Hissing sound coming from a small brass vacuum "stem" located on the drivers side on the rear plenum (pics attached) Should there be a hose attached to this? I've never noticed this thing before. CEL went on with a P0171 code AFTER I had new plugs and wires installed. The idle does go up and down every 5 seconds or so, but no noticeable performance issues with regular driving. I did replace the PCV Valve and the MAF seems to be ok. I appreciate any ideas or suggestions.

 

Never mind - problem solved instantly. No more check engine light. Thanks for all the responses