So I have a 2011 f350 super duty 6.2 liter with multiple misfires on cylinder 678 and I've recently changed all the coils wires and plugs... I've checked all the valve springs bc I thought it was a broken valve spring due to hearing some engine noise that I never heard before... It all started with hitting a rock with my cat on the driver's side cat and it became plugged and caused the exhaust manifold to start leaking so I hollowed it out but the multiple misfire is still there and the check engine light flashes and the codes I get are misfires on the 3 cylinders mentioned.... The engine noise is new as I've never had engine noise before.... I normally tow a huge tool trailer everyday with it and it currently has 179,000 miles on it... Never had any issues prior to the issues I'm having now... The truck fires right up and runs but smokes pretty good but it's not an oil burning smell, it's like a mix between oil and unburnt fuel smell... I'm lost on what the cause of this is and any insight would be greatly appreciated if anyone has any ideas on what this could be... I've had both valve pans off and checked the valve springs and looked at the timing chains and everything looked good to me... IDK much about the vvt things and don't know if they could cause this or not, the diesel sound at idle has me concerned as I've never heard any noise like that before other than the injector noise... I've checked the injectors and switched them around and the problem stayed in the same place... Any insight would be very appreciated bc I'm at my wits end with this truck... It's been the most reliable vehicle I've had and I wish I could figure it out

 

Get a head gasket test kit.

 

Yep, what @Truckwardo said ^^^^^^^

. . . and/or check compression in all cylinders and post results here.