Occassionally when using my windshield wiper, the left one will go all the way down and hit the top of the hood. My mechanic had tighten it but it just happens from time to time. Any suggestion?

 

It could be that the little gripping teeth on the post are stripped off. There is a nut under the end cover 12mm or 14mm, that snugs the arm onto the post.

If that is the problem, you can try to rig it; cut up a small strip from an aluminum cola can, wrap it around the post, then put the wiper arm back on and tighten it down. That might give it enough grip to ride where it is supposed to.

This problem happens alot in the winter when people turn on the wipers and the wipers are frozen to the windshield.