I have a Merc 500 made in belgium in 1981. When operating at higher speed for 20-30 min. it starts to run steam on tell-tale pipe. It runs water normaly there just to the moment when the vapor starts. When i kill it, and start it a few minutes after, it starts normaly, except that steam starts imediately. the impeller is ok, and when i removed the lower unit, and tried to flush the engine, water was runing both thru tell-tale pipe, and thru the exaust. One notice, though, when compared to the tell-tale pipe flow of my other merc 500 (twin setup), the one in question is much stronger. Thank You in advance, Niksa.