Mercruiser 888 overheat

"i have a mid 70's 888 For

"i have a mid 70's 888 Ford with an overheat problem i've changed the impeller, and have good water flow at the engine. i have also changed the thermostat. i found that when the engine starts to heat up, and i remove the starboard side hose, on the inside of the exhaust elbow, that i get lots of steam, and then water. i put the hose back on and the engine cools down and runs normally. if i shut the engine down for a couple minutes, and then restart it, it starts to warm up again. i have a digital thermometer and have found no blockages. i always thought that the water circulating pump would pull water down through the hose, running from the thermostat housing, but when i disconnect the top of the hose the water just sits there.
i appreciate any help and ideas anybody has on this subject
Cheers!"

That water pump issue does not

That water pump issue does not sound right. I would change out the circulating pump first and see where you are at.

Rod

Follow Rod's advice; look

Follow Rod's advice; look for rust sludge inside the block ports when you remove the pump. The age of your engine dictates that a block flush is needed using a garden hose w/a long nozzle on high pressure narrow stream spray. Keep the block drains open as the sludge will plug them.

Your T'stat hsg. may also be plugged with rust sludge. Same goes for the exhaust mans. and risers; best to remove them and clean them; many ports and passages will be narrowed restricting flow. I would take the mans. and risers to a radiator shop for cleaning and pressure testing or just replace them due to age.