Man Takes GMC Truck to Utah Mechanic. Then He's Told This One Mistake Will Cost $15,000 to Fix

One wrong move at the gas pump turned into a $15,000 nightmare for a truck owner—and his mechanic is making sure no one else makes the same costly error.

What started as a late-night fueling mistake ended with a nearly brand-new truck needing its entire fuel system replaced.

What Happens When You Put DEF in Diesel Fuel?

In a series of videos with more than 360,000 views across four parts, Dave’s Garage Center (@car.repair.yt) reposted videos of Dave from Dave's Auto Center (@davesautocenter) walking viewers through one of the most expensive fueling mistakes a diesel truck owner can make: accidentally putting diesel exhaust fluid (DEF) in the fuel tank.

The customer, Travis, was driving his GMC truck with only 24,000 miles on it when he realized he'd put a gallon of DEF in his diesel tank. It was late on a Saturday night, and he was trying to get back to Colorado with his family.

"I looked at Google to see what they said, and they said, 'Oh, you know, if you drive it, you know, change your fuel filters off then you'll be okay,' but that was the wrong decision," Travis explained in a phone call with Dave.

Travis drove approximately 80 miles with the contaminated fuel before the truck started smoking badly and making terrible knocking noises. He eventually got it to a dealership on Sunday morning, where it sat for three weeks. The dealer dropped the tank and replaced the fuel filter housing for around $3,400, but it didn't fix the problem.

When Dave inspected the truck, the news was grim. The fuel that sat in the tank had turned into a crystallized, corrosive sludge.

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"This is DEF that gets in fuel," Dave says in the video, showing the contaminated fuel.

"If you come in close, you'll see the crystals on it. And I just can't stress enough: that stuff is not a good lubricant. It's so corrosive that it wipes out your fuel system."

How Did They Diagnose and Address the Issue?

During the inspection, Dave's team performed a compression test and used a borescope to examine the cylinder walls. They discovered scoring on the cylinder walls, unusual spray patterns from the injectors, and what appeared to be oil ring failure in at least one cylinder.

"We've got some nice scoring there on the cylinder wall," Dave notes while examining the footage. The piston showed signs of wear, and the area around the rings looked dimpled and pitted from the corrosive DEF.

Despite the cylinder damage, Dave recommended focusing on the fuel system first rather than rebuilding the entire engine.

"If it was my truck or your truck, would you rebuild the motor for a scratch in the cylinder wall?" Dave asks his team. "I'd see how long it'd go."

The repair required flushing and replacing the entire fuel system: both fuel tanks, all fuel lines, the high-pressure fuel pump, all eight injectors, and the fuel rails.

The total cost—a whopping $15,000.

"When this happens, you've gotta completely flush and clean out the entire system. I'm talking fuel tanks, both line fuel lines, the feed line, the return line, the pump, all the injectors, all the feed lines for the injectors. I mean, it needs a major complete flush,” Dave explains.

Gallery: GMC Sierra (2024)

What Exactly is DEF?

Diesel exhaust fluid is designed to reduce emissions by being injected into a truck's exhaust system, not its fuel system. DEF is primarily composed of urea and deionized water, neither of which provides the lubrication that modern diesel fuel systems require, International Used Truck Centers reported.

As Dave demonstrated in his videos, DEF sitting in diesel fuel crystallizes and forms a sludge that can clog fuel filters, damage injector tips, and destroy high-pressure fuel pumps. The corrosive nature of DEF can also etch and score cylinder walls if it makes its way into the combustion chamber, as happened with Travis's truck.

The key to minimizing damage is catching the mistake before starting the engine. Dave emphasized this point repeatedly throughout his video series.

"If you put DEF in your tank and you know you've made that mistake at the gas station, easy, simple: Don't turn the key on. Don't do anything. Get the batteries disconnected. Get a tow truck," Dave said. "Get the truck to a place that can remove the tank, completely flush it out, and you're good. You wouldn't have to replace the injectors and all of this stuff."

In a follow-up example, Dave showed a customer who had put DEF in his tank but had the presence of mind not to turn the key on. The tow truck driver removed the fuel pump relay and fuel pump control module relay before transporting the vehicle, preventing any fuel from moving through the system. That customer avoided the damage and expensive repairs.

"I'm doing this because I'm trying to make you a little smarter, a little richer," Dave says. "If you … end up putting DEF in your fuel tank and you remember you've done it, you know you did it, do not turn the key on, and you will save yourself over $10,000."