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Can You Put Old Gas In A Car? (Answered By Mechanic)

Can You Put Old Gas In A Car? (Answered By Mechanic)
Written By: Kris Jackson, ASE-Certified Mechanic
Kris Jackson has been a mechanic since 2010 after graduating from UTI. He’s worked with several master mechanics and holds several ASE Certifications. You can read more about Kris here.

Have you ever been faced with the option of adding old gasoline to your car? Maybe it’s the emergency stash that slipped your mind or the leftover gallon you used for last year’s yard maintenance…regardless you might be wondering if you can trust that old fuel. 

You should avoid putting old gas in your car. It will cause poor engine performance but you can also expect the possibility of engine damage, your fuel system gumming up, and fuel system corrosion all of which will be much more costly than just buying new gasoline. 

Now old gasoline isn’t one of the the worst things you can put in your gas tank but let’s take a closer look at what can happenswhen it finds its way into your car.

1. Poor Engine Performance

Gasoline is designed with a specific octane rating. You know those mysterious numbers to choose from at the fuel pump…

These numbers are the measurement of the fuel’s ability to withstand compression in an engine without detonating. In other words, if the octane is lower than what your car needs it will fire when it’s not supposed to. 

Gasoline loses its octane over time due to evaporation and chemical degradation. Even when gasoline sits in a storage container or a vehicle’s fuel tank the more volatile components in the gasoline can evaporate. This leaves behind a lower-octane mixture. Chemical reactions also occur between the gasoline and air, causing the octane level to decrease. 

This means that as gasoline gets older, it combusts incorrectly in your engine. Incorrect combustion can result in reduced engine performance and efficiency. That’s why using fresh gasoline is important for keeping your engine running smoothly.

2. Engine Damage

Not only does aged gasoline cause poor performance from your engine it can also lead to more serious issues like damage to your engine. A lower octane gas due to age can cause the fuel to incorrectly ignite and cause something called “knocking” or “pinging”

This knocking and pinging is again from the fuel combusting incorrectly and can damage engine components like pistons, valves, and spark plugs.

The accumulation of carbon deposits from incomplete combustion can further harm engine parts and reduce performance. Using old or degraded gasoline can lead to costly engine repairs and reduce the overall lifespan of your engine. It’s important to use fresh and high-quality fuel to ensure your engine’s reliability.

This video gives a detailed explanation of what “Knocking” is. 

3. Fuel System Gumming Up

Old fuel can clog and gum up your fuel system from chemical changes as it breaks down over time. When gasoline sits in a storage container or your car’s fuel tank for an extended period, it can develop sediments and varnishes causing the fuel system to gum up. These substances can stick to the walls of the fuel tank, fuel lines, and various components of the fuel system like fuel injectors or carburetors. 

This build-up of gummy deposits restricts the smooth flow of gasoline into the engine. Here’s how it can cause problems:

Clogged Fuel Filters

Sediments and gummy residues from old fuel can clog the fuel filter. When the filter gets clogged, it limits the amount of clean fuel reaching the engine, which can affect engine performance.

Fuel Injector Issues

If you have a fuel-injected engine, the gummy deposits can clog the tiny nozzles of the fuel injectors. This restricts the precise spray of fuel needed for combustion, leading to engine misfires and reduced power.

Carburetor Problems 

In older cars with carburetors, old fuel can easily clog the tiny passages inside a carburetor, affecting the air-fuel mixture. This, in turn, messes with engine performance and may result in rough idling or stalling.

Hesitation and Poor Acceleration

The gummy deposits can cause hesitation and sluggish acceleration as the engine doesn’t receive a consistent and clean flow of fuel.

4. Fuel System Corrosion

Gasoline can cause corrosion and rust in fuel systems primarily due to the presence of water and oxygen. Here’s how it happens:

Moisture Absorption 

Gasoline, especially when it contains ethanol (a type of alcohol commonly blended with gasoline), can absorb moisture from the surrounding environment, including humidity in the air. Ethanol is hygroscopic, meaning it readily attracts and mixes with water.

Water Separation

When gasoline, particularly ethanol-blended gasoline, absorbs too much water, it can reach a point where the water and ethanol separate from the gasoline. This is known as phase separation. The separated water and ethanol mixture is heavier than gasoline and tends to settle at the bottom of the fuel tank.

Corrosive Properties

The separated ethanol-water mixture is corrosive. It can corrode metal components within the fuel system, including the fuel tank, fuel lines, and metal parts of the fuel pump. This corrosion weakens things and may lead to premature failure of components

Rust Formation

In addition to corrosion, water in the fuel system can cause rust to form on metal surfaces. Rust occurs when iron and steel components come into contact with both oxygen and water. The combination of water and oxygen triggers a chemical reaction that leads to the formation of iron oxide, or rust.

Damage and Contamination

Corrosion and rust can weaken and damage fuel system components, leading to issues like fuel leaks, clogged fuel filters, and poor performance. In addition, the presence of corrosion and rust can contaminate the fuel, further contributing to engine problems and decreased fuel system efficiency.

5. Heavy Emissions

Whether you’re environmentally conscious or just wondering if old fuel is okay for your car, a byproduct of all the issues up to this point is higher emissions. From incomplete combustion to the effects of accumulated carbon deposits, these issues are bad not only for your vehicle but could even be a legal issue depending on the emissions regulations in your area

Let’s go into a little more detail on why old gas can lead to increased emissions.

Incomplete Combustion 

As gasoline ages, its chemical composition changes, which can lead to incomplete combustion in the engine. Incomplete combustion means that not all of the fuel is burned efficiently, and this can result in the production of higher levels of harmful emissions, such as carbon monoxide (CO) and unburned hydrocarbons (HC).

Increased Levels of Carbon Deposits

Old gasoline can leave carbon deposits in the engine and exhaust system, including the catalytic converter. These carbon deposits can interfere with the proper functioning of the catalytic converter, which is responsible for reducing emissions of pollutants like nitrogen oxides (NOx) and converting them into less harmful compounds.

When the catalytic converter isn’t working as it should, emissions of NOx and other pollutants can increase.

Lower Octane Ratings 

Over time, the octane rating of gasoline can decrease, making it more prone to engine knocking. To prevent knocking, the engine control system might adjust by using a richer air-fuel mixture (Using more gasoline), which can result in higher emissions of pollutants.

5 Ways To Tell If Gasoline Is Too Old

Detecting whether gasoline has gone bad or become too old can be crucial in preventing potential problems in your vehicle. While gasoline does not come with a built-in expiration date, there are several signs that can help you determine if it’s no longer suitable for use:

1. Smell Test: 

Fresh gasoline has a recognizable smell you’re most likely familiar with. If gasoline has gone bad, it might develop a sour or varnish-like odor. If you detect a strong, unpleasant smell, it’s an indicator that the gasoline may be too old. Anytime fuel work is being done on a classic car in the shop it’s very noticeable if the fuel has been sitting for some time just by the smell.

2. Appearance: 

Old gasoline can appear discolored (usually a brownish color) or cloudy. Fresh gasoline is clear and bright. If you notice that the gasoline has taken on a cloudy or off-color appearance, it’s a sign that it might be past its prime.

3. Water Separation:

Gasoline can absorb water from the environment over time. When this happens, the water can separate from the gasoline and accumulate at the bottom of the container or fuel tank. You can check for water separation by looking for a distinct layer of water at the bottom.

A lot of times in the field if you have doubts about the fuel you simply take a sample and let it sit for a few minutes. It tends to separate quickly.

Here is a video that explains and shows testing for water in a gasoline sample:

4. Sediments or Particles: 

Inspect the gasoline for the presence of sediments or particles. Old gasoline can develop contaminants, which can settle at the bottom or float in the fuel. Sediments or particles are a clear indication of degraded fuel.

Some gas cans have a built-in filter to catch things like this.

5. Storage Duration: 

Consider how long the gasoline has been stored. Gasoline begins to degrade after a few months, with the rate accelerating in the presence of heat, oxygen, and moisture.

I Used Old Gas…Now What?

Depending on how much old gas you put into a tank with known good gas will determine how your car reacts. You most likely won’t be able to tell a difference if only a little bit is mixed. If you’re filling your tank up from empty with old gas then you might be asking for problems. It’s best just to avoid using it at all if you can.

Even cars that sit for extended periods of time experience all these issues. Many times in the shop we will have to drain classic cars of their fuel tank and put in fresh gasoline to get the vehicle running better.

Closing Thoughts

Old gasoline, whether forgotten in storage or sitting in our vehicles’ tanks, can have some consequences. It can impair engine performance, lead to increased emissions, and even cause fuel system issues.

The quality and age of gasoline matter more than we realize. The key takeaway from this is that you should prioritize fresh, high-quality fuel so that your vehicle runs smoothly, efficiently, and with reduced environmental impact.

By using gasoline that meets the manufacturer’s recommendations and by regularly maintaining the fuel system, you can help reduce the negative effects of old gasoline.

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