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Check from where the cooling water is leaking into the combustion chamber
1. Blown head gasket
2. Pin hole in the exhaust manifold between the 'gas side' and the 'water side'.
3. Injection elbow mounted too close the the boats water line.

If this engine is fresh water cooled and the fresh water also cools the exhaust manifold, you can use a fluorescing dye in the cooling water which will 'glow' when 'black light' is shown into the water. The fluorescence will show that carbon monoxide from the combustion chamber is present in the water.

Exhaust manifolds when they age and due to formation of 'slab rust' can easily 'back leak' into a combustion chamber. Easy to check - remove the manifold and do a pressure hold test.
Pressure hold ... apply air pressure to the water side of the manifold with the water inlet and out blocked. Apply a press. gage and gas-tight shut off valve between the air source and the manifold. Youre looking for the gage to slowly show a drop in air pressure over time with the shut off valve closed after pressurizing. Can be done with water pressure but isnt as sensitive as air pressure decay.

Until you find where the water is leaking into the engine and affect its repair , I suggest that you do NOT use the starter to turn over the engine; but rather, 'gently' and without FORCE hand crank the engine by hand to verify that the piston rings are not 'frozen' in their grooves on the piston(s). If theyre frozen and you 'force' the engine you will SURELY break the rings. Once you verify that the rings are 'free' by easy hand turning the engine, then you can proceed with the use of the starter. Use a 'strap wrench' on the crankshaft pulley with the compression release 'open' to check .... 'gently' rocking back and forth to free the rings; do not 'force' a frozen engine unless you want to put in new piston rings.
 
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