I have a cobalt that idles rough and idles a little high at startup when its cold. When I put it into reverse or any gear it gets worse. Ive cleaned the MAF sensor and Throttle. I've heard the it could be a coolant temp sensor or MAP sensor. After I get a block down the road its cleared up and drives and shifts perfect. What can I do to find out exactly what's wrong. I have replaced spark plugs. Its got a code for the downstream o2 sensor according to my Blue Driver but no check engine light.
What's the specific code?
How many miles are on it? Are you the original owner? If not how many miles were on it when you got it? And does it have the 2.2 or 2.4 ecotec?
93000 miles second owner I have put 2000 on it since I bought it its been doing it since then. It has 2.2 ecotec.
All ecotec engines are picky. Things gotta be 100% or they really run like crap. If your getting an O2 sensor error it can make the fuel system run really rich. If you can wipe your finger in the tail pipe and its thick with black soot that’s the clue. If that’s the case Replace it and you should be fine. Also ecotec motors hate being neglected. Change your oil every 3-4k using synthetic and it will last you a long time. Push it to 6k or more on the GM schedule and eventually your timing chain will barf on you.