Enable serial logging from ESXi 5.1

During installation

If an ESXi 5.1 host does not successfully install, serial line logging can be configured when booting the installer using advanced kernel boot-time parameters. This takes effect for the current boot, and does not persist across future reboots.
To enable serial-line logging during installation:

1. At the Loading ESXi installer screen, press Shift+O.
2. At the prompt, append the advanced option to specify a serial port after runweasel:
   
   > runweasel debugLogToSerial=1 logPort=com1
   
   
3. Press Enter to boot.
4. Logging starts after the first loading progress bar finishes.

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2032350