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. I checked the trims and both long term numbers are pegged at 25% at idle but they seem to drop down when I am cruising on the highway.

Rachel Nguyen · Accepted Answer

The issue was a vacuum leak at the intake manifold gaskets. I performed a smoke test and saw smoke billowing out from the base of the manifold near cylinder 7. I replaced the intake manifold gaskets (GM Part #12626354) and torqued the bolts to 89 lb-in in the proper sequence. Fuel trims are now back within +/- 3% and the idle is smooth as silk.

Luis Hernandez · Answer

Check your intake bellows for cracks. These LT engines are known for the rubber accordion section of the intake tube drying out and splitting, which lets unmetered air in behind the MAF.

Ashley Park · Answer

Don't forget to check the EVAP purge solenoid. If it is stuck slightly open, it will pull vacuum from the tank constantly and lean out the mixture at idle.

Marcus Bell · Answer

Since the trims improve at higher RPM, you definitely have a vacuum leak. Spray some brake cleaner around the intake manifold gaskets and the PCV hose connections to see if the engine RPM changes.

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

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