I was not doing any 401k contribution since beginning, so now, I want to contribute make a 100% pre tax contribution. By doing so, there will be no FIT although SS and Medicare will be deducted from my pay check. Contributing 100% towards 401K is within the stipulated limit of $16500 per calendar year. Please suggest whether I can make 100% contribution towards 401K?

Plans vary, so you need to ask your employer's 401(k) plan administrator what the rules are for making contributions. Be aware that you make contributions through payroll deduction, so if you are going to earn at least $16.5K between now and the end of the year it may be possible. The HR department at work can tell you whether you can make your 401(k) contribution of sufficient size to max out the $16.5K limit by the end of the year.

Also note that any matching program from your company may have a per quarter or per pay period limit, so you may not get full match (if one exists) for your 100% contribution.

In general, I doubt 100% would be allowed by an HR department.