Color and lighting look great on these! You did a great job pulling out great tones without added help from reflectors and lights. Since I know that you have far greater skill than I do in posing and lighting I'll refrain from making any comments in that direction. I will add a little comment regarding planning for the shoot though.
Having been under the hood of quite a few cars and trucks in my day, I don't think I've ever seen grease strains get on a person "naturally" the way that they are on the model above. Usually they get there from bumping into grease covered parts, wiping the face with the back of hands and arms and so on. These look to me like they were deliberately applied by finger tips, and had they just been smeared with the back of a hand they would look more authentic to me. I also keep wondering were the grime came from. Engines and cars don't get much cleaner than what shows in the photo.
Now having said all that, I wonder to myself just how many other people would really notice that detail unless they too had the honor of cleaning such grime off of themselves.
So that's my best input on how to improve these photos. I hope it helps.
Here's the rule 10 photo because I know you'll ask for it. Though it makes me laugh putting this photo anywhere close to the ones you have above. I'll honor it though, because it gives everyone else who looks at your photos the perspective needed to put me in the proper experience level.