Here’s What Happens If You Put Diesel in a Gas Car

Just because it's common for a green handle to indicate the diesel hose doesn't mean it's the rule. Mistakes are made and the wrong fuel gets put into cars and trucks every day.

Now, diesel and gasoline have a lot of similarities. Both are liquids, both are used for internal combustion engines, and they are the two most common types of fuels used on the road. And in situations where fail-safes may fail to save you, it's easy to see how someone could mistake the two. 

What happens when you mistakenly put diesel in a gasoline car, though? Is it really worth melting down over? What about when you put gas in a diesel vehicle? Ultimately, the results can be catastrophic. But rather than pointing and laughing at someone for getting the two mixed up, it's best to educate people so that they know how to get themselves out of this situation. 

Relax. The engine isn't going to explode. Damage can occur by continuously running the mix, but it’s not likely to be fatal if it's only run for a short time. 

As similar as gasoline and diesel are, there are some key differences between them. Gasoline is a much more volatile fuel that burns more quickly, whereas diesel is a thick, slow-burning, oil-like fuel. The differences between the fuels can be seen in how engines designed to run on either burn them. 

Both gasoline and diesel engines rely on combustion to create the energy needed to drive the piston downward during the combustion cycle. Gasoline engines use a combination of compression and ignition provided by spark plugs to burn fuel. Diesel engines rely solely on compression, with glow plugs and other devices to help with cold starts. That means those engines need to squeeze that fuel extremely tight to create the kind of conditions that are needed for the fuel to light off. The compression ratios in diesel engines are crazy high because of it. 

There’s a lot of science behind it, but the key takeaway is that diesel is much harder to burn than gas. In turn, a mixture of the two isn’t going to burn very well in a gas engine—and it may not burn at all. It's also likely that the diesel will clog up the fuel filter and injectors because it's thicker than gasoline. 

Accidentally putting diesel in a gas car is a pain, no doubt, but filling a diesel-powered vehicle full of gas can be way worse. 

A diesel engine is highly reliant on the fuel's oil-like nature. Not only is it responsible for combustion but it also lubricates the fuel pump and injectors. Gasoline, being a much thinner fluid, creates excess friction and accelerates wear in these components. The increased wear leads to metal shavings flowing through the system and they will be on a warpath, damaging anything they touch on their way to the cylinders. 

The faster burning rate of gasoline creates even more problems. Gasoline engines do rely on spark plugs, but that doesn't mean the high compressions of diesel engines won't ignite it. It's actually quite the opposite. 

Pre-detonation can occur in gasoline engines when too low of an octane rating is used for too high of a compression ratio. The pressure and heat created by the high compression causes the fuel to combust before the piston reaches top dead center. The same will happen in a diesel engine, and the results can be catastrophic. Unfortunately, this can all happen quite rapidly as well, meaning running the engine for any period of time can be dangerous. 

The first thing you need to do is resist the urge to start that bad boy up. Ideally, you'll catch the problem before you try driving it and all you'll need to do is empty the tank and forget that it ever happened. 

If you have run the fuel up to the engine, then you'll also need to flush the fuel system. In the case of a gasoline car with diesel in it, you may also need to replace the fuel filter, clean the injectors, and replace the spark plugs.

Diesel cars with gas in them, again, present a whole new list of issues. You'll need to run through everything and inspect it for damage. If the gasoline ran through the pump, it's indeed possible that shavings of metal have made their way through the entire system, and you'll need to replace the entire thing to prevent them from damaging the engine. 

A: It's hard to put a measurement on how much diesel could damage your engine. While a little bit likely won't cause any real damage, it's not something to gamble with. You should take precautions to remove any amount of fuel from a system it doesn't belong in. 

A: It really depends on the vehicle you're working with. Quotes range anywhere from $20 to several hundred dollars. You may be able to save a lot by simply doing the work yourself, as long as you have the right tools and are savvy enough. 

A: No. Diesel doesn't act like an octane booster if you accidentally add it to gasoline. In fact, the ratings you see on diesel fuel is not an octane rating. It's a cetane rating, which is something different entirely.