Just replaced valve covers on Nissan Maxima 2002 due to misfire. Now it smokes and burns oil every start? What's should I do .

Blue smoke is burning oil. Caused from bad rings, bad valve seals and valve guides are worn out. Since you have power, I would suspect the valves. Head job is the only way to fix this. Just a note, this is not common on 10 year old Nissan. To do a quick check for the piston rings. Start the car. Take the oil fill cap off. If you feel oil shoot out a foot above the opening, the rings are shot. Also when rings go bad, you have oil blow out about every gasket and seal. If this is the case, replace the engine or rebuild it.

This engine may not be on time. When timing belt was replaced maybe they didn't installed correctly and now is not synchronized (is not on time) this is the main reason engines experience misfire after replacing timing belts. If you take valve cover out and notice antifreeze in there, you have a blown head gasket.

Get two things done, and these tests will tell the condition of the piston rings, and valves. Compression test, and a leak down test.

White smoke is usually the sign of a blown headgasket.. I would have the coolant system pressure tested before any serious damage is done.

On the DOHC: The Spark Plug Tube Seals (Washers) on the Valve Cover has developed a leak.
Install new seals.
Remove the Valve Cover to Install the Seals.
If the Valve Cover Gasket have not been replaced recently, go ahead and replace them as well, making sure you clean thoroughly BOTH surfaces before installing any new Seal Washers and Gasket.

Let me me know if this helped, or if you have additional information or questions. Feel Free to contact me on FixYa.com!

ps...I'm replacing (for free!) the Valve Cover Gaskets today on a 1994 Nissan Maxima. Looks like we both will have fun!
If you need the Valve Cover Torque Sequence / Specs let me know on the Comment Section of this Post.

The best test is a compression test.
Blue smoke means it's burning oil, due to worn out piston rings.
White smoke means it's mixing antifreeze and oil due to a bad head gasket or cracked head or block.

If you love the car as you say than be prepaired to shell out the bucks for a rebuild or replacement engine, because thats the only fix for your situation. You may not see any blue smoke but believe me its burning it, or you got a leak witch you say you don't. Its going someware and most likely its out the exhaust.

You must have a leak under your hood. Find the leak and fix it.

Replace your valve covers do to clogged pcv piping

this might help

http://www.autozone.com/N,315599/initialAction,repairGuide/shopping/repairGuide.htm?pageId=0900c15280075f05

Not the same year, but real close,

Also check maxima.org, or

http://www.courtesyparts.com/betasite/maxima-a33b-2000-2003/genuine-nissan-parts/-c-1956_1957.html