99 c40trlx cooling question

im working on a 40 hp yammy. customer asked me to replace impeller. when i removed water pump housing , the impeller was completely disintegrated. i replaced and tested in water tank...all was fine. after a few weeks he called me back and said it wasn't peeing good all the time and that it was steaming. so i pulled thermostat cover ,exhaust plate, and the lower unit to determine the problem...i found nothing wrong with water pump/impeller. there was a piece of rubber lodged in the spring of the thermostat.. found several small pieces in the water passages behind exhaust plate. and a piece in the pee tube...i blew all passages with air , replaced thermostat and reassembled. i took it to the lake today to test and all was good except the steam coming from exhaust ports. is this normal or should i look some more

I would pull the lower and the thermostat cover and back flush. Sounds like some old impellar may be partially block the exhaust housing