Factors contributing to winter hot starting difficulties encountered in some modern automotive gasoline engines were investigated in a two-phase study. These factors were evaluated first in test cars and then in a test stand engine under more closely controlled laboratory conditions. The effect of oil viscosity on an engine's hot cranking torque requirements and the ability of batteries at various charge levels to supply sufficient power to satisfy these requirements were extensively investigated; whereas the effects of viscosity index improvers, precombustion reactions, engine hot soak time, and oil temperature were only briefly investigated. The present ASTM D 445 viscosity at 210 F was shown to be inadequate for predicting the hot cranking performance of multigrade oils and a method for determining an oil's hot cranking “engine viscosity” was developed. The results show that battery condition and oil viscosity are major factors contributing to the winter hot starting problem and can cause hot starting failures without precombustion reactions contributing to the problem.