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. It seems to smooth out a bit once I get up to highway speeds, but at stoplights it feels like it wants to stall. I can hear a slight whistling sound from the engine bay when it is idling.

Marcus Bell · Accepted Answer

The issue was the brake booster vacuum line check valve and the main vacuum pump line. The plastic had become brittle and cracked near the firewall connection. I replaced the vacuum line assembly (Part #23116140) which cost about $45 and took 20 minutes to install. After clearing the codes and performing a fuel trim reset, the LTFT stayed within +/- 3% and the rough idle is completely gone.

Greg Walters · Answer

These EcoTec3 engines are known for intake manifold gasket failures, but also check the PCV hose. They tend to crack or collapse over time causing a massive unmetered air leak.

Sam Holloway · Answer

Check your long-term fuel trims at idle versus 2500 RPM. If the trims improve significantly at higher RPM, you almost certainly have a vacuum leak after the MAF sensor.

Ashley Park · Answer

Spray some brake cleaner or use a smoke machine around the intake manifold ports while the engine is running. If the idle stumbles or changes, you've found your leak location.

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

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