E320 tensioners dont fail as much as 103 engines do .( I wouldnt replace it if it is ok) Make sure the head gasket is not leaking from the back. Alot of times people do the upper cover only and the lower leaks worse than anything. If the head has to be done save yourself some money and reseal the lower cover also. I have seen this a dozen times where a guy does the upper ,or head and it turns out the lower cover is leaking also. Have it cleaned and doublechecked with a flourecent light using the dye in the oil .