Auto mechanics, should I tip you if you do a free repair?

No tip , he was just being a good guy and don't expect a tip , in fact if he is an older man, he may be offended if you tip him.

Check all the levels of fluids when possible, then again I always get an oil change when the car indicates it to me, so the levels are always correct. I only take my car to places I've read reviews about and know that the service is legit and nothing is forgotten. Although I drive a minivan, I've always used Silver or mid-grade gasoline and while I don't always go to the big brand gas stations I make sure the octane level is to my satisfaction. Otherwise I'll drive further looking for good octane. I don't do this every time, but sometimes I'll allow my car a minute or two to sit just after starting the car. I never run the car too hard and if I travel cross country I'll take a few stops to allow the car to cool. (Not sure if that's necessary) Once every 6 months have the tires rotated and checked. I've got a Honda and I'll tell you over the last 20 years with them I've spent thousands of dollars less money on Honda maintenance then on my Chevy cars.

hand him 20.00 and say thanks with a smile. mechanics do make good money but we have to invest in the tools to fix some of the strangest things. more than likely the tiny screw at the bottom is loose, a simple turn with an allen wrench will take care of the problem

No mechanics make a lot of money there doing something they love to do but it's your choice.