Port fuel injected (PFI) technology remains the most common fuel delivery type present in the marketplace for gasoline spark ignition engines. Although increasingly stringent tailpipe CO2 targets in some markets are driving the industry towards more efficient direct injection (DI) technology, in the light of ever increasing vehicle lifetimes, a legacy vehicle fleet featuring PFI technology will remain in the marketplace for decades to come. This is especially the case in some Asian markets where PFI technology is still prominent, although DI technology adoption is starting to catch up. PFI engines can, in the presence of lower quality fuels and lubricants, build up harmful 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 formulated in gasoline 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, aims to gain a deeper understanding of the negative impacts of inlet valve deposits on engine performance and efficiency. Using insightful engine metrics an increase in combustion duration in the presence of IVDs was detected, and corroborated by supporting secondary metrics such as emissions. Changes in air-flow are explored using a steady state flow bench as a potential cause for the observed changes in combustion behavior.