how to change "delay value" in system identification tool

Hi, I'm using system identification tool in Matlab R2011b. And I want to identify transfer function by using "Process Moldels". I want to change Parameter Td (time delay) automatically. I know there is "Boudns". But always the estimate relust of Td is around 2~4. For exsample, I made simple transfer function with delay (100*exp(-15s)) and prepared input(sin wave)-output dataset using it. After that I tried system identification by Process models and set bounds of Td [0 20]. But Td was always 2~4, though acculate time delay is 15. When I set Td=15 as a "known" value, the Best fits value was better. I'm wondering "process models" can't estimate time delay more than 4, automatically? I need help.