What Happens If You Use the Wrong Gas

  • By Morgan Gertler
  • 04 Oct, 2017
Today’s gas stations offer a variety of options, from unleaded fuel with a range of octane ratings to diesel fuel. Sometimes higher-octane choices have names such as Super, Plus, Super Plus, Premium, or Super Premium. With so many different types of gas to choose from, you might wonder what happens if you select the wrong one. What happens if you use the wrong gas depends on what kind of engine you have and which specific fuel you use.

Octane Ratings

Your owner’s manual will tell you the minimum octane rating that your car requires. For most run of the mill modern cars, this will be somewhere in the 85 to 87 octane range, or Regular unleaded gas. If you pump a tank of higher-octane fuel, nothing will happen. Your car’s timing might change slightly, but it will be imperceptible under normal driving conditions. Take a moment to bemoan paying extra for gas if you like, and then move on with your day.

If you have a high-performance sports car that requires higher-octane fuel (typically around 91 octane), you may notice a slight dip in performance and gas mileage. You won’t hurt the car, but it may feel a bit sluggish. If the car is modern, its computer will adjust the engine timing and fuel flow to compensate, and you probably won’t notice a lot of difference under normal driving conditions. If it’s a vintage car, you may hear an annoying knock or ping coming from the carburetor. Either way, be sure to switch back to the proper octane fuel for your next fill-up.

E85 fuel consists of 85% ethanol and 15% gasoline, and should only be used in flex-fuel vehicles. However, if you accidentally fill the tank of a non-flex-fuel vehicle with it, it’s not the end of the world. Your check engine light may come on, and your fuel mileage will drop, but the next tank of regular unleaded gas should clear it. Be careful not to make this a habit, though, because over time, the ethanol will corrode the fuel system of a vehicle not designed for it.

Diesel in a Gasoline Engine

While you can generally get away with making a mistake in octane rating, putting the wrong type of fuel into your vehicle’s engine is a much more serious matter. Diesel in a gas engine can make the car pour smoke and quickly sputter to a stop. With a compression ratio of just 9:1 or 10:1, a gasoline engine cannot ignite diesel fuel, grinding the engine to a halt.

If you realize your mistake before you turn the car on, immediately have it towed to a trusted auto mechanic, who will drain and clean the fuel tank. If you have already started the car, and especially if you have driven any distance at all, the entire fuel system may need to be cleaned or even replaced. Have the vehicle serviced immediately, and let your auto technician know what happened.

Gasoline in a Diesel Engine

Gasoline in a diesel engine can be even worse. Because the compression ratio is so much higher on a diesel engine (between 18:1 and 23:1), and because gas boils at just 105 to 410 F, compared to 500 to 650 F for diesel, driving a diesel vehicle with gasoline in it can be catastrophic.

If you notice your mistake before starting the car, call for a tow immediately. At this point, you just need to have the tank drained, cleaned, and refilled. If you have already started driving, pull over and turn the car off. At best, you are risking total failure of the injectors and injection pump. At worst, the extreme pressure can cause spectacular engine failure, breaking pistons, destroying rods, and even potentially blowing off the entire engine head. Call for a tow and let your auto technician know what happened.

Preventing These Issues

Vehicle manufacturers and fuel pump designers try very hard to make sure that no one fills their vehicles with the wrong type of fuel. Color-coding, differently sized fuel nozzles, and other visual cues can help you ensure that you are properly fueling your vehicle. However, no external system can fully overcome operator error. Every time you pull into a gas station, especially one that is unfamiliar to you, pay attention. Visually note the locations of the gas pumps and diesel pumps, and make sure you pull up to the right one. Before dispensing fuel, do another quick visual check to confirm both the type and octane rating of your selected fuel. Take another quick glance before starting your vehicle. Although it may seem like overkill, a couple of confirming visual checks can save you thousands of dollars in repair bills.
If you are tired of getting the runaround and being treated like you haven’t got a clue, we invite you to try Colchin Automotive instead. Give us a call at 303-431-5421 for all of your automotive needs!

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