keepalive strange behaviour

Hi,

I'm trying to improve the performance of a webapp behind nginx 1.9.7. My server freezes due to the high time_wait ports.
I added the keepalive sentece to the upstream and the "proxy_http_version 1.1;" and "proxy_set_header Connection "";" to the location but I'm still having this issues.

I tried with a simple node js webserver that simulates the flow o the real app (just 302 redirects) and with this server works fine.The real webapp servers are pretty much the same and are developed in node js (not by me).

What could be happening here?

Thanks!