2004 lasabre. 3.8 Oil cap on sqeals cap off no squeal but rough code po171

Engine can not breathe! .. The pcv valve is pulling constant vacuum on the engine while it is operating - therefore there must be a clear pathway for ambient air to enter the crankcase through the crankcase breather .. The squealing sound is air trying to get into the crankcase via any available entry point so this may even be the rubber lip of an oil seal making the noise.. I know this seems far fetched but I have actually had the privilege of repairing a few of these particular strange / unusual failures. One place to check is the bottom of the air cleaner assembly for possible mouse nesting material.. Otherwise you’ll need to seek *hands-on* assistance from qualified personnel. Good luck!