What Happens if You Put Diesel in a Regular Car?

In the Chamber

Diesel fuel is much easier to ignite than gasoline; it has a much lower octane rating and a higher cetane rating, so it's much more prone to detonating in your cylinders rather than evenly burning. However, a good portion of it won't burn at all. Diesel doesn't like to vaporize the way gas does; instead of turning into a fine mist as it leaves the fuel injectors, the diesel ends up squirting through the engine in a solid stream. The diesel that does burn causes knock and misfire, and the fuel that doesn't burn goes right out the exhaust pipe.

Detonation and Misfire

The one thing you're all but guaranteed when putting any quantity of diesel into an engine is detonation and misfire. The severity of the detonation and misfire depends on how much diesel got mixed in. A very small amount of diesel might just cause some high-rpm power loss and a bit of knocking under hard acceleration. Add a little more, and the engine will start noticeably misfiring and shaking at higher rpm. Add some more, and it will struggle to run at any rpm, including idle. Try running nothing but diesel, and it more than likely won't even start.

No Smoking, Please

Diesel isn't going to burn in your engine -- but it will sort of "cook" if there's enough gas present in the mixture to keep the engine running. If the diesel gets very hot, but not hot enough to catch fire, the carbon in the heavy fuel will "cook" into a sooty, black smoke. Unburned fuel exiting the engine is why a lot of older diesels spewed plumes of black smoke on acceleration. Your gas engine will do the same thing if you add a gallon or two of diesel to a tank of gas. Again, it depends on how much you put in. The more diesel in the mixture, the more the engine will smoke... if it runs at all.

Permanent Damage to the Converter

Provided you stop the vehicle and drain the fuel system as soon as the engine starts lugging and smoking, permanent damage is fairly unlikely. It used to be that adding diesel to gas was a sure-fire way to kill catalytic converters, but that isn't so much the case now. Older diesel fuels had a lot of sulfur in them, and sulfur is death for most converters. However, the newest generation of federally mandated, low-sulfur diesel fuels are far less likely to poison the converter. However, even modern diesel still has much more sulfur in it than gas does -- so it's not impossible to kill the converter if you run diesel fuel through it for long enough.

Permanent Damage -- Engine

Detonation can be fatal to an engine if it goes on for any length of time. That's especially true for supercharged and turbocharged engines. Almost all engines today have "knock" sensors that will tell the computer if there's detonation in the cylinders. The computer will respond by dialing back timing and changing the air-fuel ratio, helping to protect your engine from potentially fatal detonation. However, the computer can only work within certain parameters. It gives you a slightly wider margin of error, but it's still entirely possible to blow holes in your pistons, blow head gaskets and fry spark plugs if you continue to drive with the engine knocking.

Diesel Cleanout

Some old mechanics will tell you that adding a little bit of diesel to your gas can help to clean out the engine. There is actually a tiny grain of truth to that, since diesel contains a lot of detergents and additives to prevent carbon buildup. And it is good at breaking up carbon sludge. Some suggest putting a little in your oil 25 miles before every oil change to clean out the engine. However, both of these are extremely risky, and likely to do more harm than good, especially in modern engines. If you want to clean out your intake tract or oil galleries, there are dedicated cleaners for that: Seafoam, Amsoil Power Foam and BG44K, among others. They cost little more than a couple gallons of diesel, and are much safer, faster and more effective at scrubbing intake passages and valves than diesel will ever be. Most of these also work for cleaning oil passages too, but Marvel Mystery Oil and Lucas make dedicated additives for that.