It misfires mainly at highyway speeds... codes read P0305, P0301, P0302, AND P0306... I just changed spark plugs, it ran better but started misfiring when i got on freeway... whats the next suggestion?
The codes are telling you specifically which cylinders are misfiring. If it was P0300 it would be randomly misfiring so you would look for something that affects all cylinders....like a plugged fuel filter or weak pump or something. Since yours specifies Cylinder 1 (P0301), 2 (P0302), 5 (P0305), and 6 (P0306) I would say look at parts that are specific to those cylinders like the ignition coils or injectors. Just to finish the thought for anyone else reading this... if it was cylinders 1,3,5 or 2,4,6 then look for something that affects the rear bank or front bank like the Variable valve timing (VVT solenoids, etc).
1st thing I would do is address the possibility it is the coils...FREE! Take the 2 known good coils from Cylinders 3 &4 and switch them with cylinders 1 & 2. Clear the codes and go for a drive. When the codes reappear check if the problem has changed cylinders...should be P0303 and P0304 now. If this is the case then you can conclude the problem is the coils and go buy new ones...There are lots at the wreckers. I go to Rock Auto's website and check out what models use that part number then search the yard for good ones. Try to get OEM Denso coils.
If the same codes reappear then you know the coils are not at fault, so it is likely the injectors. You could switch around the injectors the same way you did the coils, but it is a bit more work. I would try running a good injector cleaner through them 1st... I like Gumout's injector cleaner. If/when you pull the injectors check out Youtube for a how to clean injectors video....or take them to a shop that cleans injectors. If you go to the wreckers, grab a bunch of injectors too.