http://www.flickr.com/photos/64379129@N07/6391888985/ Its the line behind the 02 sensor with a spark plug plugging the line. The mechanic that did my motor said "You don't need this", well blown headgasket later. I put it all together myself and was driving it today and noticed coolant leaking from this line. Does anyone know what this is for? Do I truely not need this? I wouldn't doubt if I do need this considering he also told me i didn't need my oil cooler for a turbocharged engine.