I just put new ford motor sports headers on my 94' F-150 5.8L. I broke the EGR tube that goes to the header, and now i have got just an open hole where it goes into the header. I am still in the process of finishing the rest of the exaust also. I put on cherrybomb glass packs and thats about it right now. I am going to run the exaust out the sides just behind the cab corners. After i had put the headers and glasspacks on i took it for a short ride down the road (to see if parts store was still open), and on my way back home my check engine light came on, and it didn't seem like the truck had much power! Is this because the EGR hole is open on the header, or because there isnt enough back pressure, both... or something completly different?

 

Probably all of the above. You'll need to either eliminate the EGR and cap off the header hole or replace the EGR tube. That should get the CEL to shut off.

Also, sometimes when a less restrictive exhaust is put on the vehicle will gain some HP, but lose some torque. That could cause it to feel like a little power is gone.