The NAPCA-Esso study summarized in this report investigated: Feasibility of the recirculation technique when used in conjunction with positive crankcase ventilation and with the Engine Modification System with Manifold Air Injection; design characteristics needed for an automatic system to provide emission controls while retaining fuel economy and vehicle performance. The goal was to determine the desirability of carrying out further studies concerning long-term operability, effectiveness and engine wear effects of exhaust recycle in combination with hydrocarbon and CO control systems. The program was divided into two phases: a laboratory engine phase where four different engines were operated over a wide range of steady-state conditions and a vehicle test phase where two fully-equipped cars were studied under transient conditions.