Can Bad Fuel Cause a Check Engine Light?

Understanding the Issue: Old Gas and Check Engine Lights

Yes, old fuel, particularly when containing ethanol, can indeed cause a check engine light to illuminate. It's important to note, however, that it is not the only cause for this warning light to appear.

Today, most gasoline on the market includes ethanol additives. While this mixture generally remains stable for a few months, issues can arise thereafter. Ethanol's impact on engine performance varies based on the type of engine, with small engines that use carburetors being particularly sensitive to problems with aging ethanol-based fuel.

The Effects of Old Ethanol Fuel

The potential range of problems caused by old ethanol fuel is wide and varied, including:

Check Engine Light: Sensor readings become non-nominal. Hard Starting: Engines may struggle to start or require more effort than usual. Rough Idle: The engine may rumble or jitter when idling. Rough Running: The engine might run sluggishly or unevenly. Poor Acceleration: The engine may struggle to increase speed quickly. Stalling: The engine may stop unexpectedly. Engine Will Not Start or Run: In extreme cases, the engine may refuse to start altogether.

In addition to these performance issues, old ethanol fuel can also affect the fuel system by varnishing carburetors and clogging injectors. These mechanical problems may require more extensive repairs, potentially leading to increased maintenance costs.

Fixing the Problem

Several solutions can address the issues caused by old ethanol fuel. Here are some common methods:

1. Fill with Fresh Fuel

If your tank contains less than a full tank of old fuel and the mixture is less than one year old, filling up with fresh fuel often works to dilute the old fuel. However, this approach carries risks. By adding fresh fuel, you now have a tank that is less than ideal, and the dilution may not be sufficient to resolve the issue.

2. Drain and Replace

Draining the tank and refilling it with fresh fuel is another method. If the carburetors and injectors have not yet been affected by the old fuel, this approach is often sufficient for resolving the problem.

3. Special Non-Ethanol Fuel

Some gas stations offer non-ethanol fuel, which is akin to what was sold in the 1960s. This fuel is stable and can remain so for many years, even decades. Additionally, it has the unique capability to dissolve and clean carburetors, injectors, and fuel lines. Running a full tank of this fuel can sometimes clean varnish from carburetors, remove moisture, and clear clogs. Although it costs more than traditional gasoline, its benefits are significant, and it can be worth the expense for those prone to old fuel issues.

Preventing Future Problems

For engines that are infrequently used or planned to be left unused for more than 90 days, there are preventative measures you can take. These include:

1. Drain Most of the Fuel

Before storage, drain most of the fuel and add at least a quarter tank of non-ethanol fuel. Run the engine for five minutes to ensure the new fuel is thoroughly distributed through the system. This non-ethanol fuel can be left in the tank for several years, and the fuel will remain in good condition. Your engine will start and run perfectly even if the fuel is several years old.

2. Use Fuel Stabilizers

There are additives like Stabil designed to improve fuel stability. While they are effective, many believe that non-ethanol fuel provides superior results.

Note that while most gas stations operate with modern fuel supply chains to prevent old fuel distribution, occasionally a gas station may receive a delivery of stale ethanol fuel, leading to the check engine light activating in your vehicle. In such cases, draining or burning off most of the affected fuel is necessary before attempting the dilution methods described above.