My 2008 Limited has recently begun running really rough. It's our 3rd vehicle and no longer gets driven very often, maybe once a month, and the last time I drove it the engine was sputtering pretty bad at one point and the check engine light was flashing. I took it to the local O'Reillys and had them put the code reader on it and it gave the P0303/ Cylinder 3 Misfire code. It's a rebuilt engine (still the 5.3L) with 20K miles on it.

Is my cylinder 3 sparkplug already toast? Or, could it be there's water/condensation in my gas that's causing the misfire? Or maybe it's something I'm not smart enough to realize??

I really hate changing spark plugs in this motor because I've snapped a few off in the past and it was expensive to get them removed at a shop.

What's my next move here? My end game is to get it running good again so I can sell it.

Thanks in advance!

 

It's not a 5.3l. It's a 5.4l.

If it's a rebuilt engine and you didn't use old two piece plugs, you will not have a snapping issue.

Next move would be to do a cylinder compression test of all cylinders and post that information.

Along with checking the wiring loom for #3 for damaged wires when swapping engines.

 

Swap coils with another cylinder and see if the misfire moves with the coil.

 

I hope this is not bad news for you, But I fought this and It turned out being broken intake rocker. Follow Ford locomoco Utube guy from Chicago area. The number three cylinder 90% of the time has a oiling issue with lifters. There are new style rockers that work better. I rebuilt entire front end of engine (timing chains, cam phasers, etc.) also installed high velocity Melling oil pump. My rocker (intake) was gone and valve hanging by one keeper, the cam shaft was destroyed. by the way changed plug, coil, injector, and none of these fixed issue. but as said above swap coil packs and see if code moves.

 

It's FordTechMakuloco. This is the video that's referred to:

 

Start by replacing the COP first...I had the P0303 several months ago and P0304 last week, Been meaning to swap all the plugs and coils but just didn't have the time. ONLY buy the Motorcraft DG511 as these engines do not like off brand coils. And if you're doing plugs, Motorcraft SP-546 one piece plugs should set you right.
And FordMakuluco's vids are really good. Here's him fixing the P0302:

 

Denso and Delphi are acceptable. The OEM Motorcraft coils are made by Denso. Delphi is GM's OEM.

There's a question whether the SP546 is really a one piece plug. If it isn't, it's been redesigned with a stronger weld between the 2 pieces to help avoid breakage and a taper to minimize carbon buildup.