At our first pit stop near Hagerstown, Maryland, I jumped out the 2023 Cadillac Escalade I was checking, dipped my credit card in the pump, and filled the tank—two-thirds empty after about 350 miles of highway driving—with premium unleaded.
I had missed all the signs. The low redline, the grumpy torque, the 25 mpg fuel economy, even the warning message on the fuel filler door. As it turns out, the Escalade I borrowed for a week’s vacation with my friend’s family powered by GM’s 277 hp, 460 lb-ft 3.0-liter six-cylinder turbo diesel. The existence of a diesel Caddy in the 2023 model year – especially after the disgrace the brand suffered with its abysmal diesel business in the ’70s and ’80s – was beyond me.
I fully accept blame for my mistake, but I was driving the truck under the influence of SuperCruisethe standard of the world superlative hands-free highway driving system. During our highway trip, it blinded me repeatedly, able to accelerate, decelerate, steer, stay on course and detect obstacles, all without the benefit of my big mitts. Even more fun, when another motorist braked at our set speed, the Caddy would automatically find a gap, change lanes, pass and return to the right lane, a skill lacking in 90 percent of human motorists. I was entranced, which rarely happens behind the wheel of an SUV the size of a shed.
Amazingly, my reverie continued for another 250 SuperCrusing miles after accidentally filling up with gas. It wasn’t until we were within walking distance of our AirBnB, on a winding single-lane road that plunged into West Virginia’s Greenbrier River valley, that the first suspicions of a problem arose. The great Slade began to tremble; as we pulled into the driveway, it erupted in a dramatic miasma of soot.
A look in the trunk while unloading our bags confirmed my worst suspicions: 600 D – for Diesel, or Duramax, or in my case, Dumb. I was so embarrassed that I didn’t tell my friend until the tow truck was on its way.
Everything I had read about this idiot 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 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 over the phone. “But while it can running on gasoline, it’s the wrong fuel mix for it,” he said. “And then all these other bad things happen.”
That “bad stuff” occurs in an increasingly negative cascade. In the best case, you realize the error 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 drain 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 feeds fuel to a high-pressure pump in the engine compartment capable of 36,000 psi. Soon after, the petrol and diesel are completely mixed; the wrong fuel is ubiquitous. This is corrosive. “Diesel actually lubricates the pump,” Barta said. “Gasoline is actually a solvent. So if you remove that lubricant and then flush it out, you’re going to get a significant amount of wear 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 chips from the unlubricated pumps enter the fuel and spread throughout the system. Filters in each fuel injector help somewhat, but if the particles are fine enough, they can get into the nozzle and interrupt fuel flow, causing what Barta called “terrible injection events.”
After this it gets really ugly. A diesel engine burns its fuel without a spark plug, under great pressure. Gas disrupts this equation and causes a late burn. This creates a huge amount of soot. This soot then ends up in the particulate filter in the exhaust system. Typically, when a DPF fills up, the engine computer briefly injects more fuel, automatically causing the DPF to burn clean. But gasoline here lacks the energy content for a good ignition. “Eventually you create so much soot that the filter gets clogged, and then you basically have a clog so the exhaust has nowhere to go,” Barta said. “Probably that’s why your engine eventually stalled.”
Repairing this properly will require all of the solutions above 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 catastrophe never resurfaced for hours on the road with the wrong fuel sloshing in the tank. It turns out our stable SuperCruising must have helped. “Our engine is quite economical,” said Barta. “So if you were just working with a low load — if you burn less fuel, you get less soot.”
At gas stations, diesel fuel nozzles have a larger diameter than gasoline nozzlesto stop customers accidentally refueling a petrol car with diesel, but you can easily slide a smaller petrol nozzle into the filler neck of a diesel. This protection is a bit counterintuitive, as it is much less harmful to refueling a gas car with diesel then vice versa. “The diesel fuel, while it doesn’t make the gasoline-powered 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 worrying about damage or long-term durability. Perhaps in response to my recent confusion, Barta added, “We never recommend that you put the wrong fuel in 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 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 getting into a car accident. It’s an accident. No one intends that.”
While the people I spoke to at GM wanted to take the flat-bed SUV to a Caddy dealership, the closest one was 130 miles away. Fortunately, there was a Chevy shop just ten minutes away that had done this repair multiple times. The Caddy had to go back to New York at the end of the week, so the Chevy dealer just drained and flushed the entire fuel system and filled the tank with good diesel – a $750 bill, but much less than I expected . (Despite my protestations, FMI, GM’s fleet management company, graciously paid the bill.) Then the service manager drove it 300 miles to make sure it was running smoothly.
I took the risk of catastrophic failure by driving the Caddy back north. But after 600 miles it didn’t even stutter and felt just as solid as it did before my fueling accident. Total fuel economy for our 1,500-mile trip, despite the gasoline, was 26 mpg.
This is not a mistake I see myself making again. Although, given our ongoing shift to electrification, it’s always possible I’ll try to pump in lead-free a celestial.