http://bible.com/1/rom8.26 Likewise the Spirit also helpeth our infirmities: for we know not what we should pray for as we ought: but the Spirit itself maketh intercession for us with groanings which cannot be uttered.
To insist on knowing what we are praying for is to be filled with intellectual pride. There are times, many times, when we don’t know what we should pray for. The wonderful thing is that the Holy Spirit sounds the depths of the heart of God and knows exactly what to pray and how to pray. Though it is unintelligible to the mind yet it is perfectly understood by God.