To your point, diesel is heavier than regular unleaded and will sink in the tank, but the lack of homogeneity causes its own problems:
- Because diesel fuel is heavier than gasoline, it can sink to the bottom of your gas tank, resulting in the injection of both gas and diesel into the intake manifold or the cylinder. Depending on the mix, you can get partially-burned diesel fuel which leaves bigtime deposits on pistons, valves and spark plugs. You get a car or truck that runs terrible, and if you keep driving it, you can cause serious damage
- If enough diesel fuel gets in the cylinder, you can hydro-lock the cylinders, resulting in a blown head gasket, cracked cylinder head or other serious problems that can lead your vehicle down the road to a quick and final death. This diesel fuel in the cylinder can also seep past the piston rings into the oil crankcase, diluting the lubricating oil. This can damage all internal engine lubricated parts resulting in major engine failure from rapid wear.
- If unburned diesel fuel makes its way into the exhaust system, it will ignite in the catalytic convertor. The fire will plug the holes in the catalyst, destroying it and leaving you with a repair job well into the four-figures.
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