I attempted my first over night cook last nght, a pork butt. I filled the BGE with fresh lump most of the way to top of fire ring. Using a single fire starter piece I lit the fire and after about 10 minutes the starter was gone and I could see nice red embers and a small amount of visible flame, so I closed the lid and set lower vent to about 1/4 open and top vents with small holes open to allow temperature to stabilize. That configuration overshot my target temperature and got to a bit over 400 done, so I tinkered for almost two hours until I got dome to stabilize under 300, at which point I had the lower vent maybe 1" open and top vent with small holes 1/2 open.
I added wood chips, place setter, drip pan, grate and butt with remote thermometer, and monitored the temperature for two hours: the dome stayed at about 275 and the probe temperature rose steadily. After two hours (at 10:30) I felt comfortable and went to bed, intending to let it go all night.
I ended up waking up at 1 so I checked and dome was 275 and probe was 160...everything seemed right on schedule, so back to sleep.
I woke up at 5, and dome had dropped to below 200 and probe had dropped from 160 down to 150. Definitely not part of the plan.
I thought maybe I had run out of lump, so I took everything off, but there was still lump and a small red area of glowing ember. I did notice that the fire didn't seem to burn evenly: the center had burned leaving a ring of wood around the edges, and there was a single lit spot which was against the edge of the Egg ( at the 3 o'clock position if the lower vent is 6 o'clock). Is that normal? I stirred to lump and opened the vents wider, we'll see what happens.
Where did I go wrong? What caused this?
LBGE & SBGE. Central Texas.