Tech Tip: PCV Crankcase Pressure Control Diaphragm

Subject: DTC P0171 (Fuel Trim System Lean) may be set in the Engine Control Module (ECM) and the Check Engine MIL may be illuminated.
Vehicles Involved: 2011-2016 Cruze; 2012-2015 Volt; 2012-2019 Sonic; 2013-2019 Encore, Trax; and 2014-2017 ELR models equipped with the 1.4L 4-cylinder engine (RPO LUJ, LUU, LUV).

Condition: These conditions may be caused by a torn Positive Crankcase Ventilation (PCV) crankcase pressure control diaphragm, allowing un-metered air to enter the crankcase.
To determine if there is a tear in the PCV valve, first remove the engine cover, verify the engine oil dipstick is fully seated and that the oil fill cap is fully tightened, and then start the engine. With the engine running, listen for a noise from the PCV valve. If a noise is heard, check if the noise gets quieter or stops with the engine oil dipstick removed.
Next, check to see if there is vacuum at the PCV vent at the front of the PCV valve by placing a piece of tissue near it. The tissue will be sucked against the vent and held there if vacuum is present. Smoke also can be used to see if it is drawn into the PCV vent port. Normal operation of the PCV crankcase pressure control diaphragm would cause a tissue to flutter against the port (smoke would not be continuously drawn into the port).
A torn crankcase pressure control diaphragm would cause the tissue to be held against the port (smoke would be continuously drawn in) due to internal engine vacuum leaking through the diaphragm. If the PCV valve is making noise and there is vacuum present at the PCV valve vent, replace the camshaft cover following the appropriate Service Information. If the PCV valve does not make noise or leak vacuum, the PCV system is not the cause of the condition. Follow the appropriate diagnostics for DTC P0171.


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