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This is similar to this question but I'm more interested in possible long-term effects.

The story is we think my son (new driver) accidentally put diesel fuel in his 2005 5.7L Hemi engine. Luckily it was only half a tank (~10 gallons) .The next morning, the car would "start" but immediately stalled. We towed it to a repair station who did an engine diagnostic and determined that the throttle body was "dirty", and after cleaning it, the car at least ran (though a little sluggishly). They did not think it was diesel fuel and thus did not drain the tank. They thought it was just a batch of "bad gas", which surprised me since he got it from a national brand station. The fact that it ran after cleaning the throttle body makes me think that it was not diesel, but the effects all sound like what you'd get by trying to run diesel through a gas engine, so I'm not certain.

I filled it up (another 10 gallons) with premium unleaded (the manufacturer recommends 89 but I've used 87 more often than not) to try and improve the "quality" of the gas (I know this isn't how octane works but was willing to pay extra to try and reduce the effects). It still ran a little sluggishly (especially at start-up) but other than that seemed fine. Since then, I've added another 10 gallons of premium to dilute the "bad gas" further, and it is running much better now.

At this point I've either got:

  • 10 gallons of diesel/bad gas (if they do not mix and the diesel is used last)
  • No diesel/bad gas left (if they do not mix and the diesel is used first)
  • A solution of roughly 25% diesel/gas that continues to dilute every fill-up.

So I'm more interested in what possible long-term effects of this 10 gallons of diesel/bad gas could have? What I've seen is that there should be no damage done, but the nature of diesel can gunk up spark plugs and other things. Is there anything else I should do to prevent/reduce serious damage?

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The result of filling diesel on a gasoline car is serious knocking that might do serious damage to the engine.

You need to empty the fuel tank and replace the fuel,do not drive the car until this is done.

The amount of damage done to the engine depends on the mixture of diesel/gas and for how long the engine have been running with this mixture,Do not start the engine until the fuel is replaced.

edit:i will leave this answer as it is for other people with the same problem,it is the best way to solve this problem.

  • As I sad we're way past that - I've already used >10 gallon of something. At this point I've either got 10 gallons left (if they do not mix and the diesel is used last), no diesel left (if they do not mix and the diesel is used first), or a 25/75 mix of diesel/gas. – D Stanley Aug 13 at 16:30
  • diesel and gas do mix so the best you can do now is to top up your tank with the highest octane avaliable and/or add octane booster. – trond hansen Aug 13 at 16:34
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Yes that is the best answer. When you try to burn diesel in a petrol engine, it might run, if it is the MPFI type, as the fuel is atomised, but it would not have started with carbs. The Calorific values of the fuels are so different, the compression ratios are different, infact they are two different beasts, that running a petrol engine any length of time on diesel, or vice-versa is detrimental.

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The only other components that may have been damaged by this are the emissions control components such as any catalytic converters fitted to the vehicle. If it were my vehicle, I'd perform a full service as diesel/petrol mix is potentially more likely to wash the bores and end up in the sump and the fuel filter should probably be replaced along with the spark plugs. I'd also be tempted to run some fuel system cleaner through the tank with the next full fill-up of premium gas. Other than that, in this instance, it sounds that you've likely gotten away without any long term catastrophic affects.

For others in a similar situation, unless you've decanted a very small amount of the wrong fuel into the tank and then topped it completely with the correct fuel (I've done this once, decanting about 1/4 of a litre of diesel) I'd suggest draining the tank rather than trying to drive the vehicle.

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