2007 Chevrolet Cobalt rough idle

There shouldn't be more than a 10% variation between cylinders. This is excessive. However, I agree with you, I would think that both would misfire and there would be a power loss. When you replaced the plugs, what condition were the old one in? Also, have you tried a wet compression test to see if the compression changes?

Thanks for your response. The plugs were lite tan in color with no sign of oil foul. The wet compression tests show no significant change in compression in any cylinder, which leads me to believe no piston ring problem. What perplexes me is, if the low compression on # 1-3 cylinder 90# is due to a valve problem why only a code 303 and no 301 code, not even a 301 pending. In your expert opinion would a valve problem cause this engine to miss- fire only at idle? I have move the plug, coil, fuel injector to an alternate cylinder cleared the code, after 3 ignition cycles code comes back 303". Is there an answer to this problem other than (It's a Cobalt?) I will do a leak down test as soon as time permits. Thank you for your time and expertise. J.W. Simms.

With that big of a variation between cylinders, I'm surprised it runs good at any RPM. To be honest, if the wet compression test really didn't make a difference, then it has to be related to the valves or there is a timing issue (but that would noticed at all speeds). And yes, weak compression can cause an issue at an idle. I think you have a couple stuck or burnt valves or a leaking head gasket (in two places).