The best way to grill a pork loin roast in over indirect. Light half the grill and place on the indirect heat side, fat side up. For a 2 to 4 lb. single loin will take 1 to 1 1/4 hours. A larger double loin that is 3 to 5 lbs. will take 1 1/4 to 2 1/4 hours. The internal temperature will reach 160 to 170 degrees.