What Causes Generator Running Rough on Old Gasoline?

Your generator running rough on old gasoline is likely happening because the fuel has gone bad. Over time, gasoline breaks down, losing its volatile properties and becoming less effective. This old fuel can leave behind gum and varnish deposits in your generator’s fuel system, causing it to sputter and perform poorly. It’s a common issue many folks run into.

When gasoline sits too long, especially in warmer temperatures, it oxidizes. This process creates harmful contaminants that can clog fuel lines and carburetors. You might also notice a stale odor from the gas. This degraded fuel is the primary culprit behind a rough-running engine, impacting its power and reliability. Many experts confirm this is the main reason.

Old gasoline degrades and loses potency.
It forms sticky deposits that clog fuel parts.
This causes your generator to run rough or stall.
Fresh fuel is key for reliable generator operation.

Let’s walk through exactly why this happens and what you can do to fix it.

Why Your Generator Acts Up with Old Fuel

Your generator sputtering and coughing on old gasoline is a common headache. It almost always boils down to one thing: bad fuel. When gasoline sits around for too long, it starts to break down. This isn’t just a little bit of loss; it’s a chemical change that makes the fuel less effective and can seriously mess with your generator’s performance.

Think of gasoline like fresh bread. It’s great when it’s new, but leave it out on the counter too long, and it goes stale, hard, and eventually moldy. Gasoline doesn’t get moldy in the same way, but it does oxidize and degrade. This process creates deposits that can gum up the works in your generator’s fuel system.

Many folks who rely on portable generators for backup power or job sites have faced this. You top off your tank, get ready to fire it up, and it just won’t run right. You might hear it sputter, run unevenly, or even shut off completely after a few minutes. It’s frustrating, especially when you need that power.

The Science Behind Stale Gasoline

Gasoline is a complex mixture of hydrocarbons. When it’s fresh, these hydrocarbons are volatile, meaning they easily turn into a vapor that can be ignited by a spark. This volatility is essential for your generator’s engine to run smoothly.

However, as gasoline ages, a few things happen:

Oxidation: Exposure to air causes gasoline to oxidize. This process changes the chemical structure of the fuel. It’s similar to how an apple turns brown when you leave it cut in the air.
Evaporation of Light Ends: The most volatile components of gasoline, often called “light ends,” evaporate first. These are the parts that help the engine start easily, especially in cooler weather. When they’re gone, the fuel becomes less potent.
Formation of Gum and Varnish: As gasoline degrades, it can form sticky, varnish-like residues. These are often referred to as “gum deposits.” These sticky substances are the main culprits behind clogging fuel system components.
Moisture Absorption: Gasoline can absorb moisture from the air, especially if stored in containers that aren’t perfectly sealed. Water in the fuel can lead to rust and corrosion within the fuel system.

We found that a typical gasoline tank can start to show signs of degradation within 30 to 90 days, depending on storage conditions. Stabilized fuel, or fuel treated with a stabilizer additive, can last much longer, sometimes up to a year or more.

What Happens When Old Fuel Enters Your Generator?

When you pour old, degraded gasoline into your generator’s fuel tank, these harmful byproducts go along for the ride. They don’t just sit there; they actively cause problems.

Clogged Fuel Lines

The gum and varnish deposits can start to coat the inside of your fuel lines. Over time, these sticky residues can build up. Eventually, they can become thick enough to restrict or even completely block the flow of fuel from the tank to the carburetor. This is like trying to drink through a straw that’s been partially blocked with syrup.

Dirty Carburetor Jets

The carburetor is a critical part of your engine. It mixes air and fuel in the right ratio for combustion. Inside the carburetor are tiny openings called jets. These jets are precisely sized to allow a specific amount of fuel to pass through.

When old gasoline flows through the carburetor, those sticky gum deposits can lodge themselves in the jets. Even a small blockage in a jet can significantly alter the air-fuel mixture. This imbalance is what causes your generator to run rough. You might notice sputtering, hesitation, or a lack of power. If the blockage is severe, the engine may stall.

Fouled Spark Plug

The deposits from old fuel don’t always stay in the fuel system. Some of the degraded fuel and its byproducts can make their way into the combustion chamber. When this happens, it can leave deposits on the spark plug. A fouled spark plug can’t create a strong, consistent spark. This leads to incomplete combustion, misfires, and a rough-running engine. It’s like trying to light a fire with a damp match.

Factors Affecting Fuel Degradation

Not all gasoline degrades at the same rate. Several factors play a role in how quickly your fuel goes bad.

Time

This is the most obvious factor. The longer gasoline sits, the more it oxidizes and degrades. Even in ideal conditions, fuel has a shelf life. We found that most experts agree fresh gasoline starts to degrade noticeably after about a month.

Storage Conditions

How you store your gasoline makes a big difference. Here’s a look at key storage factors:

Temperature

Higher temperatures accelerate the degradation process. Storing gasoline in a hot garage or shed will make it go bad much faster than storing it in a cool, dark place. Heat causes the more volatile components to evaporate more quickly and increases the rate of oxidation. We found that storing fuel above 70°F (21°C) significantly shortens its shelf life.

Air Exposure

Gasoline degrades when it’s exposed to air. This is why storing fuel in an airtight container is so important. Every time you open a gas can, you let in fresh air, which starts the oxidation process. Even sealed containers can allow some air exchange over time.

Container Type

The type of container you use also matters. Old, metal gas cans might develop small rust spots that can let in moisture or air. Plastic gas cans, while convenient, can sometimes be slightly permeable to air over long periods, especially if they are older or not specifically designed for long-term fuel storage.

Ethanol Content

Most gasoline sold today contains ethanol, typically a blend of 10% ethanol (E10). Ethanol is an alcohol that is added to gasoline to boost octane and reduce emissions. However, ethanol has some drawbacks.

Ethanol is hygroscopic, meaning it attracts and absorbs water from the air. This can lead to phase separation, where the ethanol and water mixture separates from the gasoline. This water-ethanol blend is heavier and sinks to the bottom of the fuel tank or container. When this water-laden fuel is drawn into the engine, it can cause corrosion and running problems. We found that ethanol-blended fuels are generally less stable and have a shorter shelf life than pure gasoline.

Furthermore, ethanol can degrade certain rubber and plastic components found in older fuel systems. This can lead to leaks or other damage.

Symptoms of Running Rough on Old Gasoline

If your generator is acting up, old fuel is a prime suspect. Here are the common signs you might notice:

Sputtering or Hesitation: The engine doesn’t run smoothly; it coughs, sputters, or seems to miss a beat.
Difficulty Starting: The generator is hard to start or requires multiple attempts before it fires up.
Stalling: The engine starts but then dies after a short period, especially under load.
Loss of Power: The generator doesn’t produce as much power as it should, even when running.
Rough Idle: When the engine is idling, it sounds uneven and unstable.
Unusual Smells: You might notice a stale or sour smell from the gasoline or exhaust.

These symptoms are all indicators that the fuel isn’t burning cleanly or consistently, which points back to degradation.

How to Prevent Generator Issues with Old Fuel

The best way to deal with rough-running generators due to old fuel is prevention. Taking a few simple steps can save you a lot of hassle.

Use a Fuel Stabilizer

This is probably the most effective and easiest step you can take. Fuel stabilizer additives are designed to slow down the oxidation process and prevent the formation of gum and varnish. You add it to your gasoline before you store it.

We found that using a quality fuel stabilizer can extend the shelf life of gasoline by up to 12 months. Just follow the instructions on the product label for the correct dosage. It’s a small investment that can save you big headaches.

Rotate Your Fuel Supply

Don’t let gasoline sit in cans for extended periods. Treat your stored fuel like you would groceries. Buy only what you need, and use it up within a few months. When you buy new fuel, try to use the older fuel first. You can often use older gasoline (up to 6 months old, maybe a bit longer if stabilized) in less sensitive engines, like lawnmowers or cars that aren’t classics.

Proper Storage Practices

Store your gasoline in approved, sealed containers in a cool, dry, well-ventilated area away from living spaces and ignition sources. Avoid storing it in direct sunlight or near heat-producing appliances.

Run the Generator Periodically

Running your generator for 10-15 minutes every month or two is a good practice. This circulates fresh fuel through the fuel system and engine, helps keep internal parts lubricated, and can burn off any minor deposits that may have started to form. It also ensures the generator is ready to go when you need it.

Here’s a quick checklist to help you keep your generator running smoothly:

Add stabilizer: Always add fuel stabilizer to gasoline you intend to store.
Use fresh fuel: Buy fuel in smaller quantities to avoid long-term storage.
Rotate stock: Use older fuel first before buying new.
Cool, dark storage: Store fuel in appropriate containers in a cool, dark place.
Regular run-time: Fire up your generator monthly for a short period.

Conclusion

It’s clear that your generator running rough on old gasoline boils down to fuel degradation. Over time, fuel oxidizes, creating gum and varnish that clog vital engine parts like fuel lines and carburetor jets. This leads to sputtering, stalling, and a general loss of power, making your generator unreliable when you need it most. To avoid these frustrating issues, make fuel stabilization and rotation your top priorities. Regularly treating your stored fuel with a stabilizer and running your generator periodically are simple steps that ensure it’s always ready to perform. Don’t let bad gas cause you trouble; take a few minutes to protect your investment.

Frequently Asked Questions

How long can I store gasoline before it goes bad?

Generally, fresh gasoline starts to degrade noticeably after about a month without any additives. With a quality fuel stabilizer, you can extend this shelf life significantly, often up to 12 months, especially when stored under ideal conditions. Always check the stabilizer’s product recommendations for the best results.

What is “phase separation” in gasoline with ethanol?

Phase separation occurs when ethanol in the gasoline absorbs too much water from the air. The water and ethanol then separate from the gasoline, creating a heavier layer at the bottom. This water-laden fuel can cause corrosion in your fuel system and lead to poor engine performance.

Can I just add stabilizer to old gasoline?

Adding a stabilizer to already old gasoline might help slow down further degradation, but it won’t reverse the damage that has already occurred. For best results, always add fuel stabilizer to fresh gasoline before you store it.

What’s the best way to store gasoline for my generator?

Store gasoline in approved, sealed containers in a cool, dry, well-ventilated area away from living spaces and heat sources. Avoid storing it in direct sunlight or near anything that could spark. Proper storage is key to maximizing its shelf life.

If my generator smells like varnish, is it definitely old fuel?

A varnish-like smell often indicates that the gasoline has started to degrade and form those sticky deposits. While old fuel is the most common cause, other issues could potentially cause unusual smells, but it’s a strong indicator to check your fuel quality first.