How To Tell If 2 Stroke Mixed Gas Is Bad – Easy Method


Like that in chainsaws, two-stroke engines need a mixture of gasoline and oil to function correctly.

Unlike other engines, two-stroke engines do not have an oil supply in the crankcase to cool the engine. Mixing oil with gasoline is the only way to deliver oil to a two-stroke engine. The purpose of oil being mixed with gasoline in a two-stroke engine is to provide lubrication and cooling to the internal engine components. If a two-stroke engine is run on pure gasoline, meaning no oil mix, it will be ruined internally due to lack of lubrication and excess heat.

Two-stroke mixed gasoline has a limited shelf life and will spoil over a period of time. You do not want to run spoiled mixed gasoline in your chainsaw’s two-stroke engine. Bad 2 stroke mixed gas can severely damage an engine.

How can you tell if two-stroke mixed gas is bad?

Two-stroke mixed gas is bad if the fuel has a spoiled varnish-like odor, has a murky darker appearance not indicative of oil mix color, or has been in storage for more than a month.

Do not use it in your chainsaw if any of these conditions are present with your two-stroke mixed fuel.

2 Stroke Mixed Gas Has Spoiled Varnish Odor

Both two-stroke mixed gas and pure non-mixed gasoline have a limited shelf life. In general, if gasoline has been in storage can for an extended period of time, it will spoil and become less combustible than fresh gas.

Gasoline is made of combustible compounds that will decay and evaporate over a period of time. Once the fuel has aged to the point of decay and certain compounds have evaporated, the gasoline will start to smell like an old varnish.

If your gasoline has a stale varnish smell, do not use it in your chainsaw or other power equipment.

A good rule of thumb is if your gasoline is more than one month old, do not use it in your chainsaw. There are differing opinions on how long gasoline stays viable in storage. However, Stihl, the number one chainsaw maker in the world, says the following: “STIHL currently recommends that you do not store or use fuel older than 60 days, and preferably no longer than 30 days, even with a stabilizer.”

2 Stroke Mixed Gas Has Murky Darker Appearance


As fuel ages, the smell is not the only thing that changes during the decay process. The appearance of fuel also transforms from a clear transparent state to a murky opaque mess.

Two-stroke mixed gas should be clear and transparent with the colored tint of the oil used for the mixture. As fuel ages, it oxidizes, changing the appearance from transparent to partially opaque darker color.

If your fuel changed in appearance from when you initially mixed it, do not use it in your chainsaw.

2 Stroke Mixed Gas Older Than 1 Month

When gasoline sits in a storage container for an extended period of time it will start to decay like any other perishable item. Would you drink milk that has been in your refrigerator for 1 month or longer? I hope not. It would make you sick just like the month old gasoline can make your chainsaw sick.

The decay process of gasoline is when the combustible materials that make up gasoline breakdown and start losing combustabilty thru oxidation and evaporation.

The result of this breakdown is gasoline that will no longer ignite as intended which leads to a poorly running chainsaw. In some cases, a chainsaw may not even start, if the fuel has decayed to a certain point.

Another problem with two-stroke gasoline sitting in a container for an extended time period is water. The gasoline we have today has at least 10% ethanol.

Ethanol in gasoline can literally pull water out of the air and deposit it in the fuel container. When water is mixed with fuel and oil it causes a process known as phase separation. Phase separation is when water is absorbed from the air and settles in the bottom of the fuel can. According to Stihl “Phase separated fuel is no longer suitable for use even after being shaken vigorously.”

How Long Is Premixed Gas Good For?


Premixed gasoline such as TruFuel ready to use fuels have a much longer shelf life than that of traditionally mixed fuels.

Where a traditionally mixed fuel has a shelf life of around one month a premixed gas can last from two to five years. The period of two years is once the fuel container has been opened and 5 years is for an unopened container.

Premixed fuels do not usually contain ethanol so phase separation is not a threat.

The downside to premixed fuels is the price. Premixed fuels are considerably more expensive than traditionally mixed fuels.

What Color Should 2 Cycle Gas Be?

Pure gasoline is a colorless liquid and is essentially clear. Two-cycle gas is gasoline mixed with a colored oil. The mixing of the oil and gasoline makes the fuel adopt the color of the oil mix. The oil mix can be any color but oil mixes are usually, green, red or blue.

The only way to know what color your 2 cycle gas should be is to know what color it was when it was fresh.

Remember, decay and oxidation will change the color of gasoine, both pure and mixed. If the gas does not look correct do not use it in your chainsaw.

What Can I Do With Old Premix Gas?

Premixed fuel has a very long shelf life, usually 2 to 5 years, but it’s not indefinite.

What do you with your premixed fuel once it has expired and is no longer viable to use in your power equipment?

If you need to dispose of old premixed fuel or gasoline you will need to:

  1. Locate your nearest hazardout materials disposal facility
  2. Pour old premix or gasoline into a goverment certified container
  3. Transport premixed fuel or gasoline to hazardous materials facility

Wrapping Up

Two-stroke mixed fuel has a shelf life and can go bad while sitting in your garage.

Remember, Two-stroke mixed gas is bad if the fuel has a spoiled varnish-like odor, has a murky darker appearance not indicative of oil mix color, or has been in storage for more than a month.

If your fuel exhibits any or all of these traits do not use it in your chainsaw; doing so can damage your equipment.

I hope you were able to learn something from this post.

Please be safe out there and happy cutting.

Steven R

I have been part of the chainsaw and outdoor power equipment business in one way or the other for over 35 years. There are not many things that I have not seen in the business. From repairs, sales, equipment operation, and safety I can help you with your questions.

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