Petrol In A String Trimmer? 

You can safely leave petrol in the fuel tank for a month without causing functional problems. However, if your fuel has higher ethanol content or you live in a hot and humid climate, it may begin to degrade sooner. 

Is Fuel Stabilizer Sufficient For Preventing Engine Damage? 

Fuel stabilizers mostly prevent engine damage. However, the safer and better option is to remove all the fuel in your trimmer and store it in a sealed container. Experts and manufacturing companies also recommend the latter. 

Final Thoughts

Leaving petrol in your trimmer can cause detrimental damage, making it unusable until the fuel system and carburetor are serviced or even replaced! So, you should be extremely careful while storing your trimmer and ensure that every little drop is either treated with a fuel stabilizer or removed from it. 

Now that you know how to store leftover fuel, you can safely put away your string trimmer this winter! You won’t have to worry about degraded fuel and running problems when using it again. 

Peter Toth

Hi! I'm Peter, the owner of BackyardGadget. Working around the house has always been a big part of my life. I've created this site to share my experience, and to help people choose the right tools for the job. Thank you for stopping by!

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