2015 Chevy Silverado - P0171 Lean Bank 1 and Rough Idle

Question

My truck is throwing a P0171 code and idling really rough when I come to a stop. It seems to accelerate okay, but the fuel trims are sitting at +20% at idle and drop down closer to zero when I am cruising on the highway. I can hear a slight whistling sound from the engine bay but I cannot pinpoint it.

backyard_bart · Accepted Answer

The issue was the PCV diaphragm integrated into the driver-side valve cover (Part #12642655). On these EcoTec3 engines, the internal valve fails and creates a massive vacuum leak. I replaced the entire valve cover assembly, which cost about $110, and torqued the bolts to 89 lb-in. The fuel trims immediately returned to +/- 3% and the whistle is gone.

Rachel Nguyen · Answer

Try spraying some brake cleaner or unlit propane around the intake manifold gaskets while watching your short-term fuel trims on a scanner. If the numbers dive toward negative when you spray a certain area, you found your leak.

Kevin Tran · Answer

Since your fuel trims improve at higher RPMs, you definitely have a vacuum leak. Check the PCV hose that runs from the driver side valve cover to the intake manifold, as they often crack at the quick-connect fittings.

dieselmike88 · Answer

Don't rule out a failing brake booster. If the internal diaphragm is ripped, it will suck air directly into the plenum. Pinch off the vacuum supply line to the booster and see if your idle stabilizes and the whistling stops.

2015 Chevy Silverado - P0171 Lean Bank 1 and Rough Idle

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