Before you install the chain tensioner, you must remove the inside ratcheter from the housing by pulling it out. Then push the tensioner housing in at least one click so that the detent spring will hold it from coming back --- the oil pressure will do the rest.
Have fun getting the tensioner cap screw back on while exerting pressure on the spring. It'll exercise your patience for sure
Good luck with the work!
'91 300E, 212K miles