The reduction of exhaust emissions and particulate matter from internal combustion engines remains a critical challenge, particularly under cold start and warm-up conditions, where a significant portion of total emissions is generated. In spark-ignition (SI) gasoline engines, the formation of liquid fuel films on intake ports wall, piston and cylinder wall surface significantly contributes to unburned hydrocarbon and particulate emissions. Also, the fuel film adhering to the wall can be a cause of the lubricating oil dilution. To address these issues, a novel capacitive sensor, fabricated using MEMS technology, was developed and applied to investigate the behavior of liquid fuel films formed inside the combustion chamber of a single-cylinder engine. The sensor detects changes in capacitance caused by fuel film adhesion to the sensor surface. The sensor was installed in a single-cylinder test engine along with a direct fuel injector allowing for the controlled formation of fuel films on the sensor surface. Ethanol was used as the injected fuel for film formation due to its higher permittivity compared to iso-octane, the fuel used for engine operation. This choice enhanced the sensor sensitivity to film presence. Four experimental configurations were tested, varying the sensor’s location (intake vs. exhaust side) and whether the ethanol spray directly impinged on the sensor. The engine was operated at 2000 rpm with an intake pressure of 90 kPa. The coolant temperature was varied from 20 °C to 80 °C to simulate cold start and warm-up conditions. The transition from motoring to firing operation was used to replicate transient startup behavior, and the sensor output was monitored cycle-by-cycle. Results showed that the sensor effectively captured the formation and evaporation of the fuel film. Sensor output was significantly higher at locations exposed to direct ethanol spray, particularly at lower coolant temperatures, indicating greater film accumulation. Conversely, positions shielded from the spray exhibited minimal signal variation. Additionally, sensors mounted on the exhaust side showed faster recovery to baseline values, attributed to higher wall temperatures promoting quicker evaporation. In conclusion, the developed capacitive sensor demonstrated high sensitivity and reliability in detecting in-cylinder fuel films under realistic engine conditions. Its compact design and ease of integration make it a promising diagnostic tool for studying fuel film dynamics in production engines.