2015 Tahoe - P0171 and P0174 Lean on both banks at idle

Question

My truck is idling really rough and the check engine light just came on. It seems to smooth out once I get up to highway speeds, but it feels like it wants to stall at red lights. I hooked up my scanner and I am seeing high positive fuel trims around 20 percent on both banks.

hd_trucker_77 · Accepted Answer

Found the issue after a smoke test. The intake manifold gasket had a significant leak on the underside of the plenum. I replaced the gaskets with a Fel-Pro MS97219 set which cost about 45 dollars. I followed the torque sequence starting from the middle out to 44 lb-in for the first pass and 89 lb-in for the final pass. The fuel trims are now back to +/- 3 percent and the idle is rock solid.

wrenchqueen · Answer

Check the PCV tube that runs from the driver side valve cover to the intake manifold. Those plastic lines get brittle from heat and often crack right at the quick-connect fitting, causing a massive unmetered air leak.

Pete Sanders · Answer

Have you checked the brake booster? Sometimes the internal diaphragm leaks and will pull enough air to lean out both banks. Try pinching the vacuum line to the booster and see if your short term fuel trims drop back toward zero.

Megan Rivera · Answer

Since it clears up at higher RPMs, you definitely have a vacuum leak. When you open the throttle, the vacuum leak becomes a smaller percentage of the total air intake, which is why the fuel trims look better at cruise than at idle.

2015 Tahoe - P0171 and P0174 Lean on both banks at idle

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