The black caps are plastic and quite cheapy; I used pliers, as you suggested, and pulled off 2. Underneath is what I assume are the wheel studs; I don't see anything that looks like a nut. The studs are in a round opening in a metal piece; the studs don't protrude beyond the metal piece. Those openings are too small to fit a nut into and screw onto the studs anyway. I don't know why these 2, plus a few other caps on other wheels, decided to come loose. They're not broken. The last time the wheels were worked on was when the dealer installed new brakes, back in August 2014. We've had no problems with the wheels since then. Unless there are other suggestions, I guess I'll try to put the caps back on, as that problem appears to be minor, and take the car to our mechanic after I return from a 3-hour one way trip with the travel trailer on Thursday, returning Tuesday. Again, thanks for the assistance. John K.