2015 Silverdo 5.3L P0171 and P0174 Lean Both Banks - Rough Idle

Question

My truck is idling really rough and the check engine light just came on for lean codes on both banks. I can hear a slight whistling sound coming from the top of the engine when it is running, and the fuel trims are sitting at +25% at idle but they drop down closer to normal when I rev it up. It has been stalling occasionally at stoplights.

Jake Morrison · Accepted Answer

The issue was a cracked PCV breather tube (GM Part #12639648) located at the rear of the driver side valve cover. The plastic had become brittle and snapped, creating a massive vacuum leak. I replaced the tube and also swapped the intake manifold gaskets (ACDelco 12626354) as a precaution since I had to move some wiring. Total parts cost was about $65, and the fuel trims returned to +/- 3% immediately after a KAM reset.

Luis Hernandez · Answer

Since the fuel trims improve at higher RPMs, you definitely have a vacuum leak. Check the PCV hose and the intake manifold gaskets, as these are common failure points on the Ecotec3 engines.

Ashley Park · Answer

Check your evaporative emission purge valve. If it is stuck open, it will draw unmetered air from the fuel tank into the manifold and cause these exact codes. Remove the vacuum line from the valve and see if you feel suction while the valve is unplugged.

Chris Donovan · Answer

You should check the brake booster. If the internal diaphragm leaks, it pulls extra air into the intake. Try pinching off the vacuum line to the booster and see if your short term fuel trims drop immediately.

2015 Silverdo 5.3L P0171 and P0174 Lean Both Banks - Rough Idle

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