A laboratory test simulator has been developed to analyze the intake-valve deposit formation mechanism. The characteristics of the deposits formed with the simulator were compared with those of the real engine deposits. This comparison verified that the simulator deposits ILLEGIBLE nearly equal to those of engines. The influence of each parameter such as valve temperature, oil or gasoline quality was tested individually using this simulator. The intake valve temperature influenced the location and quantity of the deposits. The deposit formation significant in the temperature range of about 0-350 °C. The high-boiling components of oil ILLEGIBLE increased the deposits. The increase oxidation products and the decrease of antioxidants in used oil caused a significant increase of deposits. The commercial premium gasoline in Japan containing practical detergents ILLEGIBLE down and decreased the deposits. Another premium gasoline affected the oil quality, in increasing the deposits.