P0300 is the code for a random intermittent misfire. This means that it the problem has to be something that effects at least 2 or more cylinders from firing properly. Even tho you just put in new spark plugs I would pull them and check all of them to make sure the gap is set correctly. If all the gaps are set correctly. By removing them you can also see if they are wet at all which would indicate that they are not firing which could mean a bad plug or coil.
I would also have your battery tested. Even if it starts the car it may not have enough juice to run the entire electrical system properly. Another part of the electrical system you should check is your coils. If you have a bad coils the spark plugs will not fire correctly.
Another cause could be a lack of fuel getting to the cylinders. I would test your fuel pressure to make sure its within specs. If its low the first thing I would do is replace the fuel filter. This should be changed about every 50k miles or so if its dirty can cause a decrease in fuel pressure. If you still have low pressure after replacing the filter you may need a new fuel pump or have clogged injectors
Lastly random misfires could be a result or air leaks. I know you stated you have no leaks but I would double check all your vacuum line and intake manifold for cracks or leaks.
2006 Cobalt LS 2.2L Ecotec
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Last edited by biniecki; 05-08-2013 at 01:24 PM.