An apparatus for separating solid material from cooling water in the cooling system of an engine block of a marine engine, comprising a marine engine block having a plurality of cylinder bores and having a cooling passage surrounding said bores, said cooling passage containing circulating cooling water, said block having a side wall bordering a side of said cooling passage and having a bottom wall bordering a bottom of said cooling passage, drain outlet means in the side wall of said block above said bottom wall and communicating with said cooling passage, a tubular separator having a first generally horizontal section disposed in sealed relation within said drain outlet means, said first section having an inner end disposed in said cooling passage and having an outer end disposed on the exterior of said block, said separator also including a second section extending generally vertically from the inner end of said first section and located between two adjacent cylinder bores, said second section having a closed lower end, port means disposed in a side of said second section above said closed end and facing one of said cylinder bores, said separator also including a third section connected to the outer end of said first section, a temperature responsive drain valve mounted on the engine at a level beneath said drain outlet means, said valve being characterized by the ability to open when the ambient temperature falls beneath a preselected value, and conduit means connecting the third section of said separator to said valve, a decrease in the ambient temperature beneath said preselected value when the engine is not operating causing said valve to open to drain cooling water from said cooling passage while any solid debris contained in the cooling water will be retained within the lower end of said cooling passage beneath the level of said port means.