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

Question

My truck started idling really rough at stoplights and the check engine light just popped on with lean codes for both banks. It seems to drive okay at highway speeds but feels like it wants to stall when I'm stopped. I can hear a slight whistling sound coming from the top of the engine during idle.

torque_terry · Accepted Answer

The issue was the PCV Foul Air Tube (Part #12639698) which had a hairline crack in the plastic bellows section causing a massive vacuum leak at idle. I replaced the tube which cost about $25 and takes five minutes to snap into place. After clearing the codes and performing a fuel trim reset with my scan tool, the Long Term Fuel Trims returned to +/- 3% and the rough idle is completely gone.

turbo_tony · Answer

You might want to spray some brake cleaner around the intake manifold gaskets while the engine is running. If the RPM stumbles or changes, you found your leak. Also, check the vacuum line going to the brake booster as those plastic clips can crack.

Jake Morrison · Answer

Check your fuel trims at idle versus 2500 RPM. If the trims improve at higher RPMs, you definitely have a vacuum leak. On these EcoTec3 engines, check the PCV tube and the intake manifold bolts as they are known to loosen up over time.

Jordan Lee · Answer

Don't rule out the high pressure fuel pump (HPFP) or a failing fuel injectors, but usually with both banks going lean simultaneously, it is an unmetered air leak behind the MAF sensor.

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

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