While El Pollo Loco doesn't give away its recipes, you can find knock-off recipes on sites such as Food.com and Epicurious. BigOven.com also has a knock-off recipe for the chain's marinade.
To create a marinade for 4 pounds of chicken, mix together 6 ounces pineapple juice, 2 tablespoons lime juice, 1 tablespoon white vinegar and 2 minced garlic cloves. Also add 1/2 teaspoon salt, 1/2 teaspoon oregano, 1/2 teaspoon pepper, 1/4 teaspoon chili pepper and 1 tablespoon vegetable oil.
To cook the chicken, marinate it in the mixture overnight. Grill the chicken over indirect heat, and on the final turn, cover the chicken in the rest of the marinade.