Why does some crack cocaine taste like diesel?

Simply because the process of extracting the cocaine involves a solvent - often cheapest to hand is diesel. As most cocaine is badly washed (chemically) you often get a hint of the solvent used

• toluene, petrol, xylene, diesel etc.

Better solvents cost more - and are easier to spot large amounts being purchased and stored. Few will bat an eyelid if someone orders a few hundred gallons of diesel - especially if there is a generator / tractor / similar nearby.

If the impurity is left behind in the cocaine, it will often remain in it after it has been treated freebased into "crack".

All this and the most exciting drug I take is a pint of Old Peculiar or a glass of Remy :)