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 as soon as I get up to highway speeds, but at stoplights it feels like it wants to stall. I can hear a faint whistling sound coming from the top of the engine bay when it is idling.

fleet_chief_42 · Accepted Answer

Identified the issue as a failed Intake Manifold Gasket set. I used a smoke tester and saw heavy leakage coming from the bottom of the manifold near cylinder 4. Replaced the gaskets with ACDelco 12626354 seals and torqued the intake bolts to 89 lb-in in the proper sequence. The total cost for the gasket kit was about $45 and it took 3 hours to swap. Fuel trims are now back within +/- 3% and the idle is rock solid.

wrenchqueen · Answer

Could be a failing high-pressure fuel pump (HPFP) if your rail pressure is dropping below 400 PSI at idle. However, your symptoms specifically describe a vacuum leak since it clears up under load when vacuum is lower.

Rachel Nguyen · Answer

These EcoTec3 engines are known for getting cracks in the plastic intake boots or having the PCV hose O-rings fail. Also, check the EVAP purge valve; if it is stuck open, it will pull unmetered air into the manifold and cause a lean condition on both banks.

Jake Morrison · Answer

Check your fuel trims at idle versus 2500 RPM. If the trims improve significantly at higher RPMs, you definitely have a vacuum leak behind the throttle body. Use a smoke machine or spray some brake cleaner around the intake manifold gaskets to see if the idle changes.

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

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