A dishwasher pump and automatic purge system which includes a wash impeller supported for rotation within a pump chamber wherein the pump chamber has a main outlet and a sample outlet port. Wash liquid pumped out through the main outlet is recirculated throughout the dishwasher interior wash chamber. A soil collector receives wash liquid through the sample outlet port. The soil collector includes a filter screen for returning filtered wash liquid back into the sump such that soils are retained in the soil collector. Pressure within the soil collector is sensed by a pressure sensor. The soil collector is purged by a drain pump when the pressure within the soil accumulator exceeds a predetermined limit level such that soils are cleared from the collector and the filter screen. When the pressure within the soil accumulator is reduced to below the predetermined limit level, the drain pump is deenergized. A control valve is provided for preventing fluid flow from the dishwasher sump to the drain pump during the purge operation while the wash pump is operating. The control valve is operated in response to fluid pressure created by the wash impeller.