If you have accidentally put diesel in a gas tank, it's natural to panic. Most people would be horrified, thinking they have done something majorly wrong. True enough, this is bad but nowhere near as bad as it could be. Now, if you had put gas into your diesel car, then you would have a reason to get upset. It sounds odd to accidentally put diesel into your gas car, the diesel fuel nozzle at a gasoline pump is not designed to fit into an unleaded gas tank. However, people do store gasoline in jerry cans and sometimes don't label them or mislabel them, making this a more plausible mix-up.
If you know that you have accidentally put the wrong fuel in the car, then don't compound the mistake by driving the vehicle while trying to think about what to do. Instead, call your mechanic immediately. At this point, the damage isn't permanent. If you drive and continue to drive, at one point the vehicle will stop and you will still have to have it towed.
There are two things you can do to try to fix the damage:
1. Try to clean up the damage yourself – have the vehicle towed home if it's not there already, and get as much of the diesel fuel out. How? By siphon if you can or any other method that you have available. Once you have as much of the diesel out as you can, fill the tank with the correct unleaded fuel that you normally use. Try to start the vehicle. It could take several attempts but it will start and it will smoke – a lot. It will continue to smoke until the last dregs of the diesel make its way through the system. It might run a little rough until everything is cleaned out. How it runs will depend on the proportion of diesel fuel relative to gas left in the tank.
2. Alternatively, have your vehicle towed to where your mechanic is and have the fuel tank drained of all the diesel fuel by a profession. They are also likely to flush out the whole fuel system. If the diesel stays in there for too long (and who knows how long is too long for your particular vehicle), it could ruin the fuel pump, lines and injectors. It is also possible that the diesel could have gummed up the spark plugs and filters. If this is the case, they will probably need to be replaced. Getting a professional to do the job will cost you anywhere from $100 to $500 and possibly a little higher depending on the extent of the damage.