The map sensor found on all 98.5 and up Cummins equipted trucks is very important as your fuel delivery rate is partially determined by the MAP.The MAP sensor is located on the drivers side of the motor just above the lift pump and rearward of the fuel filter side of the intake manifold,Its just below the IAT sensor. The MAP sensor reads the manifold air pressure and sends a signal to the ECM to regulate fuel timing,fuel control, As air pressures vary the sensors resistence changes the signal to the ECM. It has been my experience that if an exhaust brake is used that the MAP can become fouled with soot and the sensor becomes inaccurate, I personally clean mine every 10k miles with carburator cleanor, The torque spec on the sensor is 10 ft. lbs.this makes the allmighty Cummins happy and idles better as well as better overall performance including MPG.