pray (someone's) heart out

Translations
молиться изо всех сил
She sat in the chapel and prayed her heart out for her sick child.
Она сидела в часовне и молилась изо всех сил за своего больного ребенка.
Definitions
To pray with intense emotion, effort, or sincerity, often for a prolonged period or with great desperation; to pray as hard as one possibly can.