Abstract: An apparatus for cleaning internal combustion engines in which a cleaning fluid is cyclically drawn from a reservoir by a pump, flushed through the block of the internal combustion engine, and returned to the reservoir by the pump, is improved by providing it with a pair of cleaning fluid reservoir tanks. Also, tank selection valves alternatively couple one or the other of the reservoir tanks to the pump, to the exclusion of the other tank. In one application of the improved apparatus one tank can be dedicated for use to clean gasoline powered internal combustion engines to preserve the characteristic color of the cleaning fluid, while the other tank can be utilized to clean diesel powered internal combustion engines. In another application the system is operated through first flushing and soaking periods and through second flushing and soaking periods. A first tank in the pair is used to initially pump and recover cleaning fluid during the first flushing and soaking periods.