2015 Chevy Silverado 1500 - P0171 and P0174 Lean Both Banks with Rough Idle

Question

My truck is idling really rough and just threw codes for lean conditions on 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 but I cannot pinpoint it.

Tom Patel · Accepted Answer

Found the issue was a cracked PCV breather tube (Part #12673820) and a leaking intake manifold gasket. I replaced the intake manifold gaskets with a Fel-Pro set (MS97219) and swapped the plastic PCV line. Total cost for parts was about $85 and took 2 hours. Torque the intake bolts to 89 lb-in in two passes following the center-out sequence. Fuel trims are now back to +/- 3% and the idle is smooth as silk.

garagegrease · Answer

If you have a scan tool, look at your Short Term Fuel Trims while you spray brake cleaner around the intake manifold gaskets. If the numbers dive toward zero when you hit a specific spot, you found your vacuum leak. On these L83 engines, the intake gaskets are known to flatten out over time.

Pete Sanders · Answer

Verify your fuel pressure. While lean codes are usually vacuum leaks, a failing high-pressure fuel pump (HPFP) or a weak lift pump in the tank can cause a lack of fuel delivery that looks like a lean condition on both banks under load.

shoptech_carlos · Answer

Check your intake bellows for cracks first. Since it is affecting both banks, the leak has to be somewhere behind the MAF sensor but before the intake manifold runners. You should also check the PCV tube as those plastic lines get brittle and crack.

2015 Chevy Silverado 1500 - P0171 and P0174 Lean Both Banks with Rough Idle

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