Image: Jason Torchinsky

It’s thankfully a rare problem but a horrifying one: an unwitting driver accidentally dumps diesel fuel into their gasoline-powered car, somehow bypassing the differently sized nozzles. This is disastrous for your engine, so if it happens, here’s what to do next.

Hello! This is the beginning of what will be a regular feature on this internet website: A column in which I investigate your car problems, mostly to determine just how screwed or not screwed you might be.

First up! A reader whose cousin put diesel into a 2010 Toyota Camry, and drove on it for a few miles before, inevitably, it came to a stop.

The complete email:

This may not be in your wheelhouse (pun not intended), but how much damage could putting diesel in an gas engine do? My cousin accidentally put diesel in his 2010 Camry and drove on it for a couple of miles until the car came to a stop. He took it to a mechanic, which was recommended to him by his boss, to get the fuel system flushed.

Once the system was flushed the mechanic said the car wouldn’t start and did a compression test and saw that one of the cylinders was not at full compression so he told us that there must be a vent valve. Now the mechanic is saying the whole engine is trashed and it is better to get a new one. So therein lies my question, can an engine be ruined by accidentally putting diesel in it?

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Obviously this is bad, but let’s explain why.

The Problem

First, we probably need a quick briefing on the difference between diesel and gasoline. Both fuels are refined versions of crude oil, but diesel is refined differently and thus the engines designed to run them operate differently. They generally have different compression ratios, air-fuel ratios and perhaps most importantly, diesel engines use air compression to ignite the fuel rather than the spark plugs that gasoline engines use. So while they’re both fossil fuels, your gasoline or diesel engine is only made to run that kind of fuel.

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Here’s a useful video from our friend Jason Fenske at Engineering Explained:

It’s actually difficult to put diesel in your gas tank if you don’t mean to—next time you go to the gas station, check out the nozzle size of a gas pump and a diesel pump. The diesel pump, you’ll find, is slightly bigger, and won’t be easily inserted into your car’s gas opening (if it does go in, it won’t go in all the way.) This is done on purpose.

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So we’re assuming here that the reader’s cousin got the diesel in there from a gas can perhaps, possibly in some ill-advised and desperate attempt to run his car in the absence of real gas, or perhaps just a untimely mix-up. Who knows!

What To Do

Onward, to the real question at hand. Is our reader’s cousin screwed? The short answer is maybe; the slightly longer answer is that, as you might’ve guessed, if he’s not, the repairs won’t be cheap. If you do this, and you’re fortunate enough to realize your terrible mistake, do not start the engine. You can get the fuel tank flushed out before it gets into the motor and odds are you should be fine. Embarrassed, but fine. Probably don’t tell anyone you did this afterward.

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But having actually run the car, our reader’s cousin has already gone through the first necessary steps, which amount to a flush of the fuel system, including the tank, fuel lines, injectors, rail, and fuel pump. In most cases, this is all that’s required, and, soon enough, you’ll be off driving again, a bit wiser for the experience, if also a bit poorer.

How much poorer? As little as $400 or as much as $1,500, depending on the car, according to Tim Anderson, the owner of Automotive Authority in Troy, Michigan, who has repaired a number of cars that had diesel accidentally put in them.

But what if that wasn’t enough? What can happen, though, is that the car keeps running on gasoline remaining in the fuel line, or some mixture of diesel and gasoline, depending on how much of the latter was in the tank when you put in the former.

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That results in some severe engine damage, according to Chia-Fon Lee, a mechanical engineering professor and internal combustion expert at the University of Illinois Urbana-Champaign. Simply running the engine rough—as it misfires—could cause piston or cylinder head damage, leading to a loss of compression, Lee said, adding that most people will shut off their cars before the worst of it occurs because the problem—in the way the engine sounds, at least—will be pretty obvious.

The fix in the worst case? An engine rebuild.

And if your engine doesn’t get damaged that way, John Heywood, a mechanical engineering professor at MIT, said that it’s possible that deposits could build up on valve closings while running rough because of diesel, leading to a loss of compression, though he added that he’d be “surprised” if any permanent damage resulted.

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Whatever happened, things turned out badly for the reader’s cousin. In a follow-up email, the reader said his cousin did replace the engine, at a cost of $2,300. Which sucks! But it also could be a lesson for the rest of us.

What should you do then if you accidentally put diesel fuel in your gas tank? Don’t run the car, obviously, but if it is running turn it off immediately and get it towed for repair. It might yet be saved.

Email me at erik.shilling@jalopnik.com with car problems you have, and I’ll endeavor to get to the bottom of them in this space.