The Check Engine MIL may be illuminated along with DTCs P0171 (Fuel Trim System Lean Bank 1) and/or P0174 (Fuel Trim System Lean Bank 2) set in the Engine Control Module (ECM) on some 2012-2019 Enclave, Traverse; 2012-2017 Acadia; 2015-2019 XTS, Impala; 2015-2016 Colorado, Canyon; and 2015-2017 Equinox and Terrain models equipped with the 3.6L V6 engine (RPO LLT, LFY, LFX). There may be a possible vacuum leak condition in the intake manifold.
To determine if there is a vacuum leak, first record the long term and short term fuel trims with the engine at operating temperature. Plug both the manifold hole and the disconnected four air tube hole.
Next, turn off the engine and disconnect the foul air tube from the intake manifold. (Fig. 15) Plug both sides. Restart the engine and monitor the short and long term fuel trims.
Fig. 15
Fuel Trim Results
If both the first recorded long term and short term fuel trims were at a positive value above 5 and then the second recorded short term fuel trim went to a consistent negative value, replace the foul air tube.
If the first recorded short term fuel trim was at a value above positive 5 and the long term fuel trim was –2 to 2, but the second recorded short term fuel trim went to 0 or a negative value, replace the foul air tube.
If the first recorded and second recorded short and long term fuel trims did not change, perform the purge valve test.
Purge Valve Test
If the fuel trims did not change after the foul air tube was plugged, turn off the engine and make sure the foul air tube and manifold plugs have been removed. Reinstall the foul air tube to the manifold.
Locate the purge valve on the intake manifold and disconnect the vacuum hose (Fig. 16, #3) and power supply (Fig. 16, #4) from the purge valve.
Fig. 16
Restart the engine and place your finger or a vacuum gauge over the open port on the purge valve. (Fig. 17) If you feel vacuum or rear vacuum on the gauge at the port, it will be necessary to replace the purge valve.
Fig. 17
If no vacuum was found at the purge valve port, refer to the appropriate Service Information for the applicable DTCs set in the ECM.
– Thanks to Aron Wilson
