My garage told me that the tensioner is oil pressurized and the pressure is maintained by a non return valve when the engine is not running. If the valve is worn or damaged (my engine has covered 160k miles) then oil can/will leak and therefore lose pressure...how much and over what time scale being dependent on the severity of the leak, which seems plausible. My initial problem was oil in cooling water, suspected during service as oil cooler leaking but could not fit me in to do the job before a planned trip, so to be on the safe side I trusted another outfit to do it. I drove it there performing perfectly. They completed the job and after a few seconds running it went BANG! My usual garage, whom I've trusted for 30 odd years, towed it in and sorted it, but I had some sleepless nights wondering if it was new engine or scrap time.