2015 Chevy Silverado 5.3L - P0171 and P0174 Lean Bank 1 & 2 especially at idle

Question

My truck is throwing lean codes on both banks and the idle is starting to feel a bit rough when I am stopped at traffic lights. The LTFT is sitting around +18% to +22% at idle but drops down to near +5% when I get it up to highway speeds. I can hear a slight whistling sound coming from the top of the engine but I cannot pinpoint it.

Megan Rivera · Accepted Answer

The issue turned out to be the PCV Foul Air Tube (Part #12668553) which had a hairline crack near the manifold connection. I replaced the tube for about $35 and went ahead and torqued the intake manifold bolts to 89 lb-in as they were slightly below spec. After clearing the codes, my Long Term Fuel Trims returned to a healthy +2% at idle and the whistling sound is completely gone.

Carlos Diaz · Answer

Check the PCV tube that runs from the driver side valve cover to the intake manifold. They get brittle and crack at the quick-connect fittings, which will cause a massive lean condition at idle.

wrenchqueen · Answer

I would use a smoke machine if you have access to one. Plug it into the brake booster vacuum hose and see where it escapes; if you see smoke coming from the valley under the intake, your manifold gaskets are shot.

Rachel Nguyen · Answer

Since the fuel trims improve at higher RPMs, you definitely have a vacuum leak. Check the intake manifold bolts as they are known to loosen up over time on these Gen 5 small blocks, or check the brake booster line for cracks.

2015 Chevy Silverado 5.3L - P0171 and P0174 Lean Bank 1 & 2 especially at idle

Suggest a repair