Been working on my 2004 F-150 5.4L. Initially got the P0303 and P0304 CEL codes. Replaced coils on 3 & 4. Still ran rough, was not right. Got codes 300, 303, 316, & 174. Replaced all spark plugs (in case plug 3 or 4 was bad instead of coils). Still not good. Swapped coils 3 & 4 with 1 & 2. Got codes 303 and 351. Replaced coil 1.

We think it is a vacuum leak somewhere, but can't find where. Does anyone have diagrams of the vacuum lines on the 04 F150 5.4L? Could it be the Fuel Vapor Separator Tube that is connected very near cylinder 3 on the intake manifold? Any other thoughts on what may be causing the 303 to repeatedly come up, even though the plug and coil have been replaced (other than vacuum leak which I'm trying to find)?

Thanks for any help in advance.
BAM

 

Ok, update to what is wrong. We eventually pulled the valve cover off and found the exhaust valve spring loose, the rocker gone, cam damaged. Replacing camshaft, rocker, changing oil (in case any metal flakes are in the oil). Should be good to go.

 

Good job! That happens on occasion, with the rockers. Sucks, but it has happened before.

 

Jim,

Why does the 3v motor seem to chew up the roller rocker bearings on cylinders #3 and sometimes #2?

 

You talkin to me ? Yea, I thought he had a 2 valve. My mistake. So Iduno.