How Serious Is Filling Your Car With Diesel Fuel? We Talk to the Experts

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The Brockton Enterprise reports that a fuel supplier inadvertently filled an underground gas tank with diesel fuel, rendering dozens of cars in town inoperable. We talked to AAA Car Doctor John Paul and Car Talk about the issue to learn what you can do if it happens.

According to an interview the Enterprise conducted, it appears that somehow, diesel fuel ended up in the station’s underground gasoline storage tank. “Apparently they put diesel gasoline in the ground and pumped it out as regular. Now people’s cars are dead,” Jason Hopkins told the Enterprise, after his 2013 Nissan Pathfinder‘s Check Engine light came on. Hopkins works as a mechanic at a nearby garage.

It’s something that shouldn’t happen underground, but it’s something that CAN happen when you refill your car. Since diesel fuel nozzles are typically larger than gasoline nozzles, it’s not common, but it’s not entirely unheard of, either.

BestRide made some calls find out what the issues are, and how to fix them. We called John Paul because he knows what he’s talking about, with 35 years of experience working on cars, and his role as AAA’s Car Doctor. But we also talked to him because he knows that particular fuel retailer. “I filled my tank at that station about 10 days ago,” he told us.

The issue is that diesel fuel and gasoline have completely different properties. Ray Magliozzi at Car Talk tells us, “Gasoline is lighter, less dense, more flammable and more volatile. When you spray gasoline into a cylinder, it’s vaporized, so that as soon as the spark plug fires, the gasoline detonates and powers the engine.”

Diesel fuel is heavier, more dense, less flammable and less volatile than gasoline. “In order to detonate it, it has to be compressed in a cylinder to a very high pressure and temperature, at which point it detonates without a spark,” says Car Talk.

diesel pump

“If you mixed diesel with gas you dilute the gas and the engine just won’t run correctly,” says John Paul. The severity of the issue depends largely on how much diesel fuel was dumped in the tank.

“Generally, flushing the tank, fuel lines and replacing the fuel filter is the fix,” he says. “The car may run a little rough until the plugs clean up. Depending on the concentration you may need to replace the fuel pump, maybe the oxygen sensor(s) and in the worst cases the fuel injectors.”

The biggest issue comes when you try to start the engine and it won’t start at all. John Paul tells us that if you crank the engine to the point of filling the cylinders significantly with diesel fuel, you can “hydro-lock the engine and maybe bend a valve or a connecting rod.

“Then you’ll buy an engine,” he says.

For the record, diesel fuel handles are typically green or yellow, though sometimes you’ll find black.

Diesel nozzle

Craig Fitzgerald

Craig Fitzgerald

Writer, editor, lousy guitar player, dad. Content Marketing and Publication Manager at