I made a really stupid mistake earlier tonight. I was at a gas station refueling my 2009 Nissan Altima (2.5L 4-cylinder engine). I stupidly selected the only option that was available at the pump which was biodiesel fuel. Before I filled up the car, it had about a quarter tank of unleaded gas left. I filled it up to about 3/4 of a tank with the biodiesel fuel. So now the car has 1/4 unleaded fuel and maybe 1/2 tank of biodiesel fuel. I drove the car for a total of maybe 20 minutes between the time I left the station and when I got home. After 5-10 minutes of driving my car started acting a little weird and thought at first it was something else. Then later I realized I put the wrong type of fuel in the car! Ugh!

There is a gas station just down the street from where I am (a 5 minute drive at the most). I was able to drive around without too much trouble before I came home but it did start acting weird. I read that if you top it off with unleaded fuel then it should "dilute" the diesel fuel. As I keep driving, I should keep topping it off with more unleaded fuel. I still have a little more than a quarter tank to fill the car with right now. Is this even a viable option at this point? I have a AAA membership but they told me that all they will cover is the cost of towing the car to a repair shop and then will probably be paying at least $500 in repairs if not more (from what I've read). What are my options at this point? Thanks!

  • How? Diesel pumps have larger nozzles than gasoline to prevent this. Are you sure it wasn't E85? Commented May 19, 2015 at 2:06
  • Maybe call the station and confirm that it really was biodiesel and not bioethanol (E85). As @Lathejockey81 points out, it should have been difficult to put a diesel filler nozzle in your gas filler. If it was an option at a pump that also pumps regular gas, chances are it was E85. In that case, you may be able to get by with adding more unleaded fuel, but I'd recommend taking it to a shop, getting the old gas drained and put in the right fuel.
    – TMN
    Commented May 19, 2015 at 18:46

2 Answers 2


If it were my car, I'd siphon out the entire contents of my gas tank and fill up with unleaded. The worst thing that could have happened is that your catalytic converter(s) are really dirty. To remedy that, after you've filled up with unleaded, go for a Fast and Furious drive to get the cats good and hot so they burn off all the icky stuff. Don't worry about (lots of) smoke or a rough engine. That's just the last bit of diesel being burned off.

  • Diesel is "higher octane" than petrol. It would actually be harder to ignite it. Commented May 18, 2015 at 10:10
  • 1
    In googling, you are right - I was sure that was what I was told by the AA when I accidentally filled my petrol car with diesel years ago. My mistake.
    – Rory Alsop
    Commented May 18, 2015 at 10:38
  • The other way round is obviously pretty bad though. Commented May 18, 2015 at 10:55
  • No, diesel has a really poor octane rating, and that isn't even measured because it's irrelevant. For diesel, the interesting property is cetane number.
    – juhist
    Commented Sep 14, 2017 at 17:40

What I did to drain crap gas (5 year old ethanol gas) from my car was:

  1. Get a fire extinguisher, set aside in case.
  2. Jack rear of the car up in the air.
  3. Remove gas tank lid, let pressure off.
  4. Under the car, remove the fuel line from between fuel filter and the gas tank.
  5. Put on a longer bit of fuel line with the other end going into an empty gas container big enough to hold the contents of your gas tank. Or have an assistant warn you when it's about to overflow so you can turn off the ignition.
  6. Turn on the motor, let the fuel pump drain the tank. The engine will either not start or it will die immediately- this is normal, don't turn off the ignition until either the tank is empty or the container is about to overflow.
  7. When the tank is empty, reconnect original lines.
  8. Dispose of old gas (I used it to top off the huge tank in my pickup).
  9. Fill container with proper gas for your car.
  10. Fill car from container. Drive car.

It might be safe to drive around with unleaded plus whatever in your tank, but it depends on a lot of things.

For example, it might have been E85 instead of diesel. E85 in a non-flex-fuel car would run super lean which might cause ignition misfire. It also might cause damage due to the extra heat of combustion caused by the leanness. But your car might be compensating for the leanness adequately enough to prevent damage- a lean condition on E85 should have the same effect on your O2 sensor's lambda that a lean condition on gasoline would... the question is whether or not your ecu will allow a big enough correction for e85 and whether you have enough injectors to keep the engine happy at higher rpms.

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