What Really Happens When You Put Diesel in a Gas Car?

Dear Car Talk

Dear Car Talk | Jan 26, 2016

Dear Car Talk:

Will a car be damaged if a person mistakenly puts diesel into a gasoline car's gas tank? And also, what happens if you put regular gas into a diesel vehicle? How often does this happen? It happened to us recently. Thank you.

-- Carolina

Well, it's not something we see every day, Carolina. First of all, diesel pumps are labeled "Diesel," and they're usually a different color -- most often green. So most people notice that something's different before they make that mistake.

I'm not making you feel better yet, am I, Carolina?

Actually, despite the markings, it happens surprisingly often, so don't feel too bad. I guess people are just busy and distracted these days, trying to refuel the car while responding to text messages and stopping the Cheerios fight the kids are having in the back seat.

When diesel fuel does end up in a gasoline tank, the car will run for a few miles or so, until all the gasoline that's still in the fuel line gets used up. At that point, the engine will shut down -- boom.

Gasoline engines cannot combust diesel fuel, so the car would stop running.

Once that happens, it has to be towed to a shop, and the fuel system needs to be drained. We start by emptying the gas tank. Then -- assuming the car has been driven since the errant refueling -- we get all of the diesel fuel out of the fuel lines, the fuel rail and the injectors.

We'll usually remove the injectors and run them outside of the car so they just squirt out a bunch of the diesel fuel. Sometimes that doesn't work, and we have to take things apart, and, well, it can be a real mess. But it's doable. And normally, no permanent damage is done to the injectors, the seals or anything else. And because the car won't actually run on diesel fuel, there's little risk of damaging the catalytic converter.

But it's a costly and time-consuming process, as I'm sure you now know, Carolina! Depending on the car, it can cost $500 to $1,000.

Accidentally putting gasoline in your diesel tank can be even more harmful, depending on how much gasoline you've added. I mean, if you had an empty 20-gallon tank, and you caught your mistake when you'd put in only a gallon, you might be all right if you filled the rest of the tank with diesel. But if it's more than that, you could create serious problems.

Aside from being the fuel, diesel also is a lubricant. So running a diesel engine on gasoline starves the fuel-injector pump of lubrication.

And because of their different combustion properties, the gasoline detonates too early in a high-compression diesel engine and causes all kinds of damaging knocking and misfiring. So not only would you have to drain the tank, in that case, but you might need to replace other parts as well.

So I think we need to launch a new public-service campaign to reduce distractions while refueling, Carolina: "Don't Text and Pump." Want to be the poster girl?

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