Port fuel injected (PFI) technology remains the most common fuel delivery type present in the marketplace for gasoline spark ignition engines and a legacy vehicle fleet featuring PFI technology will remain in the market for decades to come. This is especially the case in parts of Asia where PFI technology is still prominent, although direct injection (DI) technology adoption is starting to catch up.PFI engines can, when operated with lower quality fuels and lubricants, build up performance impairing deposits on a range of critical engine parts including in the fuel injectors, combustion chamber and on inlet valves. Inlet valve deposits (IVDs) in more severe cases have been associated with drivability issues such as engine stumble and engine hesitation on sudden acceleration. Deposit control additives in gasoline formulations are a well-established route to managing and even reversing fuel system fouling.This study, involving an industry standard, Mercedes-Benz M-111 PFI bench engine heavily augmented with measurement equipment, was able to obtain a deeper understanding of the negative impacts of IVDs on engine performance and efficiency. By using a test cycle based on the CEC method F-20-98 but of increased severity it was established that IVDs lead to an increased combustion duration as measured based on the delta of the spark point to the center of combustion (CA50, MFB50%). This sluggish combustion behavior was corroborated by supporting secondary metrics such as engine out emissions and increases in exhaust temperatures. Changes in air-flow into the combustion chamber were investigated using a steady state flow bench as a potential cause for the observed changes in combustion phasing and resulting sluggish combustion.