Overheating at idle. 1988 Mercruiser 5.7l magnum, I know I'm the trillonth person to ask this.... Replaced manifolds, risers, impeller, water pump, thermostat, hoses to risers, cap, rotor, plugs, wires, coil flushed multiple times no blockage good water flow from exhaust ports... Don't know what else to check or look at please help before I blow it up....