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Recently picked up a 1978 Mercury 402 (40 hp, 2 cyl 2 stroke) and had no water coming out of the tell tale (pee hole). Pulled the lower unit & replaced the pump assembly (housings, gaskets, impeller, seals, etc). Reassembled, started it up; now water is streaming out of the tell tale, but I have found some additional leaks. There was a jet of water shooting onto the base of the top spark plug (Cyl block cover gasket), and water was leaking out of the hole in the top of the poppet diaphragm cover (adjacent to the thermostat in the coil mounting bracket). I did not run the engine long, maybe 3-5 minutes using a cooling muff for water supply. Running at a high idle the tell tale was streaming very hard and some water was running out of the prop, equal to or less than what was shooting out of the tell tale.
I removed both the cyl block cover and the coil mounting bracket, & cleaned up the sealing surfaces. An inspection revealed no obvious problems with the sealing surface or the gasket where the block cover leak was, and the diaphragm showed no signs of a rupture. I'm starting to believe that the leaks were a symptom of another problem, perhaps a blockage in the cooling system.
What I need is an understanding of the cooling system & water flow through the engine. Does water pressure from the pump lift the poppet valve via the diaphragm to bypass the thermostat when it is cold & closed? Was the strong flow from the tell tale an indication of a blockage? Could a partial blockage downstream of the cylinder block cause the water pressure to lift the cyl cover around the spark plug and causing the leak? Where does the hot water from the cyl block get routed back overboard? Where should I look for a blockage?
Greatly appreciate any help,
Joe in St Louis
Recently picked up a 1978 Mercury 402 (40 hp, 2 cyl 2 stroke) and had no water coming out of the tell tale (pee hole). Pulled the lower unit & replaced the pump assembly (housings, gaskets, impeller, seals, etc). Reassembled, started it up; now water is streaming out of the tell tale, but I have found some additional leaks. There was a jet of water shooting onto the base of the top spark plug (Cyl block cover gasket), and water was leaking out of the hole in the top of the poppet diaphragm cover (adjacent to the thermostat in the coil mounting bracket). I did not run the engine long, maybe 3-5 minutes using a cooling muff for water supply. Running at a high idle the tell tale was streaming very hard and some water was running out of the prop, equal to or less than what was shooting out of the tell tale.
I removed both the cyl block cover and the coil mounting bracket, & cleaned up the sealing surfaces. An inspection revealed no obvious problems with the sealing surface or the gasket where the block cover leak was, and the diaphragm showed no signs of a rupture. I'm starting to believe that the leaks were a symptom of another problem, perhaps a blockage in the cooling system.
What I need is an understanding of the cooling system & water flow through the engine. Does water pressure from the pump lift the poppet valve via the diaphragm to bypass the thermostat when it is cold & closed? Was the strong flow from the tell tale an indication of a blockage? Could a partial blockage downstream of the cylinder block cause the water pressure to lift the cyl cover around the spark plug and causing the leak? Where does the hot water from the cyl block get routed back overboard? Where should I look for a blockage?
Greatly appreciate any help,
Joe in St Louis
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