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 coming from the top of the engine when it is running.

garagegrease · Accepted Answer

The issue was a cracked PCV Foul Air Tube (Part #12661811) located at the rear of the driver side valve cover. It had a hairline crack in the plastic bellows section that was sucking air. I replaced the tube which cost about $25 at the dealer and took 10 minutes to swap out. After clearing the codes and resetting the fuel trims, the truck is idling perfectly at 600 RPM again.

Dave Kowalski · Answer

Check your Long Term Fuel Trims (LTFT) at idle versus 2500 RPM. If the trims improve significantly at higher RPMs, you definitely have a vacuum leak after the MAF sensor.

Chris Donovan · Answer

Check the intake manifold bolts. These LT engines are known for the bolts loosening up over time, which causes the intake gaskets to leak air. You might also check the PCV tube going to the intake.

wrenchqueen · Answer

Spray some brake cleaner or use a smoke machine around the intake manifold gaskets while the engine is running. If the idle changes when you spray a certain area, you've found your leak point.

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

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