Replaced impeller and thermostat. boat still overheats

So i have a 87 alpha one mercruiser (sea ray) i have replaced the impeller and the thermostat. brouhgt it to the lake for a spin it still overheats, struggles to hit 20 mph and also it shakes sometimes at low speeds.

any advice or pointers are very welcome. I would love to have this thing running befor the winter spoils my fun!

thanks
always broken

Re: Replaced impeller and thermostat. boat still overheats

Do you have any history on this boat, or is it new to you? Sounds like multiple issues to me. Do some reading up here on Exhaust Manifolds, possibly a blockage in the cooling system; water has to get out as well as in.

The vibration may be a propeller out of whack, visual on the prop? Look good? Damaged blades?

Re: Replaced impeller and thermostat. boat still overheats

what's your motor and setup? closed cooling system with antifreeze, or cooling directly from lake water?

Re: Replaced impeller and thermostat. boat still overheats

no history with the boat. the prop is fine not even a scratch. 4.3L and uses straight lake water for the cooling

Re: Replaced impeller and thermostat. boat still overheats

Have you confirmed that you are getting good water flow though your cooling system? Did you test the engine on muffs before hitting the water?

Re: Replaced impeller and thermostat. boat still overheats

struggles to hit 20 mph and also it shakes sometimes at low speeds.

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I think you need to do some engine troubleshooting, do a compression test, possibly a leakdown test, check the timing, along with a vacuum check.

Re: Replaced impeller and thermostat. boat still overheats

yes i confirmed lots of water is making it through the engine with earmuffs

Re: Replaced impeller and thermostat. boat still overheats

It sounds as though the boat is new to you. If so, what exactly do you know about it? And what have you done to it, other than the impeller and t-stat?
What about a tune up? Plugs, wires, dist cap, rotor button, points, cleaned spark arrestor, etc... And what about the fuel? Is it fresh, how long had it sat before you got it? Was it properly prepared for storage? Your carb may be dirty/gummed up as well. And there is the fuel filter / water seperator, have you checked or replaced that? Have you checked the timing? What does the oil look like? Have you changed it recently?

Basically you've got a V6 automotive/truck engine that has had some modifications for marine use. As such you can troubleshoot poor power and rough running the same way that you'ld do a gasolene car engine. Check compression, check spark, check air, check fuel, check vacuum.
There are several things that can cause poor running and overheating and you need to do some basic troubleshooting and let us know the results to effectively help you.