Through the use of things like satellite imagery, I'd imagine it would've been found by now if it did exist. Anyways, I did some research through some peer-reviewed articles. Here are my findings:

"Though there had been earlier searches for lands believed to be rich in gold, the Venezuelan historian Demetrio Ramos Perez has made a strong case for dating the first rumours of a golden king just prior to Pizarro's expedition of 1541, which was shortly followed by an equally disastrous one led by Sebastian de Benalcazar.' Both Conquistadores led their expeditions northwards from Peru, seeking an indian king whose body was said to be powdered in fine gold-dust every morning. Whatever the origins or veracity of these reports, they fuelled belief in a third, still richer indian empire after Mexico and Peru, which was also mixed up with the Spanish explorers' anxiety that the Incas had escaped with the greater share of their gold, and that Atahualpa's ransom (his weight in gold) was only the tip of a vast iceberg.2 By the time of Pedro de Ursdias expedition in 1559, El Dorado had become a golden land rather than a golden man, and the official name of a province." ^^1

So according to Perez, it didn't begin as a search for a "Golden City" as the current myth typically goes. Regardless, there's been conquistador searches throughout history, as you are sure to know. More information from the same source:

"In volume III of his Personal Narrative, Humboldt established his view that the topic of El Dorado belonged primarily to the domain of science, specifically to geography: The discussion to which I shall devote the end of this chapter is important, not only because it throws light on the events of the Conquest, and that long series of disastrous expeditions made in search of El Dorado, the last of which was in 1775..."

So we can say that as of the 1850s (which is when Humboldt wrote said Personal Narrative), the last known expedition had taken place in 1775, to no avail.

This book by John Hemming details all the myths that led to the searches, and the searches themselves, fairly well. However, to my knowledge credible archaeologists are in agreement that it never existed as described.

^^1 The Myth of El Dorado John Silver History Workshop , No. 34, Latin American History (Autumn, 1992) , pp. 1-15