Don't forget, he was also looking for the Kingdom of Prester John and a route to attack the Ottoman Empire from behind Wasn't "Prester John" just a bastardization of "Genghis Kahn?" IIRC, the Crusaders heard stories of a great warrier who was smiting the Muslims in the 13th century and put together a bunch of rumors and wild stories and concluded that there was a Christian king coming to their rescue.
I thought Prester John was to do with the Ethiopian Emperors -- the ancestors of Haile Selassie.
Historical foundation of the origin of the legend.
Otto von Freising does not mention the exact year of the battle between the Eastern conqueror and the Persian sultan; he only remarks that in 1145 it had taken place "ante non multos annos". On the other hand, there is found in the Annals of Admont (1181), part of which, as far as 1141, are a continuation of Otto's chronicle, the following note: "Johannes presbyter rex Armeniae et Indiae cum duobus regibus fratribus Persarum et Medorum pugnavit et vicit". Minute research has shown that in that year the Persian Sultan Sanjar was completely vanquished by a conqueror from the east, not very far from the ancient Ecbatana. The Arabic historian Ibn-el-Athir (1160-1233) says that, in the year of the Hegira of 536 (1141), Sanjar, the most powerful of the Seljuk princes, had mortally offended his vassal the Shah of Kharezm. The latter called to his assistance Ku Khan, or Korkhan of China (Chinese, Yeliutasche), who had come in 1122 from Northern China at the head of a mighty army. Korkhan killed Sanjar and l00,000 of his men. The Arabic versions are substantially corroborated by other Asiatic historians of that epoch: by the Syrian writer Abulfaradsch (on account of his Jewish descent called Bar Hebraeus, 1226-86), by the Arabic Abulfeda (1273-1331), the Persian Mirkhond (1432-89) etc. It is not certain whether the Spanish Jew, Benjamin of Tudela, who travelled in Central Asia in 1171, refers to this event. If so, the hypothesis based on the researches of d'Avezac, Oppert, Zarncke, and Yule becomes a certainty, i.e. the land of this uncertain and shifting legend is the kingdom of Karakhitai (1141-1218), founded in Central Asia by the priest-king of the tale. The disputed points are the name, the religion, and the priestly character of the mysterious personage.
Independently of the much earlier work of d Avezac, Oppert thinks that Ku-Khan, Korkhan or Corchan (Coirchan), as the East-Asian conqueror is called in the chronicles, could easily have become Jorchan, Jochanan, or in Western parlance, John; this name was then very popular, and was often given to Christian and Mohammedan princes (Zarncke). History knows nothing about the Christianity of Yeliutasche. Yet it is clear that the league of the West against the Mohammedans stirred up the oppressed Christians on the borders of Tatar Asia to look for a deliverer. The sacerdotal character of the legendary king still offers an unsolved riddle.
http://www.newadvent.org/cathen/12400b.htm