Mercury Optimax Diagnostic Guide., Page 8Get this manual
OPTIMAX DIAGNOSTIC GUIDE
Procedure 5
MAP SENSOR
The manifold absolute pressure (MAP) sensor is mounted on top of the air plenumThe ECM regulates fuel flow, in part, based on manifold absolute pressureWith the DDT connected run the engineQuickly open the throttle and close it again while watching the MAP sensor readingsA good sensor will quickly change readingsIf the MAP readings do not change or change slowly, it is an indication of sensor problemInspect the sensor opening for blockage by oilClear the opening of oil and reinstall the sensorAnother sign that the MAP sensor could be problem is if the MAP sensor range of values changes by more than 2.1 kPa (0.3 psi) while the engine remains at constant RPM and throttle openingExample: Running at constant 2000 RPM the MAP numbers change from 93.8 kPa (13.6 psi) down to 92.4 kPa (13.4 psi) and then up to 95.1 kPa (13.8 psi), and back againThis would indicate problem with the MAP sensorIf the readings are incorrect, replace the MAP sensor