It doesn't look like she was listed in the 1798 census records.
There was no Eva Maria born on or around 1779. Two that are listed as being born in 1774 are already matched with husbands. She may have lived in a different village, perhaps the same one as her husband, and they came to Grimm at some point after 1798.
Head of the Household Friedrich Zimmermann, age 59
Wife Eva Maria Zimmermann, age 55
Child #1 Johann Adam Zimmermann, age 10 in 1816, deceased 1835
Wife of Child #1 Anna Barbara Zimmermann, age 24
Grandchild #1 Johann Philipp Zimmermann, age 6
Grandchild #2 Johann Daniel Zimmermann, age 4
Grandchild #3 Philipp Jakob Zimmermann, age 2
Grandchild #4 Maria Charlotta Zimmermann, age 3 months
Child #2 Anna Elisabeth Zimmermann, age 16
↑The 1775 and 1798 Census of the German Colony on the Volga, Lesnoy Karamysh, also known as Grimm; Published by the American Historical Society of Germans from Russia, Lincoln, Nebraska, USA; Published date: 1995.
↑1834 Census of Grimm in the District of Saratov, Russia, dated 2 February 1835; Translated by Brent Mai, Concordia University, Portland, Oregon; Published by Dynasty Publishing, Beaverton, OR, USA; Published 2011; page 7, family #17, Friedrich Zimmermann family.