An engine flush is used to flush the gunk out of your engine. Chemicals are poured into the engine, which is then gently idled for a few minutes to make sure the cleaning agent reaches every place oil would normally go. Then the chemicals are drained, just like the oil would be for an oil change. As a matter of fact, an engine flush is often performed after the old oil is taken out and before the new, clean oil is poured in. An engine flush is recommended for older models but the newer models have built-in technologies that serve the same purpose.