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Some people are talking about a fuel station in my area and saying that they are selling contaminated 98 octane fuel.

I have fueled my car up from them with their 98, and I'm a little worried.

What are some signs of contaminated fuel that I should keep out for?

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It depends on what the contaminant is.

If it is water, see Zaid's answer.

If it is flammable liquid the damage depends on how it behaves in the fuel pump, injectors and combustion chamber. It may cause no effect to jamming the injector(s).

If it is solid contaminant, it will be caught by fuel filter or may stuck in the injector.

In all cases (except for large particles) you may get cylinders to fire earlier, later or miss a fire.

On the other hand, one tank of contaminated fuel won't kill the engine; you need to use that fuel for longer times to get into serious trouble. Or it must be really, really bad fuel - contaminant with small amount of fuel. The older engine you have, the worse fuel it can digest without harm.

Watch for misfires and unexpected power output (significantly lower, significantly higher) and throttle reaction.

Run the tank to the limit, fuel 5 litres of trusted petrol and run it to the limit again, then fill it up with trusted petrol.

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If the concern is water contamination, there will be a tendency for the engine to misfire or stumble.

This is because the fuel system will happily deliver the fuel/water mixture to the engine's cylinders, but the engine isn't seeing the amount of fuel it expects, leading to pre-ignition (misfiring) or inadequate combustion (stumbling/hesitation).

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