The upper intake manifold gasket and/or a PCV vacuum lines/hoses can develop vacuum leaks with age. This type of leak can result in a higher that normal or rough idle.
Car Problem Reports
Chevrolet Monte Carlo Intake Manifold and PCV Vacuum Lines/Hoses Can Leak Due to Age
Chevrolet Monte Carlo Problem
Engines Affected: 3.1L V6, 3.4L V6
Average Mileage: 174,032 mi (137,000 mi - 254,128 mi)
Visitor, 1999 Chevrolet Monte Carlo, 3.1L V6, 155,000 mi
The T at the intake valve cover breaks causing a vacuum leak. This feeds the cacuum to the pcv valve.Irratic idle and racing rpms.
The T at the intake valve cover breaks causing a vacuum leak. This feeds the vacuum to the pcv valve. Irratic idle and racing rpms up 2 or 3K. NAPA had the part in stock Positive Crankcase Ventilation Valve Tube Assembly (Part Number: NOE 7151782). Not hard to replace once motor has cooled down. Since you are replacing this part you might want to replace PCV value grommet (NOE 6301362) and PCV value (MPF 39307) everything was under $20 total.
Idles so low cant get enough power to get on expressway. Idles so high the rpms went from 2 to 3 without touching gas. worst car i ever owned.
Visitor, 2000 Chevrolet Monte Carlo, 3.4L V6, 137,000 mi
My car would act like it was going to stall.