They can do a great deal of damage, and it's well documented. It is true that diesel and kerosene are good solvents, but there's a big difference between a solvent and a dispersant which is what engine oil does. It's a well known and often unfortunate fact that using a fuel-based solvent on engine internals can irreparably damage seals and bearing surfaces. Galling on bearing surfaces after such a treatment leads in short order to failure, most often in the crank bearings as they are usually completely immersed during the "treatment". Those with chemistry backgrounds will recognize that fuels such as diesel and kerosene, which are petroleum distillates, alter the chemisty of engine oil inhibiting its ability to film and adhere to engine load surfaces.Originally Posted by Tone
The "myth" of the kerosene/diesel flush and the snake oils based on it are self perpetuating. But what goes on inside your engine block is no myth, and you want one thing in contact with it at all times and that's a properly specified engine oil. Plain and simple. Kerosene and diesel are good for washing engine parts and any other internal use will lead a healthy engine to the parts washer in no time.
I mentioned above that the manuals for my vintage Caterpillars specify using kerosene as an "engine wash", but there are specific procedures followed to eliminate any residue before putting the engine back into operation such as using straight mineral oil as an after-flush conditioner to help the bearing surfaces adhere to fresh engine oil. Also these old engines used leather for seals which was relatively unaffected by kerosene, which can attack and damage the seals in our engines.