In a skillet, heat oil over medium heat. Add onions and cook until soft. Add garlic, chili powder and pepper, cook for 1 minute, stirring. Add chili sauce, brown sugar, vinegar, Worcestershire sauce and liquid smoke. Stir to combine and bring to a boil.
Place pork in slow cooker stoneware and pour sauce over. Cover and cook on low for 10 to 12 hours or on high for 6 hours, until pork is falling apart.
Transfer pork to a cutting board and pull the meat apart in shreds, using 2 forks. Return to sauce and keep warm. When ready to serve, spoon shredded pork and sauce over warm buns.