My Daughter Accidentally Put 8 Gallons Of Diesel In My Mazda CX-5. She Drove A Mile Home. As Soon As We Realized The Mistake, We Had It Towed To A Shop. They Quoted Me $4,000 To Fix It. What Should I Do? Mazda Master Mechanic Responds

With most parents, loaning your car to your child is fraught with the ubiquitous worry of the possibility of an accident. And rightfully so. But how many parents think about the consequences of their child refueling a car with the wrong fuel type? Can such a simple mistake cost thousands? That was the question raised on social media when a father posted on Reddit r/MechanicAdvice whether a quoted $4,000 car repair was correct for his situation.

Wrong or Contaminated Fuel Concerns

Not long ago, we learned about the hazards of refueling your vehicle at gas stations where cars were fueled with water-contaminated gasoline. Worse yet, we discovered the wrong fuel type was pumped into gas station storage tanks due to a "cross drop" mistake: diesel was accidentally put into the unleaded gas storage tanks, and vice versa, unleaded gasoline was put into the diesel storage tanks in some Circle K gas stations.

What Happens When Diesel is Put Into a Gasoline-Only Car?

While it is true that putting diesel fuel into a gasoline-powered car can cause significant issues and potential damage, the damage is more often minimal and not an engine-destroying event that will cost thousands of dollars in repairs or replacement.

The key thing to remember is that gasoline engines are designed to ignite a highly volatile fuel with spark plugs, while diesel fuel is less volatile. Diesel fuel is oily, whereas gasoline is more refined.

With that in mind, the following makes sense when diesel is added to a gasoline engine:

Immediate Effects of Diesel On Most Gasoline Engines:

1. The engine may not start at all if a high concentration of diesel is present.
2. If the engine does start, it will likely run very poorly, misfire, or produce excessive smoke.
3. Diesel fuel, due to its oily nature, can clog fuel injectors, filters, and fuel lines.
4. The spark plugs may also become fouled and prevent proper combustion or cause erratic firing of the cylinders.
5. The fuel system, including the fuel pump and fuel injectors, might become damaged and need either cleaning or replacement.

Reddit Posts Agree $4,000 is Ridiculous

The majority of the thread posts agree that the OP was correct in questioning the quote of $4,000 for the repair and that this is not an uncommon problem and is one that can be fixed even by a DIY car owner for much, much less:

While the posts above are in agreement and reassuring, this post from a Mazda Master Tech concurs as well, and reminds us that the garage in question may have been quoting a gasoline-pumped-into-a-diesel problem (which is a much more significant repair issue), and also points out that substantial damage can sometimes occur and result in a higher than expected repair bill when diesel fuel is run in a gasoline engine:

In conclusion, the takeaway message is that accidentally putting diesel into a gasoline engine should, in most cases, not cost thousands of dollars but somewhere between $200 and $500 at most garages.

If you do add diesel by mistake, do not panic. Follow these simple rules:

1. Do not start the engine. If you realize the mistake before starting, the damage is significantly minimized. Even running driving for a short distance should not excessively damage your car.
2. Tow the vehicle to your home for a DIY fix or to a garage AFTER getting a reasonable phone price quote before towing your car to their repair facility. Note that if you DIY the repair, you will want to take into account the mess, and the problem of proper fuel disposal might not be worth the trouble.
3. The fix entails draining the fuel tank and flushing the fuel system to remove any remaining diesel fuel, replacing the fuel filter, and checking your spark plugs for fouling.
4. If this does not resolve the wrong fuel problem, then a professional diagnosis is warranted. The professional should explain to you whether your car's problems are related to diesel fuel or to some other cause.
5. If you want to attempt a DIY repair to the problem, bear in mind that the suggestions provided in the thread will not work (or work well) in all vehicle models. Be sure to check a repair manual specific for your car to determine which DIY repair techniques apply to your model so as not to create an even bigger repair problem.

If your family car has had a similar situation, we would like to hear about it. Feel free to share your stories in the comments section below on what happened, how it was resolved, and what the repair cost came to.

Timothy Boyer is an automotive reporter based in Cincinnati who currently researches and works on restoring older vehicles with engine modifications for improved performance. He also reports on modern cars (including EVs) with a focus on DIY mechanics, buying and using tools, and other related topical automotive repair news. Follow Tim on Twitter at @TimBoyerWrites as well as on Facebook and his automotive blog "Zen and the Art of DIY Car Repair" for useful daily news and topics related to new and used cars and trucks.

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