For my proliant servers I found that I had to also allow access from HP SIM to port 80/tcp on the iLO before discovery would be successful.
Here are all the ports I ended up opening:
=== HP SIM --> iLO ===
161/udp - SNMP queries
443/tcp - https
80/tcp - http
22/tcp - ssh
17990/tcp - Remote Console
17988/tcp - virtual Media for Remote Console
=== ilo --> HP SIM ===
162/udp - SNMP traps