Examples of my stuff with lines added-

The ones with lines don't look that bad but they tend to look a bit flatter and less realistic. Almost nothing in real life has a line around it, usually you get a hard edge between light and dark, not really a line.
I'm pretty sure your jeep will look a million times better if your line drawing was either more subtle or if it was selectively erased or masked out. You can use your shadows and highlights to make the shapes on the Jeep, like you already have, and not rely on the line drawing to separate the shapes.
But do keep in mind, this could be a stylistic approach you are using and not intended to be ultra realistic.
If this is the look you want, then it's 100% right

I'm sure other will put in some comments too.
Guys?