An extension request only gives you an extra six months to file the tax return.  It is not an extension for tax payments.

If you cannot pay the entire amount of taxes owed by April 18, 2017, then you should pay as much as possible and then either pay the remainder at a later date, which will incur late payment penalties.  See this IRS website for how the penalties are calculated - https://www.irs.gov/businesses/small-businesses-self-employed/understanding-penalties-and-interest

Or you can set up an installment agreement with the IRS although fees may apply.  See this IRS website for an online installment agreement - https://www.irs.gov/individuals/payment-plans-installment-agreements