Abstract: Malfunctions, such as low power performance or low compression, of individual cylinders of an internal combustion engine are determined by the apparatus herein. Measurements are made as to the values of the time intervals between successive crankshaft positions as the crankshaft of an engine rotates through at least one engine cycle. One engine cycle is defined as the number of power periods corresponding with the number of cylinders of the engine being tested. Each power period includes a plurality of crankshaft positions. At least two time interval measurements for each power period are examined to determine the difference in value. This is an index or indication of the condition of the cylinder associated with that power period. A ratio of each index to the average index for at least two cylinders is obtained. The index ratio for each cylinder is then compared with a threshold and an output may be given as to a faulty condition, such as low compression or low power, at a particular cylinder.