4. Cook Slowly
Cover the slow cooker and cook until the beef reaches your preferred level of doneness.
Because sirloin is a relatively lean cut, avoid assuming that a long cooking time will automatically make it more tender. Unlike tougher cuts such as chuck roast, sirloin can become dry if it is cooked well beyond its ideal temperature.
Use an instant-read thermometer to check the thickest part of the meat.
For a whole beef roast, the USDA recommends a minimum internal temperature of 145°F (63°C) followed by at least a three-minute rest.