I Put Diesel In My Car's Gas Tank! What Do I Do Now?

Photo: Axel Heimken (AP)

It’s springtime! Time to start working on your project car, learn a new wrenching skill, learn how things work under the hood, or just spruce up your daily driver. All month, we’ll be looking back at our best informative, maintenance and DIY articles from Jalopnik’s near 20-year history to get your ride ready for the road. Welcome to the Jalopnik Spring Tune-Up.

It’s thankfully a rare problem, but a horrifying one: An unwitting driver accidentally pumps diesel fuel into their gasoline-powered car, somehow bypassing the built-in safety feature where the diesel fuel nozzle won’t fit in the gasoline car’s fuel filler. This is disastrous for your engine, so if it happens, here’s what to do next.

A Jalopnik reader’s cousin put diesel in a (conventional gasoline-powered) 2010 Toyota Camry, and drove on it for a few miles before, inevitably, the car 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 a 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, who 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?

G/O Media may get a commission

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 from crude oil, but diesel is refined differently, and thus, engines designed to run on diesel operate differently. They generally have different compression ratios, demand a different air-fuel mixture, 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 engine is only meant to burn gasoline, and your diesel engine is engineered to burn only diesel.

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’re not supposed to—next time you go to the gas station, check out the size of the nozzle on a gas pump and compare it to a diesel pump. The diesel pump nozzle, you’ll find, is slightly bigger, and won’t easily fit into your car’s gas filler neck (if it does go in, it won’t go in all the way). This is done on purpose, specifically to prevent these kind of mishaps.

So we’re assuming here that the reader’s cousin got the diesel in there some other way — 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 an 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, as you might have guessed: If he’s not screwed, the repairs won’t be cheap.

If you make this mix-up, and you’re fortunate enough to realize your terrible mistake, do not start the engine. If you don’t start the engine, it’s much easier to get your fuel tank drained before the wrong fuel gets introduced to the engine. If you drain the fuel tank, flush it, and refill it with the proper fuel, without ever starting the engine, odds are you should be fine. Embarrassed, but fine. Probably don’t tell anyone you did this afterward.

But our reader’s cousin actually drove the car, running the engine on the wrong fuel (specifically, on a random mix of some diesel fuel plus whatever gasoline was left in the fuel tank). And according to the email, 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, fuel injectors, 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. He’s the owner of Automotive Authority in Troy, Michigan, and has repaired a number of cars after owners accidentally filled them with the wrong fuel.

But what if a system flush is not enough? What if, worst-case scenario, you drive on the wrong fuel until your car breaks down?

It’s actually understandable how our reader’s cousin got in this situation. The Camry probably ran normally for a few minutes after filling up on diesel, burning untainted gasoline that remained in the fuel lines and never contacted the diesel in the tank. But soon, the good gas is consumed, and the car starts sucking in a nasty mix of diesel and gasoline.

This can result in severe engine damage, according to Chia-Fon Lee, a mechanical engineering professor and internal combustion expert at the University of Illinois Urbana-Champaign.

First, because the engine wasn’t designed to burn diesel (or a diesel/gasoline mix), the engine will start to run rough and misfire. This on its own can cause damage to the pistons or the cylinder head, leading to a loss of compression, Lee said, adding that most people will shut off the engine before the worst of it occurs. The rough-running, bad-sounding engine makes it pretty obvious that something is going wrong.

The fix in the worst case? A complete engine rebuild.

Even if your engine doesn’t get damaged to the point of not running, you may cause problems that only become obvious down the road. John Heywood, a mechanical engineering professor at MIT, said that it’s possible that deposits could build up on valve closings while running rough on diesel, leading to a loss of compression, though he added that he’d be “surprised” if any permanent damage resulted.

Whatever happened, things turned out badly for the reader’s cousin. In a follow-up email, the reader said his cousin had to replace the Toyota’s 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, or at the very least, turn it off immediately and get it towed for repair. It might yet be saved.