There is a tech bulletin about fueling problems when the PCM switches from closed loop(cold) to open loop(warm) operation, this may be your problem the computer needs a re-flash at the dealer.
3.1L ENG IDLE SURGE ON COLD START - NEW FLASH PROM TECHNICAL SERVICE BULLETIN Reference Number(s): 67-65-31, 67-65-31A Related Ref Number(s): 67-65-31, 67-65-31A ARTICLE BEGINNING IDLE SURGE ON COLD START (NEW FLASH PROM) Model(s): 1996 Buick Century 1996 Oldsmobile Cutlass Ciera Section: 6E - Engine Fuel & Emission Bulletin No: 67-65-31A Date: November, 1996 VEHICLES AFFECTED 1996 Buick Century and 1996 Oldsmobile Cutlass Ciera with 3100 V6 engine (VIN M - RPO L82).
CONDITION Some owners may comment that the engine idle will fluctuate on the initial cold start of the day prior to the vehicle being shifted out of Park position. This typically occurs at the point the emission control system goes from "open" loop to "closed loop" status. This condition is most noticeable in warmer ambient temperatures. CAUSE The idle surge is caused by the difference in fueling between "open" and "closed" loop operation.
CORRECTION A new flash prom has been released to correct this condition. The following changes have been incorporated in the new flash prom: 1. Time delay increase to closed loop. 2. Smaller fueling corrections in the transition from open to closed loop. NOTE: This bulletin supersedes Technical Service Bulletin 67-65-31, dated September, 1996 and revises Correct Labor Operations Warranty Information. Page 1 of 2 3.1L ENG IDLE SURGE ON COLD START - NEW FLASH PROM -1996 Oldsmobile. 10/18/2008
Saturday, October 18th, 2008 AT 7:14 AM