A car's engine is full of varying sensors that communicate specifics about the environment surrounding the devices, so the on-board computer can alter functions for peak performance. The manifold absolute pressure (MAP) sensor is an integral part of this communication system.
The MAP sensor evaluates the pressure within the intake manifold assembly. If a car requires more power, more air will infiltrate the system, causing pressure to lower. The MAP sensor then reacts by reporting the low pressure and the engine allows more fuel into the system to balance the fuel-to-air ratio for more performance power.
If a MAP sensor is wet, the moisture can affect how the device reports pressure levels by not recognizing the appropriate changes within the intake manifold. In addition, the circuitry within the sensor can fail with water infiltration, necessitating a repair or replacement of the device.
The driver will feel the effects of a wet MAP sensor. The car will idle roughly or even surge. Overall power may be diminished due to the fuel-to-air ratio confusion within the engine chamber without the accurate reports from the MAP sensor.
