[mnez]

Try capping both ends with a couple of those rubber vacuum caps from a an auto parts store as a test. As I understand it, that valve only functions in cold weather to draw some warm air from the oil tank when the temp switch on the manifold opens to let the air pass into the second small hose. The end at the oil tank should be capped if not in use. I also see the big vacuum line has been capped. The other end needs to be capped as well.