Abstract: An engine crankcase gas blow-by sensor. In a closed crankcase ventilation system, crankcase gases are caused to flow through a venturi which includes high pressure and low pressure taps. The high and low pressure taps are coupled to a differential pressure transducer which produces an output that is proportional to the volumetric flow of crankcase gases through the venturi. The use of a venturi in conjunction with a differential pressure sensor offers a low resistance path for the flow of crankcase gases and allows continuous monitoring of blow-by without exceeding the operating pressure limitations of various oil seals. Such a sensor is particularly suited for closed crankcase ventilation (ccv) systems, as it doesn't require venting of crankcase gases to the atmosphere, but works well in open crankcase ventilation systems also.