Intake valve deposits ( IVD's ) have been found to cause driveability problems in some modern port fuel-injected engines. Due to the poor repeatability and the lengthy time required for the existing testing procedures, only limited information is available on how and why intake valve deposits form.For a better understanding of the deposit forming mechanism, a short IVD sampling procedure was developed to investigate deposit formation in a single-cylinder CFR engine. To facilitate deposit sampling, an intake valve was modified to accommodate removable sampling coupons. A measurable amount of deposits can be accumulated on a coupon in two hours.It was found that both fuel and lubricant can contribute to the formation of deposits on an intake valve. The fuel-derived deposits came from the high-boiling fractions of the fuel and they were derived from liquid phase fuel. The lubricant can either keep the valve clean by forming a protective layer, or it can form heavy deposits depending on the type of lubricant being used.Valve temperature is the most important physical factor in the deposit forming process. Injector spray pattern and targeting have a large influence on the valve temperature and liquid fuel composition on the valve tulip, consequently, they have large effects on the deposit level. Valve rotation, exhaust gas recirculation, and valve surface material have little effect on deposit formation.