What Happened When I Accidentally Filled A Diesel Cadillac Escalade With Gasoline?

Photo: Cadillac

At our first pit stop near Hagerstown, Maryland, I jumped out of the 2023 Cadillac Escalade I was reviewing, dipped my credit card into the pump, and filled the tank—two-thirds empty after about 350 miles of highway driving—with premium lead-free.

I missed all the signs. The low red line, the grumpy torque, the 25 mpg fuel economy, even the warning message on the fuel filler flap. As it turned out, the Escalade I borrowed for a week-long vacation with my friend’s family was powered by GM’s 277 horsepower, 460 lb-ft 3.0-liter six-cylinder turbo diesel. The existence of a diesel Caddy in the 2023 model year – especially after the infamy the brand suffered with its horrendous diesel fun in the 70s and 80s – was a mystery to me.

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I fully accept the blame for my mistake, but I was driving the truck while intoxicated – from SuperCruise, the world’s superlative hands-free driving on the highway. It blinded me repeatedly on our highway trip, being able to accelerate, decelerate, steer, stay on course and detect obstacles, all without the benefit of my big mitts. Even more amusingly, when another motorist braked our set speed, the Caddy would automatically find an opening, change lanes, pass and return to the right lane, a skill 90 percent of human motorists lack. I was entranced, which rarely happens behind the wheel of a barn-sized SUV.

Amazingly, my reverie continued for another 250 SuperCrusing miles after I accidentally topped up with gas. It wasn’t until we were within walking distance of our AirBnB, on a winding single-lane road that plunged into the Greenbrier River valley in West Virginia, that the first suspicions of a problem arose. The great Slade began to shiver; as we pulled into the driveway, it loomed in a dramatic mass of soot.

Photo: Cadillac

Photo: Cadillac

A look inside the trunk while unloading our bags confirmed my worst assumptions: 600 d – for Diesel, or Duramax, or in my case, Dumb. I was so ashamed that I didn’t let my friend know until the tow truck was on its way.

Everything I’d read about this idiotic mistake suggested that just starting the engine on the wrong fuel would render it permanently useless. So how did I manage to drive the equivalent from Detroit to Chicago without noticing?

“It’s kind of a compliment to me and our team for this engine, to say it didn’t even look like a diesel,” John Barta, assistant chief engineer of GM’s smaller Duramax engines, told me on the phone. “But While It” can driving on gasoline, it’s the wrong fuel mix for it,” he said. “And then all these other bad things happen.”

Those “bad things” occur in an increasingly negative cascade. In the best case scenario, you realize the mistake before you start the engine. “Now your problem is just in the tank,” Barta said. “If you can stop there, it’s a lot easier to deal with. Have it towed to a dealer. They can empty the tank and put fresh diesel in, and you’re good to go.”

When you start the vehicle, the low-pressure fuel pump in the tank supplies fuel to a high-pressure pump in the engine compartment that can deliver 36,000 psi. Very soon after, the petrol and diesel are completely mixed; the wrong fuel is ubiquitous. This is corrosive. “Diesel actually lubricates the pump,” said Barta. “Gasoline is actually a solvent. So if you remove that lubricant and then flush it out, you get a significant amount of wear and tear in that pump.” After only a few miles in this condition, draining and flushing the entire fuel system is necessary.

The more exposure to gasoline, the more severe the wear becomes. Eventually, metal shavings from the unlubricated pumps enter the fuel and spread through the system. Filters in each fuel injector help somewhat, but if the particles are fine enough, they can get into the injector and interrupt the flow of fuel, what Barta called “terrible injection events.”

After that it gets really ugly. A diesel engine burns its fuel without a spark plug, under great pressure. Gas contaminates this equation and causes late combustion. This creates an enormous amount of soot. This soot then ends up in the soot filter in the exhaust system. When a diesel particulate filter becomes full, the engine computer usually injects more fuel for a short time, which automatically burns the particulate filter clean. But gasoline here lacks the energy content for a good ignition. “Eventually you create so much soot, it clogs the filter and then you basically have a clog, so the exhaust has nowhere to go,” said Barta. “Probably that’s why your engine eventually stalled.”

Properly repairing this will require all of the above repairs, plus the replacement of the low and high pressure fuel pumps, injectors and fuel lines. Not a cheap procedure.

The Escalade's 3.0-liter Duramax diesel is so smooth and quiet that I mistook it for a gasser in the worst possible way.

The Escalade’s 3.0-liter Duramax diesel is so smooth and quiet that I mistook it for a gasser in the worst possible way.

That all makes sense. Still, I wondered how this disaster never surfaced during hours on the road with the wrong fuel sloshing in the tank. It turns out that our steady-state SuperCruising must have helped. “Our engine is quite economical,” said Barta. “So if you just cooperate at low load — if you burn less fuel, you get less soot.”

At gas stations, diesel nozzles have a larger diameter than petrol nozzles, to prevent customers from accidentally filling a petrol car with diesel, but you can easily insert a smaller petrol nozzle into the diesel filler. This protection is a bit counterintuitive, as it is much less harmful to fill a gas car with diesel than vice versa. “The diesel fuel, while it doesn’t make the petrol car run well at all, is at least not corrosive to the system,” Barta said. A simple flush of the fuel system will fix a gas-powered car without causing long-term damage or durability. Perhaps in response to my recent confusion, Barta added, “We would never advise you to put the wrong fuel in your engine, and our warranty coverage does not cover these accidents.”

GM doesn’t really keep up with the incidence of this kind of idiocy among its customers, but Barta noted that mistakes like mine aren’t unheard of. “I mean, is it happening? Yes, it happens. I don’t have numbers on how often it happens,” he said. “But I think it’s in all our diesel products. It’s like having a car accident. It’s an accident. Nobody intends to do that.”

While the people I spoke to at GM wanted to take the SUV in a flatbed to a Caddy dealer, the closest was 130 miles away. Fortunately, there was a Chevy store just ten minutes away that had done this repair several times. The Caddy had to go back to New York at the end of the week, so the Chevy dealer just emptied and flushed the entire fuel system and filled the tank with good diesel—a $750 bill, but way less than I expected. (Despite my protests, GM’s fleet management company FMI graciously paid the bill.) Then the service manager tested it 300 miles to make sure it ran smoothly.

I risked catastrophic failure by driving the Caddy back north. But in 600 miles, it didn’t even stutter and felt as solid as it did before my fueling accident. Overall fuel economy for our 1500 mile trip, despite the gas, was 26 mpg.

This is not a mistake I see myself making again. Although, given our continued shift toward electrification, it’s always possible I’ll try to pump lead-free into a Celestiq.

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