High rpm overheat 1985 170 mercruiser

"boat sat for two years. Ran O

"boat sat for two years. Ran OK at tirst...overheated...replaced water pump impeller..no missing blades. Engine ran good until operating high rpm...overheated...removed hoses from exchanger (4") and blew air thru, no obstruction. Taking on some water in bilge mixed w/coolant...possibly from overflow. Coolant res. is full and does not exchange. Running on flushette, exhaust is loud...suspect plugged manifold? New boater, used boat...any help?"

Where are you running this at

Where are you running this at high rpm?
If you're running it on a hose you're asking for trouble.

If the test is being done on a hose put the boat in the lake / river whatever and try it.

Then if it runs hot replace the thermostat.

"Chic: Heed Charli's advi

"Chic: Heed Charli's advice. Just don't go far from the dock until you are certain it is not overheating; cruise around near the dock--watch the gauges and look inside the engine compartment. "New boater"; don't assume the plug is in! Check the belt tension; loose belt, no circulting coolant.

Exhaust will always be loud at the rear of the boat when run on a "flushette" or muffs and a hose. Loud exhaust from the engine compartment while under way spells an exhaust leak; but a plugged manifold? Check the manifold area for the noise problem to see where it is coming from. Could be a rubber coupler, rusted thru manifold or riser gasket which would explain water in the bilge or engine/manifold gasket.

Your eyes will tell you alot if you take your time when looking; use a mech's mirror (has an extending arm) to see under or behind parts.

"New boater" - If you check things with the engine running, STAY AWAY FROM THE FRONT AND REAR OF THE ENGINE AND ITS MOVING PARTS. Fingers, hands and arms are removed easily.

My friend is a farmer whose adult son was killed when his shirt sleeve was caught in the PTO on his tractor. It wrapped him around the shaft in a flash like a recoiling yo-yo.

Blowing air thru the exchanger only tells you it is not 100% plugged. One hose ( inlet raw water one)will be cool while the outlet will be hot if the exchanger is working. Run your hand along the exchanger and see if their are hot or cold spots. Cooling tubes that run antifreeze thru them will grow (especially when it sits two years)crystals of calcium inside of the tubes by electrolisis which will narrow the tube and decrease the heat transfer capability--overheat.

Taking on water in the bilge mixed with antifreeze--if the engine is only partial fresh water cooled, is the exhaust manifold raw water petcock closed tight? Antifreeze has a number of places to leak from; check the weep hole on the bottom of the front circulating pump for any sign of seepage. It will either run out when the engine heats up and pressure builds in the cooling system or just dribble. Replace if there are signs of these conditions."