Can E15 Gas Damage Your Engine?

While gas stations are required to post warning labels announcing the fuel's ethanol percentage, you may or may not have noticed the signage, especially if it's missing or obscured. For those not in the know, E15 gas is typically your unleaded 88, containing up to 15% alcohol, which is slightly higher compared to regular 87 pump gas that contains up to 10% ethanol – check out our breakdown if you're wondering why gasoline has ethanol and how it affects your engine.

Whether E15 damages your engine largely depends on the age and model year of the vehicle. According to the EPA, E15 gas is approved for use in passenger vehicles (model year 2001 and later), as well as in cars that are flex-fuel capable. As you can tell, pre-2001 cars and trucks are not necessarily E15 compliant. Filling those vehicles with E15 can bring about numerous problems, including fueling issues, corrosion, improper combustion, high cylinder temperatures, and damaged rubber parts. You may even notice a loss in power, in addition to starting issues and clogged fuel filters.

It's also worth pointing out that, per the EPA, E15 is prohibited from being used in lawn mowers, motorcycles, chainsaws, and boats. Additionally, heavy-duty engines used in delivery trucks and school buses are also prohibited from using E15. But why, you ask? Cars and trucks that are E15-compliant have upgraded components and other hardware tweaks. But more importantly, the sensors and computers inside them are programmed to adjust the fuel parameters and ignition timing to compensate for the ethanol blend's high oxygen levels.

Filling up with E15 won't ruin your car overnight. Let's just say it's not as bad as putting diesel in a gas car. Ethanol, being a form of alcohol, burns at a higher temperature than gasoline. It's not uncommon for old carbureted engines to run lean after after using E15, especially in the case of engines that aren't computer heavy. This, as you may know, can cause the engine to experience pre-ignition or detonation. Additionally, per a research paper published by the National Renewable Energy Laboratory, older engines that don't come equipped with hardened valve seats saw extended wear after running on E15.

Ethanol is more corrosive than gasoline, so fuel hoses, seals, and gaskets of incompatible engines won't like using E15 either. Perhaps the biggest concern with ethanol is its affinity to absorb water. Not only does ethanol's hygroscopic nature cause corrosion, but it can also lead to other issues, especially during storage. Understand that ethanol can literally pull water vapor from the surrounding air, leading to phase separation, which means gasoline floats above water inside the fuel tank.

Something else to consider is vapor lock. Since ethanol is more volatile than gasoline, these evaporated molecules (usually from excess heat) can block the fuel system from sending the required amount of fuel to the engine. Furthermore, the fuel's octane levels are reduced as ethanol evaporates — gas can go bad if it sits in your car for long enough, and it can also lose so much octane that the engine may find it difficult to even start. In short, if the owner's manual says E15 is fine, then it's okay to use, but don't experiment.