What are the consequences of not making a full payment along with the Extension?
The IRS has some strict rules regarding filing an extension without making a full payment of the additional tax liability.
If the IRS determines that the taxpayer has made an inadequately low payment with his extension when in fact he should have paid substantially more, then the IRS have been know to void the extension altogether. This results in substantial late filing penalties and interest.
Each situation is unique and is based on the prior history of the taxpayer making timely payments. But, the IRS has tightened up on taxpayers making low payments with extensions and then paying the balance due by October 15th.
The IRS is particularily vigilant on these abusive practices of taxpayers.