I was having a problem all last week with cold starts. The car would barely respond when I hit the gas and then once I could make it repond it would miss and spit and sputter and smoked terrible. I wasn't getting a service engine light at all untill about 5 days of this problem. The problem would go away after the car went to closed loop when it warmed up some. Ran like a champ. But what I found out was I had 2 codes (and forgive me I didnt write them down) about Coolant temp sensor high voltage. I knew I had a good sensor. I just replaced a few months back with a new GM unit. It ended up being the harness to the sensor that was the problem. The terminals in that connector were so spread apart that they wouldn't make contact with the ones on the sensor. All I did was push them back together some and the made contact and no more problems. Hope this helps someone.