1972 mercruiser water issue

Hi guys, I have a 1972 mercruiser 153 ci 2.5 L 120 HP. I've replaced the impeller in the lower end a couple times. I have a thermostat that just tells me the temperature of the engine. Usually sits around 160-180 when it's running fast but when I'm idling it likes to go to the red over 200. Any thoughts? Should I get a thermostat kit that goes in the water pump? The old suction hose had a kink so replaced that, it seemed to help the overheating issue but it still seems hot when running at idle. Could it be a water pump issue? I feel like the impeller is good.

What about the water pump on the engine ? Clogged hoses ? Yes you should be running a thermostat in the engine. Is there a scale buildup inside the engine block...or mabey a blown head gasket ? What are compression numbers ?

The motor was completely rebuilt so compression is great. I just ordered a thermostat as I took it apart and it doesn't have one. I just find it weird when I'm driving like even 15 mph it stays cool and as soon as I idle it goes hot

If you are sure the water pump assembly is assembled correctly, It's probably the water seal in the upper unit where the water pump output pipe connects. When that seal goes bad, it leaks like a sieve internally. Which means not all of the water leaving the pump makes it to the engine at lower speed.

Merc water impellers do not pump well at low speed, but work OK at higher speeds, say above idle, if the bottom is chewed out. The water pump base can melt as well.

Also, If an Alpha OD is in salt water, barnacles can build up in the water intakes under the impeller.

I would check the bottom of the impeller for gouges. Check the stainless steel (SS) wearplate for scratches, as well as the SS cup in the upper waterpump housing. Look under the wearplate for melting of the grooves of the pump base. look into the water intake holes and see if there is junk in there. it can be scraped out after the pump base is removed.

Replace all SS stuff if scratched or worn. Replace the pump base if melted. The thermostat housings have changed over the years, so I am not sure what to look for. My '88 alpha had a SS insert into the cast iron thermostat housing, that held the thermostat in place. Not sure what you have.