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Ok I am at a loss. My 1988 245 vista overheats anytime I put it at about half speed. I just had the impleller replaced and it cools downs if it is at idle. The thermostat was replaced about two years ago. I have water going out the exhaust like it is suppose to. Any ideas before another trip to the mechanic

If it's not something more serious like a head gasket etc, sounds like a restriction in the engine / the recirculation pump is not pushing water around the engine ( they usually leak before they fail to pump though). The water going to the exhausts is pumped by the raw water pump, it goes into the top of the thermostat housing and straight out again to the exhaust manifolds. Was the impellor changed because the old one broke up? If so, you may have bits of the old one dotted withing the cooling channels ( and thermostat). First thing I'd check is the thermostat in case it's clogged (if so, try flushing the engine out in case there's more debris elsewhere).

Graham

at idle are the tops of the exhaust manifolds cool or warm to the touch? is the alarm going off, or are you looking at the gauge?

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Do you have a VovoPenta?
This may sound like overlooking the obvious but on VolvoPenta engines there is a wash out hose that has a blue twist top cap with a gasket in it. If this cap is off, not tightened or cracked it will allow the coolant system to suck in air causing the impeller to cavitate; this may cause the engine to overheat.

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2008 Four Winns V248
2004 Stingray 220CS

If it's an 88 it should be a Merc or an OMC...having had the OMC...the impeller in the drive must be put together right...the housing gasket has to fit correctly and be sealed with OMC gasket sealer or similar for it to pump right. The OMC test for the impeller is to put the boat in the water...disconnect the raw water supply hose that runs to the thermo housing...point it up in the bilge and have someone start the engine (very briefly) it should shoot water up in the air a couple of inches...if not check the impeller installation and the water intakes for obstructions. Here with salt water moored boats we get barnacles in the water intakes...I had to remove them last year and probably will this year too...
If your water flow to the therm housing is adequate...check for clogged risers (and make sure all the passages in the thermo housing are open and not blocked by corroded cast iron)...as was noted check the temp of the risers when the engine has been run a while...if they are too hot to keep your hand on that's a good sign they may be clogged...

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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