MCM 5.0 MPI / Bravo I coupler ??

2002 Mercruiser 5.0 MPI with Bravo I outdrive in a 24' wooden runabout. 500 total hours, careful preventative maintenance, always in fresh water. Engine alignment is one finger slides the bar in perfect, not a drop of water or any rust inside the bellows.

I've got a vibration driving me crazy. Full left or full right or centered steering and full up trim (not tilted up to trailer) I get a very noticable vibration proportional to engine speed. I can turn it on and off by playing with the power trim switch. I can feel it going forward at idle speeds and at 3000rpm vibrates the whole boat. The noise/vibration is very apparent even in neutral at more than about 1500 rpm. Going straight with the drive level everything seems pretty normal even at WOT. Top speed is the same as before the problem started.

In spite of the pristine interior of the bellows I figured it's got to be U-Joints because the problem only occurs at significant drive angles. I replaced the U-joints (the old ones looked almost brand new). Of course I wouldn't be writing for advice if this fixed the problem - new U-joints had no effect on the problem.

Yesterday I replaced the gimbal bearing even though the old one rotated smoothly. That made no difference either. I took the old bearing apart and it looked like new.

I just hate to replace the next most likely part - the coupler. $500 plus R&R the engine. It looks fine peeking in the hole with a flashlight. I swabbed some of the grease out with a rag on a stick looking for aluminum filings - nothing. But what else could it be? The only fact (other than the bad vibration) is that I idle most of the time, probably 400 of the 500 hours are just putting around. The greater the U-joint angle the greater the shaft acceleration and deceleration every 90 degrees. Could the long hours of idling have caused the aluminum coupler splines to open up so the shaft can really bang back & forth?

The fact that the problem is just as apparent in neutral suggests it isn't anything in the drive itself. No water in the drive oil; nothing but a tiny bit of sludge on the little magnet on the drive oil drain plug.

I appreciate your time reading this long post. Any suggestions? Similar experience?

 

Because you started out by saying you slide in the bar with one finger, the soft aluminum splines may be the cause. But it may also be that the whole coupler can slightly rotate in the flywheel. It is held in place by a number of small screws, secured almost a decade ago with a drop of Loctite. Several 100's of idling hours may have worked them loose.

Very difficult to diagnose without engine removal. If there is a substantial play you may be able to feel or hear it with the alignment tool inserted.

 

Well, it was the coupler. Just launched the boat and the vibration level is back to normal.

The old coupler splines looked OK but a measurement showed the distance across the 'valley' between splines was .070" on the new one, .100" on the old one.

Looking at my motor log I see that I have 630 hours with an average speed of 7.8 mph. I believe all that idling is was wore out the coupler.

 

Keep it greased. That will increase the life of the coupler.