The IQLOGDIR16 environment variable defines the location of various log files. IQLOGDIR16 is not set by the installation program.
The server log is in the file servername.nnnn.srvlog (where nnnn is the number of times the server has been started) in the directory specified by $IQLOGDIR16.
If IQLOGDIR16 is not set to a valid, write-enabled directory, then most utilities, including start_iq, use the default location $IQDIR16/logfiles for all server logs.