Sea raider-------Put some of this white stuff in water.-----Put it on the small burner of the stove.----See what happens as it boils.----Just like a science experiment.
Question : Does water boils constantly inside a powerhead for that to theoretically happen ? besides we're dealing with salt not freshwater which turns the science experiment into a useless one..
Made a short investigation, It's not salt layers, crusts, it's pure saltpeter potassium/sodium nitrite forming over time in all metal water paths due to repetitive heat/cold operating cycles. Doesn't matter much year of fabrication, brand nor HP model. If in pursuit to remove 100% of it will need to be removed mechanically scrapping it gently off and patiently as in pics.
Going back to the original post, find that on the 9.9/15 HP models, the thermostat which is old technology must work fully clean as not to seize compared to modern car type ones which works much better and has less seizing issues when dealing with saltpeter formations.
There's an interesting direct flush port at middle of the exhaust cover, flushes better than using muffs, barrel, but won't remove extreme saltpeter formations. Will need to buy a OMC adapter for that...
So guys, if you have changed a new impeller or complete water pump along a new thermo and OB keeps experiencing overheat issues specially at speed..
it's proper time to have some DIY fun time work and tear the darn powerhead down..LOL!!
Happy Boating