What Gasoline Does to a Diesel Engine

Pumping diesel gas
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To keep people from accidentally fueling up a diesel engine with gasoline, most diesel fuel pumps are distinguished by green markings and green fueling nozzle handles. In addition, the inside of a diesel vehicle fuel door has a “Diesel Fuel Only” label. But what happens if you inadvertently fill your diesel car or pickup with gasoline?

Whether you’re new to diesel ownership or have always had both diesel and gasoline-powered vehicles in your personal fleet, it can be oh-so-easy to accidentally misfuel your diesel tank with gasoline. Filling a fuel tank is such an ordinary and mundane task that just a moment’s inattention (did you really need to read that text message?) can cause you to grab the wrong nozzle and pump away.

It's bad enough if you realize the mistake right away and can get the car towed to a car dealership or independent repair shop to have the tank drained—a $500-$1,000 nuisance.

But what if you don’t even realize the mistake and end up driving away with a tank full of gasoline? Chances are you won’t get very far, perhaps just a mile or so. That’s when the diesel in the fuel line gives way to the fresh batch of gasoline on the way from the tank and the engine starts to run “funny.”

Of course, it all depends on how much diesel fuel remained in the tank before the gasoline was added and how new and sophisticated the diesel engine is.

How Much Gas Does It Take to Harm a Diesel Engine

In a 2007 or newer “clean diesel” engine, any amount of gasoline will probably damage the sensitive emissions control components (DPF, OxyCat, and SCR) and system. In older engines with much less sophisticated emissions systems, a lightly diluted (say 90% diesel/10% gasoline) mix would likely pass through with little to no detriment. It might simply cause reduced engine power, perhaps a bit more noise, and possibly a sharp warning from the emissions sensors that detect something other than pure diesel exhaust.

It’s a high concentration of gasoline that spells real trouble. Whether a modern clean common rail diesel (CRD) or an old indirect injection unit, burning straight gasoline or highly diluted diesel fuel will almost certainly result in catastrophic damage to the mighty diesel engine.

Do's and Don’ts

If you are fortunate enough to discover you were pumping gasoline rather than diesel before driving away, here are the do's and don’ts.

  • DO NOT start the engine, even just to move the car from the pumps.
  • DO NOT turn the ignition on, even just to unlock the steering wheel. This could activate an electric fuel pump and feed tainted fuel into the engine injectors. DO tell the station attendant you cannot move the car and pay for the fuel dispensed.
  • DO call your roadside service provider and request a tow to either the vehicle’s brand dealership or an independent repair shop.
  • DO have the fuel tank drained and receive confirmation that the contaminated fuel was limited to the fuel tank.

If you don’t notice the misfueling error until the car has been driven, stop as soon as it is safe to and call your roadside service provider to request a tow. Unfortunately, the price to repair the damage will be very high and this is a situation that will not be covered by your automaker’s warranty.

What Gas Does to a Diesel

The problem of fueling a diesel with gas is multifaceted; it is a function of the completely different burn characteristics of the fuels (volatile and explosive gasoline versus high flash point diesel fuel) and the peculiarities of engine design in regards to how fuel is ignited (spark ignition versus compression ignition).

Gasoline is formulated to resist auto-ignition in a spark engine (depending on the octane), so this fuel introduced into a diesel engine either won’t ignite or will, more likely, ignite at the wrong time causing severe detonation—literally a shock wave throughout your cylinder. Though diesel engine reciprocating components—pistons, wrist pins, and connecting rods—are built to withstand enormous explosive force, the shock wave effects of uncontrolled detonation can easily destroy them.

If by chance major engine damage is avoided, there are other serious consequences.

Diesel fuel itself acts as a lubricant for the fuel pump and delivery system as well as the valve train. Running thin, low viscosity gasoline through a diesel fuel system would starve it of lubrication and cause those sensitive components to rub together, eventually destroying them.

Additionally, the entire fuel system is negatively affected. That means the fuel pump, fuel filter, and fuel injectors will likely need replacement. In the worst-case scenario, it might be cheaper to just replace the engine and components.

Good News for Newer Diesel Vehicles

Gasoline vehicle fuel filter openings were made smaller in diameter beginning in the early 1980s; this was in response to the mandatory use of unleaded fuel to protect catalytic converters and the negative effects of lead to human health. That’s why the smaller diameter gas filler nozzle fits into the larger filler opening of diesel cars.

Then in 2009, BMW launched its clean diesels in the U.S. with a “misfueling protection device”—essentially a gas cap replacement with a diameter specific to diesel nozzles—as standard equipment. Audi followed in 2011 with a similar device, and beginning with 2013 vehicles, Volkswagen redesigned its fuel fillers to accept only diesel fuel. Today, nearly every diesel vehicle—car or pickup—will only accept diesel fuel.

What Diesel Fuel Does to a Gasoline Engine

Fortunately, this scenario is highly unlikely since larger diesel filler nozzles won’t fit in narrow gasoline filler necks. But if you do manage to get diesel fuel in your gasoline tank, the engine will probably not even start, and if it does, it’ll run terribly and probably smoke like a chimney. Engine damage will most likely be minimal, but a thorough and expensive fuel system flush will certainly be in order.