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 notice a distinct whistling sound coming from the engine bay when I first start it up in the morning, but it seems to quiet down once the engine warms up. Fuel economy has taken a massive hit over the last two weeks.

Jordan Lee · Accepted Answer

The issue was the PCV Tubing / Foul Air Pipe (Part #12639698) which had a hairline crack at the plastic quick-connect fitting. I performed a smoke test and saw smoke pouring out from the rear of the intake valley. Replaced the PCV pipe for about $45 and cleared the codes; fuel trims are now back within +/- 3% and the whistling noise is completely gone. Tightened the intake manifold bolts to 89 lb-in while I was back there just to be safe.

Sarah Thompson · Answer

On these L83 engines, it is very common for the intake manifold gaskets to start leaking. Since it is affecting both banks, the leak is likely at the main plenum or the PCV hose connection points behind the manifold.

backyard_bart · Answer

Check your fuel trims at idle versus 2500 RPM. If the trims improve at higher RPMs, you definitely have a vacuum leak downstream of the MAF sensor. Look closely at the intake bellows for cracks.

Ashley Park · Answer

Double check your brake booster hose. Sometimes the check valve or the hose itself splits near the firewall, creating a massive vacuum leak that triggers lean codes for the whole system.

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

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