I do tri-tip a lot. I don't inject or marinade. I use rubs. Usually cook 2 tri-tip each time so I rub one with Minced garlic, fresh ground black pepper, kosher salt, a little onion powder and cumin. The other just some Montreal seasoning. Wrap them up for a few hours and throw them in the smoker for 3.5 hrs at 250 or until the center is at 145F