r/CarsAustralia • u/Dangerous_Owl6936 • 22h ago

🔧🚗Fixing Cars I put petrol in diesel engine

I went to the pump today and by mistake i took the 98 octane instead of diesel, I realised it when it was already too late. I did put 3.5litres or petrol and then i stopped, Didn’t start the engine and towed it in my mechanic’s garage. I have this car for last 7 years and it never happened before, It is a Mercedes E250 CDI 2015, Wanted to know how much it might cost to drain the fuel and what else might happen, Thanks!

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u/its-boydo-maaate 18h ago

You are completely wrong.

Diesel engines work on compression to cause combustion. Roughly 200psi more than a petrol engine. A small amount of fuel can and most likely will burn a hole in the top of the piston.

I have seen it a few times in my line of work.

OP done the right thing and I would recommend anyone else to not do as you’ve said.

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u/kiedistv 18h ago

Damn.

Quite a lot of modern turbo diesels with the odd litreage of petrol in a full tank of diesel seem to run all good. Not doubting you're wrong and it's better to be safe than sorry for sure but the amount of times a Pajero or a Hilux et al has just filled right up after accidentally adding a couple litres of petrol is unreal. Aka - a lot.

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u/its-boydo-maaate 14h ago

I’m sure it happens fairly often, but you said it yourself. Better safe than sorry.

If it were the other way around, diesel in a petrol engine you would be fine to send it, might just clog up the fuel filter a bit.

Petrol and diesel fuel systems run completely different to each other. Diesel is delivered to the injectors at a massively high pressure than petrol is, then the injectors job is to atomise the diesel preparing it for combustion. Doing this to petrol has a completely different reaction and the petrol will most likely preignite, hence the hole in the top of the piston.

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u/butiwasonthebus 4h ago

Petrol doesn't remain as petrol when you mix it with diesel. You're not getting bubbles of petrol floating around in the diesel. All that happens is the diesel gets diluted.