GMC Vacuum Leak
This bulletin applies to 2015-2017 GMC Terrain vehicles. A customer may bring their vehicle to the shop with a complaint of an SES light on. The technician may find DTCs P0171 and/or P0174 stored in the ECM. The possible cause is a vacuum leak in intake or purge.
Ensure that the engine is at operating temperature and monitor and record long and short term fuel trims. Plug both the manifold hole and the disconnected foul air tube hole.
Shut the engine off and disconnect the foul air tube form the intake manifold. Detach the foul air tube from its connection and plug both sides.
Restart the engine and monitor short and long fuel trims. If both the previously recorded long term and short term fuel trims were at a positive value above 5 and now the short term has gone to a consistent negative value during this test, replace the foul air tube. If the previously recorded short term is at a value above positive 5 and the long term is between -2 and 2, but during the test the short term has gone to a zero or negative value, replace the foul air tube.
If the fuel trims in steps 1 or 2 did not change, proceed with a purge valve test.
PURGE VALVE TEST
Shut off the engine and ensure that the foul air tube and manifold plugs have been removed and re-install the foul air tube to the manifold.
Locate the purge valve on the intake manifold.
Disconnect the vacuum hose and power supply from the purge valve.
Restart the engine and place your finger over the open port on the purge valve.
If you feel vacuum on the purge valve port, it needs to be replaced. If no vacuum was felt at the valve port, further diagnosis is required.
Note the location of the purge valve.
Disconnect the purge valve vacuum hose and power supply.
Place you finger over the purge valve vacuum port and feel for vacuum.