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Gassy Duramax

You’d be in a much better position if you hadn’t run the vehicle at all, as you could get away with only draining the fuel tank and there would be no chance of gasoline having made its way further into the fuel system. Because it was run, and for long enough to be moved even that relatively small distance, some gasoline has likely made its way well into the low pressure side of your truck’s system. You do note that you shut the truck off. In other words, it did not shut itself off, so to speak, and you don’t mention any odd behaviour on its part. These two things are good signs.

Unfortunately, while the fill nozzles on fuel station diesel pumps are a larger diameter to prevent them from being inserted into the smaller filler neck holes on gasoline powered vehicles, the smaller gasoline nozzle will quite happily fit into a diesel vehicle’s larger filler. Your wife is by no means the first person to make this mistake.

For what it’s worth, I’ve also encountered one (very much gasoline-powered) Lincoln Navigator where one of the owner’s children filled it with diesel — with some effort, we were told — because it was the least expensive fuel at the pump …. It did not turn out to be an overall savings for the family, as you might expect.

In the case of gasoline vehicles filled with a sufficient amount of diesel (such as this Navigator), the typical outcome is that the vehicle loses power, runs poorly, or more often just stalls out and dies. Draining the tank and flushing the fuel system usually sorts these mis-fuelings out with no further repercussions.

Diesels, particularly modern diesels with common rail injection systems (which operate at substantially higher fuel pressures), do not fare as well.

A gasoline-contaminated diesel might make its displeasure known by knocking, smoking, running poorly, or stalling out, but even without obvious signs calling attention to the mix-up, damage may already have occurred.

There are key differences between the two fuels; gasoline engines use a spark to ignite a compressed mixture of air and fuel, while diesels compress air to a high enough pressure that fuel sprayed into the cylinder spontaneously combusts.

That’s why the two fuels don’t interchange functionally. However, the bigger issue is that gasoline is effectively a solvent, and diesel, being much more closely related to oil in the hydrocarbon world, is a lubricant. The Bosch-supplied “CP4.2” high pressure pump used in your truck to both pull the fuel out of the tank and pressurize it to as much as 29,000 psi (2000 bar) has tight clearances and considerable potential for internal friction.

Unfortunately, the CP4.2 has a deserved reputation for being unforgiving of low-lubricity as results from misfueling or being fuel-starved. Metal debris from a damaged pump can find its way to the injectors (also high-precision, fuel-lubricated items), rails, and throughout the fuel system, potentially resulting in five-digit repair costs.

Working in your favour is the amount of plumbing between the tank and high pressure pump, which holds a fair bit of fuel. Definitely replace the fuel filter, checking it for contamination — a gasoline odour will be the giveaway — and you need to blow out and then flush the fuel feed lines.

For added peace of mind you could replace the pump’s fuel control actuator with an aftermarket “system saver” part, which could both reveal any damage and prevent future debris from making it beyond the pump.

I’m optimistic that your brief, no-load running didn’t damage anything.

Ask a Mechanic is written by Brian Early, a Red Seal-certified Automotive Service Technician. You can send your questions to wheels@thestar.ca. These answers are for informational purposes only. Please consult a certified mechanic before having any work done to your vehicle.