Thanks in advance for taking the time to read and weigh-in.
Serial #: OT834066
2004 Optimax 135 XL SWB
798 hours
I've noticed for almost a year now, that my motor presents a vibration that is rhythmic in nature. Every 5 seconds the motor will shake, then smooth out, then shake...over and over. This is felt both out of gear and in gear. The faster I go (or higher in rpm) the shorter the interval between vibrations gets.
Again, it's done this for about a year, and I've finally decided to dig into the issue. I replaced the plugs, no change. Clipped a timing light to the plug wires to see if I could find a coil mis-firing at the intervals, no luck. Cleaned the crank position sensor teeth, thinking it was a timing issue, nothing.
Fuel pressure while at idle is 87, Air is 77. Thought this might be a touch low. Hooked regulated air to the shrader valve. Air dumps through the exhaust at 77 psi, anything lower than that it holds and I cannot hear any leaks through the spark plug holes.
Tracker looks good after carefully inspecting.
So, I decided to check the heartbeat and do a compression test. Here are the results from the first go-around (running temp/throttle wide open/all plugs out/by the book):
1: 83
2: 85
3: 90
4: 105
5: 90
6: 70
Yep, good ol' #6. Not wanting to believe it, I soaked the cylinders in seafoam and Marvel Mystery oil overnight. It made clouds of white smoke the next day. I warmed it up and tested again to see if I could loosen up a stuck ring:
1: 90
2: 90
3: 100
4: 105
5: 95
6: 80
While #6 came up a little, it's still the black sheep.
What's the general opinion? Time for a rebuild? Are the vibrations symptoms of having a weak cylinder? Boat runs great otherwise, but definitely would want to stop short of completely blowing her if this is a good warning...
Here's a video for a better example.