Dry pork loin
usually means overcooked, as you suspect, but there are other factors which can have some impact on the finished product. For USDA inspected pork, the minimum recommended finished temp is now 145*. If smoked at a higher chamber temp of 250* or so, the carry-over will bring it up several more degrees from where you pull it out of the cooker at, so @ 142-143* finished temp, carry-over will bring it over 145*. I typically finish my loins in the upper 150's giving a carry-over of 160*+ for med-well and it holds it juices nicely at that temp.
Speed (chamber temp) of cooking a loin may or may not play a role in moisture retention in the meat. Pork loin is already tender, so like younger poultry, it doesn't really benefit from low & slow cooking (with the exception of old roasting hens), so if a higher chamber temp may be to your liking, it won't hurt anything, other than give a bit less smoke reaction time.
I have cooked loins indirect on a
weber kettle relatively hot and fast (275-300*), and incidentally at well above 160* (upper 170's to lower 180's) and they still had a fair amount of moisture. Two things help these loins out: a dry cooking environment (dry???)...I'll explain below), and a fat-cap.
http://www.smokingmeatforums.com/t/...gold-w-recipes-hope-you-dont-mind-some-q-view
Was the loin trimmed lean, or was the fat-cap intact? Fat-cap can play a major role in moisture retention by self-basting the loin as the fat renders-down. I've brined loins before and this can help a little, but it's not a huge factor in overall moisture retention...it helps some, but the biggest benefit I've noticed added flavor to the interior meat
Prep methods don't generally effect moisture a lot, such as mustard with dry rub, although a bacon wrapped loin is a tasty way to baste the loin for more moisture retention:
http://www.smokingmeatforums.com/t/94631/bacon-wrapped-loins-in-the-vault-q-view
I have noticed benefits from using a dry smoke chamber with leaner meats for better moisture retention. A dry smoker chamber allows the surface meat fibers to dry and tighten-up, sealing in the natural juices. The draw-back to a dry only smoke chamber is reduced smoke reaction time. To offset this, I use a wet to dry smoke chamber for the best of both worlds.
Lots of reading here, but the1st 7 threads are pertaining to dry only, or, wet to dry smoke chamber...when you have some time to read about a new trick or two, these may give you some ideas what you may want to try for juicier meats:
http://www.smokingmeatforums.com/ne...luvofsmoke&output=all&sdate=0&containingforum[0]=90&replycompare=gt&numupdates=&newer=1&type=all&advanced=1
Chamber temp gauges are also not accurate in most cases. You should be able to verify the chamber temp by placing a digital probe near the door temp gauge stem and most of these gauges can be calibrated by turning a small hex "nut" on the the stem to correspond with your digital probe reading. Your probe should be verified as well by placing the probe tip in boiling water and verifying that the temp is close to what a rapid boil is for your elevation (altitude). Here's a chart listing the temp/elevation:
http://www.hi-tm.com/Documents/Calib-boil.html
A little bit of discussion on thermometer checks, started by Dutch:
http://www.smokingmeatforums.com/t/...ur-thermometer-and-the-boiling-point-of-water
Anyway, there's generally not just one thing that can make a loin dry-out a bit (if not severely overcooked), but there can be several things going on at once, and everything I mentioned can be a contributing factor at some point...preventing some of them can be a part of your routine, if you want to take a few extra steps, and you can work it into your routine in time.
Good smokes to ya, Mike!
Eric