The first thing to recognize is that this scenario isn't possible in most cars. In any car manufactured in the last 25 years or so, there's a plate under the gas cap that prevents anything but the small unleaded-gasoline nozzle from fitting into the tank. When unleaded gasoline first appeared, this plate helped to prevent drivers from putting the leaded gasoline nozzle in -- the unleaded and leaded nozzles were different sizes. The diesel nozzle is even bigger than leaded nozzles were, so it would never fit in the gas tank of most cars. However, most motorcycles and trucks don't have this plate, so it's easy to make this mistake if you're driving one of those vehicles. And if you're driving an older car, there's no plate either.
So say that you somehow filled a gasoline tank with diesel fuel. If you've ever compared gasoline to diesel fuel, you know that they smell different. They also feel different -- diesel fuel is oily. Like oil, diesel fuel doesn't evaporate like gasoline does. Plus, diesel fuel is heavier. A gallon of diesel is about a pound heavier than a gallon of gasoline.
A lot of diesel pumps are separated from the unleaded pumps, which could also prevent pumping the wrong fuel.
Photo courtesy stock.xchng
If you had a gas tank full of diesel fuel, the fuel injectors in your engine would inject the diesel fuel into the engine's cylinders. The spark plugs would fire, but nothing would happen after that. Because the diesel fuel doesn't evaporate very well, the spark plugs would have nothing to ignite, and the engine would never start.
To solve the problem, drain all of the diesel fuel out of the gas tank and refill it with gasoline. Then you would have to keep cranking the engine for a while to get the diesel out of the fuel lines and the injectors. Eventually the engine would start and run fine. There would be no damage.
One obvious question from this discussion is: If diesel fuel won't burn in a gasoline engine, why does it burn in a diesel engine? There are two big differences between gas and diesel engines:
First, diesel engines have no spark plugs.
Second, they have much higher compression ratios. When the diesel engine compresses the air during its compression stroke, the air gets extremely hot
Stop pumping the fuel
Assess how much of the wrong fuel you put into the tank (is it more or less than 10% of the tank capacity?)
Don’t start the engine (if you need to move away from the pumps, push the car to a parking bay)
If you've put the wrong fuel in your car, don't turn on the ignition or start the engine as this could compound the mistake by circulating the contaminated fuel and increasing the risk of damage to your car.
The ‘Gasoline in Diesel’ mistake is a far more common scenario than the reverse. This is because the Diesel nozzle is much bigger than Leaded Gasoline nozzles which are thus unable to fit in the gas tank of most cars (with the exception of some older models). The Unleaded and Leaded Gasoline nozzles are also different sizes with the Unleaded being the smallest - the use of the wrong fuel in an Unleaded car is therefore the least likely scenario to come across.
On discovering you have put the wrong fuel in your car, the first thing you should do is check with your local dealer on what action to take (if your car is still under warranty). The dealer may advise towing the car to a garage and having the tank drained and the seals and filters changed. If you take action yourself or even start the engine after using the incorrect fuel, you may invalidate your warranty.
Assuming your car is out of warranty or you have been given the go-ahead by your local dealer, here is what to do next.
If you've pumped more than 10% of your tanks capacity with the wrong fuel (Gasoline in our example) you will need to drain the tank and refill it with Diesel.
If you've added less than 10% Gasoline then top up with Diesel and run normally unless you have received advice to the contrary from your local dealer or manufacturer.
The main damage in mis-fueling cases is caused, not by the act of pumping the wrong fuel into the tank, but of starting the engine and driving away. Diesel is obviously a propellant but it is also used a lubricant for components. The solvent nature of Gasoline can strip out the lubricant and cause metal components to grind together. This is where the damage can become very expensive if you accidentally put Gasoline into a Diesel car.
Putting the wrong fuel in your car is not an uncommon mistake and certainly not uniquely American. In 2008 a UK newspaper reported that there are “an estimated 120,000 drivers put the wrong fuel in their car every year - about 13 an hour - and end up paying out a total of £50 million in repair bills.” End of Article