Just had a compression test done on a mercruiser 350 5.7 tournament ski EFI. All cylinders checked out around 145 except for the last one #8 back starboard which was only at 35. The boat was sitting for three years. Any suggestions on alternative options then a repower or rebuild?
Thanks
What brand of boat?
Look at the intake manifold- does it have a vacuum line to the base of the throttle body that's attached to the #8 cylinder's runner? This can cause a lean condition in that cylinder and if it has this, you may have a worn valve/seat. There's often another place on the intake where this hose could be attached so the vacuum is provided by all cylinders.
If you have a compression gauge, remove the valve stem and insert the hose as if you were doing a compression test. Connect the hose to a compressor and use the pressure to build in the cylinder, so you can listen for where the air escapes. Remove the spark plugs for this test to make it easier to turn the engine and remove the fuel pump fuse or relay if you use the starter to turn it, but just bump it briefly- you're trying to close the valves in this cylinder. If you hear air escaping from the exhaust, turn the crank and listen for it to stop, then it should come out of the intake. Keep turning the crank and it should eventually stop escaping completely.If it does stop escaping through the exhaust and intake but you still hear it coming from somewhere, remove the oil filler cap- if it's coming from the crankcase, it could be bad rings, scored cylinder or a problem with the piston. If the air begins to escape from an adjacent spark plug hole, it's probably a bad head gasket.
