[brcidd]

Don't you guys forget that the fuel pump relay is in parallel to the oil pressure switch circuit - and that the fuel pump hot wire may be arcing inside this sending unit/fuel pump switch-- I had a 454 C/K Dually do this so bad that the engine bucked and carried on- until I unplugged the sending unit/switch assembly- my big hint was that the oil pressure gauge would fluxuate- while this happened. I pulled out the sending unit/switch and two terminals were melted- from the internal short-- The 454 engine runs really hot and melts those units down by the exhaust manifold- since that time I have seen two other 454's do the same thing.....just something to have you check... you'll know it is in parallel if your sending unit is a three terminal unit- 1 terminal for the gauge and 2 terminals for the fuel pump switch......The sending unit on a 5.7L I believe is behind or to the side of the distributor........unplug it and see if your fuse blows.....

Also- since your truck has been running badly- this may be the culprit that caused that as well- my dually ran like crap intermittently- and once would barely get me home-- then I had problems with the battery running down overnight (pulling amps)-- then I did amp draw tests- found the Ecm-B fuse was the 1-2 amp draw- and looked at the circuit- and pulled the oil pressure sending unit/switch- to see the melted terminals-along with the oil pressure gauge fluxuation- the switch is fed by a battery hot at all times- and would pull some amps that tried to run the fuel pump- even though the truck was shut off- there was arcing going on inside the switch- even when the truck was running- that was shorting to ground- thats why it ran so bad- the fuel pump was not getting enough current while running and too much (any at all is too much) when shut off---- I threw that sending unit/switch assembly over the barn roof when I finally figured it all out.......