2015 Chevy Silverado P0171 and P0174 Lean Both Banks - Rough Idle

Question

My truck is idling really rough and the check engine light just popped on with lean codes for both banks. I can hear a slight whistling sound coming from the top of the engine when it is running. It seems to drive okay at highway speeds but almost stalls at stoplights.

torque_terry · Accepted Answer

The issue was the PCV diaphragm built into the driver side valve cover. The internal membrane ruptured, causing a massive vacuum leak and that whistling sound. I replaced the entire valve cover assembly (Part #12642655) which includes the new PCV valve and gasket. Torque the bolts to 89 lb-in in a criss-cross pattern. After clearing the codes and resetting fuel trims, the truck idles perfectly.

Sarah Thompson · Answer

Check your fuel trims at idle versus 2500 RPM. If the trims improve at higher RPM, you definitely have a vacuum leak. Common spots on these EcoTec3 engines are the intake manifold gaskets or a cracked PCV hose.

Jordan Lee · Answer

Spray some brake cleaner or use a propane torch (unlit) around the intake runners while the engine is idling. If the RPM stumbles or changes, you've found your leak location.

Marcus Bell · Answer

Check the high pressure fuel pump. Sometimes the internal seals fail and it can affect the mixture, though usually that causes a rich condition. On these 5.3s, I would look closely at the brake booster line connections first.

2015 Chevy Silverado P0171 and P0174 Lean Both Banks - Rough Idle

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