Why is the response truncated at 16k with php + fastcgi?

The accepted answer is useless, so I went with the comment about checking write permissions, which did the trick for me. I'm running Nginx as a different user than default, so permissions had to be changed on the default temp path. For RHEL installs this is /var/lib/nginx/tmp/fastcgi and it has default permissions of 700:

chown -R webuser /var/lib/nginx/tmp

I ran into this problem on Scientific Linux 7, using the package from the SL repos. This step was not necessary for me on Scientific Linux 6 using the Nginx-provided package, which has 755 permissions on /var/cache/nginx