misfires help
 

P0300-random misfire
P0301-cylinder 1 misfire
P0302-cylinder 2 misfire
P0303-cylinder 3 misfire
P0304-cylinder 4 misfire
P1399-misfire in any cylinder

I've tried figuring this out myself and doing research but still no luck getting rid of them. It's my first car so I don't have much experience. So far all I've done/learned is how to clean the IACV valve, replace the intake manifold gasket, change distributer, replace bushings with energy suspension bushings, replace catalytic converter and other simple tasks/maintenance. It's a completely stock ls motor, if anyone could suggest anything I would really appreciate it, thanks.

Missfire on all four usually points to a fuel supply issue.

Check fuel rail pressure. may have a weak pump.