I have what I would consider an extremly high vacuum on my 2000 dodge ram 5.9. The engine pulls high enough cacuum that you let go of the oil fill cap about an eight of an inch from the fill hole it will be sucked down and held in place BEFORE egaging the threads. It dosen't seem to have any other issues that I could find when I scanned it other than one oxygen sensor is puting out a low reading usually one can fix that by just cleaning them with a wire brush. With the sensor being weak I am getting 10 to13 M.P.G. Anyone have any ideas?

 

Maybe PCV valve stuck open drawing a lot of vacuum? Maybe the plenum gasket is blown causing an internal vacuum leak? You could check your PCV valve and do the TSB test for the plenum gasket. Bout all that comes to mind for me.