2015 Chevy Silverado 5.3L - P0171 and P0174 Lean Both Banks

Question

My truck is idling very rough and the check engine light just started flashing when I stopped at a red light. It seems to run okay at highway speeds, but as soon as I slow down, the RPMs dip and it feels like it wants to stall. I hooked up my cheap scanner and saw long-term fuel trims are sitting at +25% on both banks.

Sam Holloway · Accepted Answer

Found the issue was a leaking intake manifold gasket. I performed a smoke test and saw smoke pouring out from the base of the manifold near cylinder 7. I replaced the gaskets with a Fel-Pro set (Part #MS97219) and torqued the manifold bolts to 89 lb-in in the proper sequence. The long-term fuel trims immediately dropped back down to +2% and the idle is now perfectly smooth. Cost was about $55 for the gaskets and two hours of my time.

Sarah Thompson · Answer

Check the PCV hose running from the driver side valve cover to the intake manifold. These are known to crack or pop off, causing a massive unmetered air leak that the computer tries to compensate for by dumping fuel.

Pete Sanders · Answer

Since you have lean codes on both banks, look for something common to the whole system. Check the intake bellows for cracks and make sure the oil fill cap is tight, as a vacuum leak anywhere after the MAF will cause this.

garagegrease · Answer

If the fuel trims improve when you rev the engine to 2500 RPM, it is definitely a vacuum leak. If the trims stay high even at high RPM, you might be looking at a failing high-pressure fuel pump or a clogged fuel filter/pickup in the tank.

2015 Chevy Silverado 5.3L - P0171 and P0174 Lean Both Banks

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