I have a '76 marlin with the GM 153 Mercruiser 120 hp, boat used to run at 140 degrees all day. Overheated one day, but never got above 200 deg. Had impeller pump and housing replaced by local boat shop. Still overheating. Pulled thermostat, no help. Checkked circulation pump ok. Pulled head and found piece of impeller stuck in one port on top of engine block. Repaired that and put back together, still overheating. Checked flow from impeller pump seems ok, checked flow going into exhaust manifold and when I removed this hose, water and exhaust began blowing out of manifold. Does this indicate a problem with the manifold or riser?
