From the US Dept. of State web page:
ENTRY/EXIT REQUIREMENTS: A valid U.S. passport is required for all U.S. citizens, regardless of age, to enter Guatemala and to depart Guatemala for return to the U.S. Even if dual nationals are permitted to enter Guatemala on a second nationality passport, U.S. citizens returning to the United States from Guatemala are not allowed to board their flights without a valid U.S. passport. Certificates of Naturalization, birth certificates, driver's licenses, and photocopies are not accepted by Guatemalan authorities as alternative travel documents. While in Guatemala, U.S. citizens should carry their passports, or a photocopy of their passports, with them at all times.
Say's nothing about how much time is left, but i'd be getting another passport soon!
If i recall, Guatemala only gives you a max. of 90 days, so i think you might be alright. One way to find out