2015 Tahoe - P0171 Lean Bank 1 and Rough Idle at Stoplights

Question

My dash just threw a CEL and I am getting a P0171 lean code on my scanner. The truck idles really rough when I am sitting at a red light but seems to smooth out once I get up to highway speeds. I also noticed a slight whistling sound coming from the engine bay when the hood is up.

Brian O'Connor · Accepted Answer

The issue turned out to be the brake booster vacuum line check valve and a hairline crack in the intake manifold plenum. I replaced the intake manifold gasket set (ACDelco 12626354) and the vacuum hose assembly. Torqued the manifold bolts to 89 lb-in in two passes as per the service manual. Total cost for parts was about $115 and it took about two hours to swap. Fuel trims are now back within +/- 3% and the idle is smooth as silk.

road_rambler · Answer

On these EcoTec3 engines, the PCV tube that runs from the valve cover to the intake is notorious for cracking at the plastic connectors. Check the underside of the hose where it is hard to see.

wrenchqueen · Answer

Since you mentioned a whistling sound, grab a bottle of brake cleaner or a propane torch (unlit) and spray it around the intake manifold gaskets and vacuum lines while the engine is idling. When the RPM stumbles or changes, you have found your leak point.

Dave Kowalski · Answer

You should check your fuel trims at idle versus 2500 RPM. If the Long Term Fuel Trims drop significantly when you increase the RPM, you definitely have a vacuum leak somewhere behind the throttle body.

2015 Tahoe - P0171 Lean Bank 1 and Rough Idle at Stoplights

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