www.iFourWinns.com

This can be caused by rust buildup in the thermostat housing...specifically in the two outlets where the hoses connect to the manifolds.

My Horizon is also a 2000, and about 3 years ago I used a drill to open up the two outlets due to some rust buildup. I don't remember the exact size of the drill I chose, but I think it was close to 7/16". I will probably replace it within the next year. As you said, it could also be in the manifold, but it is easier to start with the thermostat housing.

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"Knot Easy" 2000 Horizon 240 Volvo 5.7GS /SX
tow: 2017 Honda PILOT EXL-AWD
prev. boats:
'87 Chaparral 198CXL 4.3 OMC Cobra
'69 Jetstar 16ft Ski Boat, 115hp Yamaha
'68 Aluminum Jon Boat, 3hp Sears
'64 Water Wings

On many Mercruisers it is common for the starboard side manifold to run hotter. On my OMC which uses the exact same thermostat housing as many Volvo engines, the starboard side runs a tiny bit hotter (maybe 10*) usually at idle they are both at 90-100*F, the hottest they get after coming off plane is about 135*F. Yes the passages in the 'stat housing can neck down due to corrosion, I have found that you can ream them out but in salt water it gets to a point where there is enough flaking rust that its better to replace it. What temps are you getting on the risers at idle, and after coming back to idle after coming off plane?

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88 Four Winns 200 Horizon
4.3 OMC Cobra-4bbl
2002 Walker Bay 10/2012 Suzuki 2.5
2008 Walker Bay 8

1998 Jeep Grand Cherokee 4.0/Selectrac
2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II