Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Diagnosis And Testing
Turbocharged System
DIAGNOSTICS
The Powertrain Control Module (PCM) monitors the dirty air (PCV Valve) and clean air (Make-Up Air) sides of the Crankcase Ventilation System for disconnects. The monitoring strategies are explained in the component sections below.
DIAGNOSTICS - DIRTY AIR (PCV) SIDE
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 and likely idle quality issues. 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 a DTC (P04FC) is set.
Since this diagnostic uses the calculated Gas Flow/Air Flow ratio to make a determination of a failure, any issue that effects the pressure in the Intake Manifold can cause this fault to set.
DIAGNOSTICS - CLEAN AIR (MUA) SIDE
If the Make-Up Air (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 Crankcase Pressure Sensor connects to a fitting on the Clean Air Tube that is also connected to the MUA Tube. The 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 a DTC (P2C90) is set. The PCM also monitors the sensor and circuits for electrical faults such as opens or shorts.
Non-Turbocharged System
DIAGNOSTICS
Apply parking brake or block wheels before performing any test or adjustment with the engine operating.
- Remove the make-up air hose from the air cleaner body. Hold a piece of stiff paper (parts tag) loosely over the end of the make-up air hose.
- After allowing approximately one minute for crankcase pressure to reduce, the paper should draw up against the hose with noticeable force. If the engine does not draw the paper against the hose end Inspect the PCV valve and hose for blockage.
- Turn the engine off. Remove the PCV valve. The valve should rattle when shaken.
- Replace the PCV valve and retest the system if it does not operate as described in the preceding tests. Do not attempt to clean the old PCV valve.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.
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