Theory Of Operation

SYSTEM DESCRIPTION:  There are two sides to the Positive Crankcase Ventilation (PCV) system. The dirty side of the system consists of the PCV Valve, installed on the Valve Cover Oil Separator, and the PCV Tube that connects between the PCV Valve and Intake Manifold. The clean air side of the system consists of the Make-Up Air (MUA) Tube that connects between the MUA Oil Separator in the Valve Cover and the Fresh Air Hose, and Crankcase Pressure Sensor.

NATURALLY ASPIRATED OPERATION:  When the engine is running naturally aspirated, clean air flows from the Fresh Air Hose, through the MUA Tube and MUA Oil Separator, to the crankcase. The crankcase vapors are pushed into the other Oil Separator in the Valve Cover and drawn into the Intake Manifold through the PCV Valve and PCV Tube where they are burned.

BOOST OPERATION:  During boost operation, a check valve in the PCV closes blocking flow through the PCV Tube. The crankcase vapors flow out through the MUA Oil Separator and MUA Tube into the Fresh Air Hose on the clean air side, then into the Turbocharger, and eventually into the Intake Manifold where they are burned.

CLEAN AIR (MUA) SIDE DIAGNOSTICS:  If the MUA Tube is disconnected on the clean air tube, PCV vapors will leak into the atmosphere whenever the engine is in boost. The Crankcase Pressure Sensor is used to diagnose a disconnect in the MUA Tube. The Powertrain Control Module (PCM) monitors the pressure pulsations in the Crankcase Pressure Sensor signal to determine whether or not the hose is connected. If the MUA Tube is determined to be disconnected the P2C90 DTC  is set. The PCM also monitors the sensor and circuits for electrical faults such as opens or shorts.

DIRTY AIR (PCV) SIDE DIAGNOSTICS:  The dirty air side of the system does not have a sensor for detecting a hose disconnect. A disconnected PCV Tube will cause a measurable vacuum leak. However, the range of authority of torque management can keep engine rpm in control despite a disconnected PCV Tube causing a vacuum leak. To diagnose this condition an algorithm using the Gas Flow/Air Flow ratio is calculated and compared to a calibrated threshold. If the diagnostic is determined to be failing the P04FC DTC  will set.