The tensioner itself is a ratcheting device. You should remove it and first check the spring. It usually shows wear, it gets worn along the sides, or it is just plain weak. Then the ram in the center should be cleaned and reinserted from the backside. It should hold when yor try to push it backwards through. I noted on a new one that it did not hold when shoved hard. But it should hold to moderate shoving. If not, replace the assembly.
The banana slide should be replaced as a matter of course in my humble opinion, it is easy and cheap to do. With 250k on the clock it will be worn and just do it. That slide has wrecked a few engines when it came apart. Gets brittle with age. Will stop a potential catastrphic event from happening if you get to it now.