I have a 2003 cavalier with the 2.2L ecotec. In the cold weather it runs terribly and doesn't get better even when engine has been warmed up. The engine will shudder when I give it gas from a low rpm before picking up, it runs noticeably noisier, shudders when engine braking, and poor mileage + power. This problem hasn't suddenly appeared either, every winter it strikes, but I noticed every year it gets worse. Used to only occur in extreme cold (-30c) but now occurs when its much warmer (-10c). There are no check engine codes.
A few things I've tried:
I had the car in a heated garage to warm up, after complete warm up drove it (few hours highway driving) with no issues. Parked and cooled off, next day back to running rough.
I have replaced the spark plugs, ran injector cleaner through it. I suspect it is a sensor, maybe in the exhaust? I assume my water temp sensor works as my dash gauge is working. I should mention the car runs perfectly in warm weather.
So I am wondering, where should I start? Thanks!