The ultimate cause of the long-existing smoking failure of an engine type was explored by an explorative study of comparison analysis of flow tests

fuel nozzle structure analysis and spring failure mechanism analysis. The proof analyses of fuel nozzle and manifold flow tests on a smoking engine and a non-smoking engine show that the ultimate cause of engine smoking failure is the un-uniformity of manifold flow at the idle condition resulting from back pressure decrease on some fuel nozzles. The proof analysis of the structure of 14 nozzles of a smoking engine shows that the ultimate cause of the drifts of nozzle and manifold flow characteristics are the loss of the pressing mechanism of the seal spring

and the ultimate cause of that is the excessive un-evenness of the spring end leading to un-uniformity of pressure on the uneven end during nut screwing

which leads to abnormal fall-down and plastic deformation of the spring.