hey guys, need some help i have an 03 xls. it shooting a random miss fire and running lean codes P0172,P0174,and P0316. i changed spark plugs and wires. I also did the coil cap, and cleaned the MFA. i noticed that i had a leak in the right rear corner valve cover. is it possible. that oil is get in the manifold? it idles real bad. and lacks power at times.(its worse in reverse). should i replace gasket? and im think may the upstream o2 could be bad cause of the leak. any advice on this would be real grateful.