A comprehensive program was conducted to evaluate the effects of intake valve and port deposits on spark-ignition engine performance. The performance parameters tested included power, exhaust emissions and driveability. Testing utilized single-cylinder and multicylinder laboratory engines as well as vehicles. The results show that all performance aspects tested can be adversely affected by intake valve and port deposits.In addition, the swirl designed into many fast burn engines was disrupted with both synthetic and real intake valve and port deposits. This swirl upset led to some of the poor driveability performance.