Mixing E85 with 87 octane gasoline in a non-Flex-Fuel vehicle can cause significant engine issues and is generally not recommended.
There’s a lot of chatter at the pumps these days about different fuel types. You might see E85 next to the regular 87 octane and wonder if you can just mix them to save a few bucks or for some perceived performance boost.
Let’s clear up the confusion and talk about what’s really happening under your hood with these fuels.
Understanding Your Fuel Options: E85 and Regular 87
First, let’s break down what these fuels actually are. It’s not just about the octane number you see on the pump.
Regular 87 octane gasoline, the kind most cars use, typically contains up to 10% ethanol. This blend is known as E10.
Ethanol is an alcohol derived from corn or other plant materials. It acts as an oxygenate, helping fuel burn cleaner.
E85 is a high-ethanol blend. It’s usually 70-85% ethanol and 15-30% gasoline. This is a very different fuel composition.
The “85” in E85 refers to the maximum ethanol content, though it can vary seasonally.
Octane rating measures a fuel’s resistance to knocking or pre-ignition. E85 typically has an octane rating around 100-105, much higher than 87 octane gas.
Can You Mix E85 And 87 Gas? The Engine’s Adaptation
The short answer is: it depends entirely on your vehicle. Most cars on the road are designed to run on E10 or less ethanol.
These vehicles are often called “non-Flex-Fuel Vehicles” (non-FFVs). Their fuel systems and engine computers are calibrated for traditional gasoline blends.
Flex-Fuel Vehicles (FFVs) are specifically engineered to run on any blend of gasoline and ethanol, from pure gasoline (E0) all the way up to E85.
You can usually identify an FFV by a yellow gas cap, a “Flex-Fuel” badge on the vehicle, or a sticker inside the fuel door.
FFVs have special components built to handle the corrosive nature of high-ethanol fuels. They also have sophisticated engine management systems.
These systems include a fuel composition sensor. This sensor detects the ethanol content in the fuel tank.
The vehicle’s Engine Control Unit (ECU) then adjusts fuel delivery and ignition timing accordingly.
A non-FFV lacks these specialized components and the ability to adapt to such a wide range of ethanol content.
Mixing E85 and 87 gas in a non-FFV is like asking a sprinter to run a marathon in deep snow; they’re built for different conditions.
What Happens When Non-Flex-Fuel Vehicles Get E85?
Putting E85, or even a significant mix, into a non-FFV can lead to several problems. Your engine relies on a precise air-to-fuel ratio for efficient combustion.
Ethanol has less energy per gallon than gasoline. This means the engine needs to inject more E85 to produce the same power as gasoline.
A non-FFV’s ECU cannot compensate for this difference. It will try to adjust fuel delivery, but its range of adjustment is limited.
This results in a “lean” condition, where there isn’t enough fuel for the amount of air. The engine runs hotter and less efficiently.
Symptoms of a lean condition include:
- Reduced engine power and acceleration.
- Rough idling or stalling.
- The “Check Engine Light” illuminating on your dashboard.
- Potential diagnostic trouble codes (DTCs) like P0171 or P0174 (System Too Lean).
Beyond performance, E85 can cause physical damage to fuel system components in non-FFVs.
Ethanol is a solvent. It can corrode or degrade certain plastics, rubbers, and metals not designed for high ethanol exposure.
This includes fuel lines, fuel pumps, fuel injectors, and seals. Over time, these components can fail, leading to leaks or blockages.
The fuel pump might work harder, leading to premature wear. Fuel injectors can become clogged or damaged.
Cold starting can also become an issue. E85 does not vaporize as easily as gasoline in cold temperatures, making ignition difficult.
Finally, you’ll likely experience a noticeable drop in fuel economy. Because E85 has lower energy content, your vehicle will consume more of it to travel the same distance.
| Characteristic | Regular 87 (E10) | E85 (High Ethanol) |
|---|---|---|
| Ethanol Content | Up to 10% | 70-85% |
| Energy Content | Higher | Lower |
| Corrosiveness | Low | High |
How Your Vehicle’s Fuel System and ECU Respond
Your vehicle’s fuel system is a complex network. It includes the fuel tank, fuel pump, fuel lines, fuel filter, and fuel injectors.
In a non-FFV, these parts are manufactured using materials resistant to the small amount of ethanol in E10.
However, they are not designed to withstand the prolonged exposure to the higher ethanol concentrations found in E85.
The ECU, or Engine Control Unit, is the brain of your engine. It constantly monitors various sensors, including oxygen sensors in the exhaust.
These sensors tell the ECU if the engine is running rich (too much fuel) or lean (too little fuel).
The ECU then makes small, rapid adjustments to fuel delivery, known as “fuel trims,” to maintain the ideal air-fuel ratio.
FFVs have a much wider range of fuel trim adjustment. Their ECUs can significantly increase fuel delivery when they detect high ethanol content.
Non-FFVs have a narrow fuel trim range. When faced with E85, the ECU will try to compensate, but it will quickly hit its limits.
Once the ECU can no longer adjust enough, it triggers the Check Engine Light. This is the car’s way of telling you something is wrong.
Continued operation under these conditions can lead to carbon buildup, catalytic converter damage, and other expensive repairs.
Accidental Mixing: Steps to Take
Mistakes happen at the pump. If you accidentally put E85 into a non-FFV, your course of action depends on how much you added.
For a very small amount, say a gallon or two in a nearly full tank of regular gas, you might be okay. The ethanol concentration will still be low.
Your best bet is to immediately top off the tank with premium 91 or 93 octane gasoline. This helps dilute the ethanol further.
Drive gently, avoiding heavy acceleration. Monitor your engine for any unusual noises or the Check Engine Light.
If you put a significant amount of E85, or filled the tank completely, do not start the engine.
Starting the engine circulates the incorrect fuel throughout the system, increasing the risk of damage.
You will need to have the fuel tank drained by a qualified mechanic. This is the safest way to prevent costly repairs.
Some vehicles have a drain plug on the fuel tank, while others require removing the fuel pump assembly to drain.
After draining, refill with the correct 87 octane gasoline. It’s a hassle, but far less expensive than replacing damaged fuel system components.
Always double-check the fuel door label before pumping. This small habit can save you a lot of trouble and expense.
| Amount of E85 Added | Recommended Action |
|---|---|
| Small (1-2 gallons) | Top off with premium 91/93 octane, drive gently, monitor CEL. |
| Significant (Half tank or more) | Do not start engine, have tank drained by mechanic. |
Protecting Your Engine: Fueling Best Practices
The simplest way to protect your engine and fuel system is to always use the fuel type specified by your vehicle’s manufacturer.
This information is found in your owner’s manual and usually on a sticker inside your fuel filler door.
Using the correct fuel ensures your engine runs as intended, maintains its efficiency, and avoids premature wear on components.
While E85 might seem cheaper per gallon, the lower fuel economy in an FFV often negates the cost savings. For a non-FFV, the potential repair costs far outweigh any perceived savings.
Think of your vehicle’s engine as a finely tuned instrument. It performs best when given the fuel it was designed to consume.
Sticking to the recommended fuel type keeps your ride running smooth and reliable for years to come.
It’s about understanding your vehicle’s needs and respecting its engineering.
Can You Mix E85 And 87 Gas? — FAQs
What is a Flex-Fuel Vehicle (FFV)?
A Flex-Fuel Vehicle (FFV) is designed to run on any blend of gasoline and ethanol, from pure gasoline to E85. These vehicles have specialized fuel system components and an engine computer that can detect and adapt to varying ethanol concentrations. Look for a yellow gas cap or a “Flex-Fuel” badge.
Can E85 damage my non-Flex-Fuel vehicle’s engine?
Yes, E85 can cause significant damage to a non-Flex-Fuel vehicle. Its high ethanol content can corrode rubber and plastic fuel lines, seals, and other components not designed for it. It also causes the engine to run lean, leading to reduced performance, Check Engine Lights, and potential long-term engine damage.
What happens if I accidentally put a small amount of E85 in my regular gas tank?
If you’ve only added a small amount (like a gallon or two) of E85 to a nearly full tank of regular 87 octane, your vehicle might be able to handle it. Immediately top off the tank with premium 91 or 93 octane gasoline to further dilute the ethanol. Drive gently and monitor for any engine warning lights or unusual performance.
Will mixing E85 and 87 octane improve my car’s performance?
No, mixing E85 and 87 octane in a non-Flex-Fuel vehicle will not improve performance; it will likely harm it. The engine will struggle with the incorrect fuel mixture, leading to reduced power, rough idling, and potential engine damage. Only FFVs can properly utilize E85 for its higher octane potential, and even then, performance gains vary.
Where can I find out if my car is a Flex-Fuel Vehicle?
The easiest ways to determine if your car is an FFV are to check for a yellow fuel filler cap, a “Flex-Fuel” badge on the vehicle’s exterior, or a sticker inside the fuel filler door. You can also consult your vehicle’s owner’s manual or check the manufacturer’s website using your VIN (Vehicle Identification Number).
Certification: BSc in Mechanical Engineering
Education: Mechanical engineer
Lives In: 539 W Commerce St, Dallas, TX 75208, USA
Md Amir is an auto mechanic student and writer with over half a decade of experience in the automotive field. He has worked with top automotive brands such as Lexus, Quantum, and also owns two automotive blogs autocarneed.com and taxiwiz.com.