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What Happens If Your Put the Wrong Fuel in Your Car

By Larry Carley Copyright AA1Car.com

When you pull up to a fuel pump in a service station today, there may be a variety of different types of fuels from which to choose. If you put the WRONG fuel in your car, you could be in for real trouble!

color coded fuel nozzles
Fuel nozzles may be color coded to reduce the risk of fuel mix-ups.

Different Grades and Types of Fuel

Unleaded Gasoline is usually available in three grades: Regular 87 octane, Midrange 89 octane, and Premium 91 to 93 octane. Any of these unleaded grades may be used in any vehicle with a gasoline-powered engine. However, engines with higher compression ratios, turbochargers or superchargers generally require higher octane Premium fuel. If the filler cap or flap on your vehicle has a label that says "Premium Recommended" or "Premium Required" do what it says. Using a lower octane fuel will reduce performance and may increase the risk of engine-damaging detonation when the engine is accelerating rapidly or is under a heavy load (as when towing or carrying extra weight).

Ethanol Alcohol is added to many grades of gasoline. Most of the Unleaded Gasoline in the U.S. contains up to 10 percent ethanol alcohol(E10), and up to 15 percent (E15) in some states. Ethanol is an octane-boosting additive and it helps to reduce air pollution by adding oxygen to the combustion process. It also helps American farmers, too!

E10 Gasoline can be safely used in all 1995 and newer vehicles, while E15 is approved in the U.S. for use in ALL 2001 and newer passenger cars and light trucks, and in any Flex-Fuel gasoline-powered vehicle. Some rubbers and plastics in older vehicles (pre-2001) are not alcohol-resistant and may be damaged if gasoline containing too much alcohol is used in their fuel system.

E85 is a mixture of up to 85 percent ethanol alcohol and at least 15 percent gasoline. The exact mixture of E85 can vary somewhat based on current fuel costs and refinery output. E85 should NOT be used in a vehicle that is not a FLEX FUEL capable. It should only be used in FLEX FUEL vehicles that can handle any mixture of gasoline and ethanol alcohol (straight gasoline, E10, E15 or E85).

Diesel Fuel is for use in diesel engines only. Diesel fuel is actually a light oil that is designed to self-ignite inside a high compression diesel engine. It will not work in a gasoline engine. There are also different grades of diesel fuel: Number 2 summer grade diesel, and thinner (lower viscosity) #1 winter grade diesel. Number 1 diesel is recommended for colder winter temperatures to reduce the risk of fuel gelling and freeze-ups.

Biodiesel is another type of diesel fuel that contains a blend of oils derived from renewable plant sources (usually soybeans). The most common blends include B2 (2% biodiesel), B5 (95% biodiesel) and B20 (20% biodiesel). B2 and B5 can be safely used in most automotive and light truck diesel engines. But B20 may or may not be approved for your diesel engine. If you don't know, check your owners manual or your vehicle manufacturer to see if they approve B20 for your engine.

What Happens If You Use The Wrong Fuel?

To reduce fuel mix-ups, the fuel nozzles on dispenser pumps are labeled and often color coded so people don't fill up with the wrong fuel. The diameter of the tube on a diesel nozzle is also larger than those on gasoline and E15/E85 nozzles so it cannot be accidentally inserted into the filler pipe on a gasoline-powered vehicle. A restrictor ring inside the filler neck will only accept a gasoline-sized nozzle (unless somebody has punched it out to enlarger the opening!). Even so, with so many distractions today (including those annoying pumps with TV screens), mix-ups sometimes happen.

Although it does not happen very often, fuel delivery truck drivers have been known to accidentally put the wrong fuel in the underground tanks at a gas station (diesel into a gasoline tank, or gasoline into a diesel tank). If this kind of mixup happens, you will likely notice your vehicle is not running well immediately after filling it up. You should contact the gas station right away and let them know you suspect something may be wrong with their fuel. Do NOT drive your vehicle. Have it towed to a service facility so the fuel tank can be drained and the fuel system flushed out to remove the wrong fuel. The gas station that sold you the bad fuel should be held responsible for the cost of repairs.

If you realize YOU accidentally put the WRONG fuel into your vehicle, what happens next depends on what type of fuel you put in your tank.

Diesel Fuel into a Gasoline Tank

Because of the different nozzle sizes, you really have to try hard to make this kind of mix-up. If you only added a couple of gallons to your tank before realizing your mistake, and the tank was at least half full, your engine will run but not very well until the fuel is used up. It likely won't hurt anything. But if you filled the entire tank with diesel, you probably won't get very far before your engine quits. The best advice is don't start the engine. Have your vehicle towed to a service facility. The fuel tank will have to be drained and the fuel system flushed out to get rid of the contamination.

If you did not start your engine after filling the tank, or only drove your vehicle a short distance, it is unlikely the diesel fuel will have damaged the fuel pump or injectors. There should be no need to replace any of these expensive parts. Only the fuel filter might need replacing.

Gasoline or E85 in a Diesel Truck or Car

DO NOT start your engine or attempt to drive your vehicle!

This kind of mixup is fairly common because gasoline pump nozzles are smaller in diameter than diesel pump nozzles, so they will fit into the fuel tank filler tube on a diesel-powered vehicle. If you mistakenly grab the wrong pump nozzle when filling up, or you don't know the vehicle you are driving has a diesel engine, you will contaminate the fuel tank in your vehicle with gasoline.

Trying to run a diesel engine on gasoline or E85 can damage the high pressure diesel injection pump and fuel injectors (which are lubricated by the oils in diesel fuel but are lacking in gasoline). In other words, diesel fuel is a lubricant while gasoline is a solvent. Eliminating the lubricant qualities of the fuel will result in internal wear and damage in the high pressure fuel pump.

This kind of fuel mix-up can also damage pistons and rods because of the different rates at which gasoline and diesel fuel burn (gas burns too fast and builds up pressure too quickly in a diesel engine).

Gasoline in a diesel engine will also dramatically increase the amount of soot in the exhaust, which can overload and clog the diesel particulate trap in the exhaust system. This can plug up the exhaust, creating a severe restriction that may cause the engine to stall after driving some distance.

If you accidentally put gasoline into a diesel-powered vehicle, DO NOT start the engine and DO NOT drive away. Call a tow truck and have your vehicle towed to a service facility so the fuel tank can be drained and the fuel system flushed to get rid of the contamination.

If you did not realize your mistake, started the engine and drove for some distance, at the very least, you will have to flush the entire fuel system and replace fuel filter/water separator. And if you drove far enough to damage the components in your fuel system, you may also have to replace the expensive higher pressure fuel pump, and maybe even the injectors and/or diesel particulate trap if it is clogged.

E85 in a vehicle that is not FLEX FUEL capable

What happens next when you have made this kind of fuel mix-up depends on how much E85 you added to gasoline that was already in your tank. A couple of gallons of E85 mixed with half a tank of gasoline should not make a lot of difference and probably won't harm any rubbers or plastics in a 2001 or newer vehicle. However, your engine will run somewhat leaner than normal which may cause a loss of performance and misfires. If you filled your entire tank with E85 and you don't have a FLEX FUEL vehicle, the best advice is don't drive your vehicle. Have it towed to a service facility so the tank can be drained. If you attempt to drive a non-FLEX FUEL vehicle on E85, it will run poorly and misfire because the fuel system is not calibrated to handle the additional alcohol (which requires a richer fuel mixture and changes in spark timing).

Other Contaminants Accidentally (or Intentionally) Put In Your Fuel Tank

This kind of fuel contamination includes motor oil, automatic transmission fluid (ATF), antifreeze, power steering fluid, brake fluid, water, vodka, beer, urine, sugar, dirt, rocks, you-name-it. If the substance is not a combustible motor fuel that your engine and fuel system are designed to handle, it does not belong in your tank. Period.

A small amount of a combustible liquid such as motor oil, ATF or even vodka will mix with the fuel in your tank. The amount of dilution will depend on how much fuel was already in your tank and how much other stuff was dumped into it. A pint or quart of a combustible liquid added a tank that is already half or more full probably won't cause much harm and will eventually burn out as the contaminated fuel is used up. It would be more of a concern in a diesel engine that might suffer injection pump or fuel injector damage if the fuel were diluted too much with something other than diesel fuel.

If a noncombustible liquid were added to your fuel tank (antifreeze, water or something else), it would probably not mix with the fuel and would settle to the bottom of the tank. When drawn into the fuel system, it would cause misfiring or stalling. Water also corrodes steel tanks and metal components in the fuel system. Diesel fuel systems have fuel filters/separators that can trap moisture, but gasoline fuel systems do not. Depending on the amount of contamination, the fuel tank may have to be drained to get rid of the junk in the fuel.

Water is a common contaminate of both gasoline and diesel fuels. Water can seep into leaky underground fuel storage tanks in filling stations, and cause engine performance problems and misfiring.

If sugar, dirt or other solid material is dumped into a fuel tank, it can gum up the fuel filter and damage the fuel pump. Draining the fuel tank may not remove all of the contamination, so it may be necessary to remove the fuel tank from the vehicle so it can be steam cleaned and flushed out.

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Bad Gas Update

Gasoline Fuel Treatments

Fuel Octane Ratings & Recommendations

E85 Ethanol Alcohol Fuel

What is E15?

Diesel Fuel Treatments


Gas Cap Loose?

Troubleshoot & Cleaning Fuel Injectors

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