Hello all, new member

I just bought a 2008 STX 4x4 with a 4.6, 109000 miles. It has a rough idle thing going on.

What I've done:
Cleaned throttle body
Cleaned MAF
Cleaned PCV valve
Replaced plugs with Motorcraft OE plugs

It's throwing a P0420 - one of the cats is new, one is not. The one that's not is being replaced tomorrow. It also has a pending code of P0316 - misfire at startup in first 1000 RPM's. It doesn't have a code specifying which cylinder.

Can a bad cat make it misfire? What else can I do to try to get it to idle smoothly? Acceleration seems normal (as good as it can be with a 4.6) but the rough idle annoys me.

Any suggestions would be welcome.

 

A plugged cat can make it run like total crap.

Did you use dielectric grease in the cop boots when you changed the plugs? Might want to throw a bottle of Techron in the gas in case the injectors are getting dirty.

 

IAC??? does it ever turn off after rough idle

 

No IAC on a 2008, it has an electronic throttle.