An internal combustion engine operating on natural gas (NG) spiked with siloxanes has been studied experimentally with the goal of understanding the impact of siloxane impurities on engine performance. These impurities are shown to completely decompose during NG combustion in the engine to form silica microparticulates. These coat the internal metal surfaces in the engine (e.g., the piston heads) as well as the engine's oxygen sensors and spark-plugs, and they also collect in the engine oil. A certain fraction of them, furthermore, are carried out of the engine in the flue-gas, and they deposit inside a catalyst monolith bed placed downstream of the engine resulting in its severe deactivation. These engine studies are consistent with prior fundamental studies by this team that indicate that siloxane impurities readily decompose in the NG combustion environment to form silica particulates that coat exposed metal surfaces. They also point out the critical importance for engine performance of adequately removing these siloxane impurities from NG prior to its use.