Hello new to the boards. Have a an 11 f350 6.2 gas throwing p0171 and 0174.
Driving the truck it has spark knock pretty bad. Ive parked it at this point. Its a tow truck so I need it fixed and back on the road. Ive checked the hoses going to the pcvs they appear to be ok....also upon starting the truck it just sounds weak.....ok guys where do go from here?

 

Most of the time a P0171 and P0174 is caused by either a vacuum leak--more than likely the PCV hose and valve or a dirty or defective MassAirFlow sensor. Low fuel pressure can also cause this condition. check your intake from the filter box all the way to the manifold to make sure properly installed.
So listen under the hood, with the car running, for a hissing noise. if you have one of these, then you have a vacuum leak and need to replace the hose. if not you might still have a small hole, but can't hear it yet.