2002 Nissan Altima 2.5 liter. I got the cylinder 3 misfire code, so I put all new spark plugs and all new ignition coils and now I just have the P0300 code. The thing that is throwing me off is that ever since I put the new spark plugs and new coils the car is running worse. Now it seems like it is going to quit and it has no power. I have no coolant in oil or anything like that. Anybody have any idea on something that I can check before I bring this to someone? The car has 160,000. The pre cat is removed and replaced with headers. New valve cover gasket installed 2 years ago. Thank you for your help.

Hey there, sorry to hear about the issues. I'm not too familiar, but I did find a neat video that may help with a few ideas. It is here: https://www.youtube.com/watch?v=OyvdWeyDHf4
Also, the FSM will have a diagnoses procedure, likely in the EC chapter. Check this link here and get your manual downloaded: http://www.nicoclub.com/archives/nissan ... nuals.html

Those new plugs may have the wrong gap, depending on what you bought. Can you check them again? The O2 sensor after your missing cat might need replacement. It probably sends a different signal also, since there's no CAT in front of it anymore. You could put the OEM manifold back on with the CAT for diagnoses purposes. I take it you aren't getting any other codes besides P0300? Valve cover gasket seems like a non-issue, however, vacuum leak could be and if the valve cover leaks, you have vacuum leak I think.