I had the exact same problem with my 96 5.9.Bought a code reader from autozone(same one they use)and came up with p0123 &p0505. Was told that its my idle air control sensor-p0505,causing p0123 throttle position sensor highoutput to trigger. Didnt mean to hijack the thread just thought this input would help further and maybe someone can chime in on my situation.

 

wow...thats bad to you? I get 10.4 in town and 12 on the highway with my 360 and 3.55 gears.....and losing a quart of oil shows a plenum leak?