At our first pitstop near Hagerstown, Maryland, I jumped out of the 2023 Cadillac Escalade I was reviewing, dipped my credit card in the pump, and filled the tank — two-thirds depleted after about 350 miles of highway driving — with premium unleaded.
I’d missed all the signs. The low redline, the grunty torque, the 25-mpg fuel economy, even the warning message on the fuel filler door. As it turns out, the Escalade I was borrowing for a week-long vacation with my boyfriend’s family was powered by GM’s 277-hp, 460-lb-ft 3.0-liter turbodiesel straight-six. The existence of a diesel Caddy in the 2023 model year — especially after the ignominy the brand suffered with its abysmal diesel dabblings in the ‘70s and ‘80s—was beyond me.
I accept the blame entirely for my error, but I was operating the truck under the influence — of SuperCruise, the Standard of the World’s superlative hands-free highway driving system. In our freeway trek, it dazzled me repeatedly, able to speed up, slow down, steer, stay a course, and detect obstacles, all without the benefit of my big mitts. Even more entertaining, when another motorist was inhibiting our set speed, the Caddy would automatically find an opening, change lanes, pass, and return to the right lane, a skill lacking in 90 percent of human motorists. I was enraptured, which rarely happens behind the wheel of a barn-sized SUV.
Amazingly, my reverie continued for another 250 SuperCrusing miles after my accidental gasoline fill-up. In fact, it wasn’t until we were within walking distance of our AirBnB, on a winding one-lane plunging into West Virginia’s Greenbrier River valley, that the first inklings of a problem occurred. The big Slade started to shudder; as we pulled into the driveway, it conked out in a dramatic miasma of soot.
A peek at 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, I didn’t let on to my boyfriend until the tow truck was en route.
Everything I’d read about this moronic error suggested that merely starting the engine on the wrong fuel would render it permanently inoperable. So how did I manage to drive the equivalent of Detroit to Chicago without noticing?
“It’s kind of a compliment to me and our team for this engine, to say that it didn’t even seem like a diesel,” John Barta, assistant chief engineer of GM’s smaller Duramax engines, told me over the phone. “But while it can run on gasoline, it’s the wrong mix of fuel for it,” he said. “And then all this other bad stuff happens.”
That “bad stuff” occurs in an increasingly negative cascade. Best-case scenario, you realize the error before starting the engine. “Now your problem is just inside the tank,” Barta said. “If you can stop there, it’s a lot easier to deal with. Get it towed over to a dealership. They can drain the tank and put fresh diesel in, and you’ll be good to go.”
If you start the vehicle, the low-pressure fuel pump in the tank feeds fuel into a high-pressure pump in the engine compartment capable of 36,000 psi. Very soon after, the gasoline and diesel are fully mixed; the wrong fuel is ubiquitous. This is corrosive. “Diesel actually lubricates the pump,” Barta said. “Gasoline is basically a solvent. So when you remove that lubricant, and then wash it away, you end up with a significant amount of wear in that pump.” After just a few miles in this state, 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 un-lubricated pumps enter the fuel and propagate through the system. Filters in each fuel injector help somewhat, but if the particles are fine enough, they can pass into the nozzle and interrupt fuel flow, causing what Barta called “terrible injection events.”
After this, things really get ugly. A diesel engine combusts its fuel without a spark plug, under intense pressure. Gas fouls this equation, causing late combustion. This generates a tremendous amount of soot. This soot then enters the diesel particulate filter in the exhaust system. Typically, when a DPF fills up, the engine computer briefly injects more fuel, which burns the DPF clean automatically. But gasoline lacks the energy content for proper ignition here. “Eventually, you create so much soot, it plugs up the filter, and then you basically have a blockage so that the exhaust has nowhere to go,” Barta said. “Probably, that is why your engine ultimately stalled.”
Repairing this properly requires all of the above fixes, plus replacing the low- and high-pressure fuel pumps, injectors and fuel lines. Not a cheap procedure.
That all makes sense. Still, I wondered how this cataclysm never reared its head during hours on the road with the wrong fuel sloshing away in the tank. As it turns out, our steady-state SuperCruising must have helped. “Our engine is pretty fuel efficient,” Barta said. “So if you were just tooling along at low load — if you’re burning less fuel, you’re going to get less soot.”
At gas stations, diesel fuel nozzles are a larger diameter than petrol ones, to stop customers from inadvertently filling a gas-powered car with diesel, but you can easily slip a smaller gasoline nozzle into a diesel’s filler. This protection is a bit counterintuitive, because it’s far less damaging to fill up a gas-powered car with diesel than vice versa. “The diesel fuel, although it will not let the gasoline-powered car run well at all, at least it’s not corrosive to the system,” Barta said. A simple flush of the fuel system will fix a gas-powered car without worry for long-term damage or durability. Perhaps in response to my recent mix-up, Barta added, “We’d never recommend you put the wrong fuel into your engine, and our warranty coverage does not cover these accidents.”
GM doesn’t really track the incidence of this kind of idiocy among its customers, but Barta noted that mistakes like mine are not unheard-of. “I mean, does it happen? Yes, it happens. I don’t have any rates on how often it happens,” he said. “But I think it happens in all of our diesel products. It’s just like getting into a car accident. It’s an accident. Nobody plans to do that.”
While the folks I spoke with at GM wanted the SUV flat-bedded to a Caddy dealer, the closest one was 130 miles away. Fortunately, there was a Chevy shop just ten minutes away that had handled this fix multiple times. The Caddy needed to make it back to New York at the end of the week, so the Chevy dealer simply drained and flushed the entire fuel system and refilled the tank with good diesel — a $750 bill, but way less than I’d expected. (Despite my protestations, GM’s fleet management company FMI graciously paid the tab.) Then the service manager test-drove it 300 miles, to make sure it was running smoothly.
I took the risk of catastrophic failure driving the Caddy back north. But in 600 miles, it did not even stutter, and felt just as rock-solid as it had before my fill-up mishap. Overall fuel economy for our 1,500-mile trip, despite the gasoline, was 26 mpg.
This is not an error I see myself making again. Though, given our ongoing shift to electrification, it’s always possible that I’ll try to pump unleaded into a Celestiq.