Bravo One XR gear lube leak

[gizard]

So we have a triple install and one of the drives is leaking gear lube. It only leaks noticeably if the boat is run under heavy load, 75% plus throttle. All propshaft seals have been replaced and also tubes etc that pass / connect through the transom and there is no gear lube ever in the engine bay.

The only thing we think could be a problem is the prop shaft thrust bearings under load which allow slight movement under heavy load?

Has anyone had a similar experience?

 

[Matt]

I’d do a proper pressure test on it and see what you find:

 

[gizard]

Sorry forgot to mention that it had a pressure test and it did not leak...

 

[gizard]

Problem is we don't have a way to test whilst it is under load, which might reveal the problem...

 

[Matt]

Almost impossible to diagnose something as detailed as this via the internet.

But, for example, if it's only >75% throttle - is the top end overheating, which in turn causes the drive to over-pressurise and blow oil out? Do you have drive showers? If you've had the seals done on the bottom end, then I am sure the mechanic will have checked the runout on the prop shaft and various backlash, shims etc are all spot on.

Stick some of these on the top of the drive uppers and compare the temperatures you're seeing between drives? If the troublesome one is hotter, that would give a direction to investigate

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[gizard]

Ahh that is a good thought over heating and blowing oil under load thanks very much, will do this. Yes we do have drive showers.

I assume just put the temp gauges right on the top, although as long as they are all in the same place on each drive it should give a good indication of the difference.

 

[gizard]

What temperature range is normal on the top of the casing?

Thinking further I guess this over heating could point to 3 areas:
- Exhaust gas higher for some reason on this engine than the others (CMD 4.2 Turbo diesels - through drive exhaust)
- worn gears or bearings somewhere in the drive (although this has been going on for a while and we have not seen any excess metal on the drain plug)
- might be as simple as the drive shower not functioning correctly

 

[gizard]

I have found this which might help:

https://www.offshoreonly.com/forums/drives-lower-units/283503-bravo-1-temps.html

for that I am guessing 71-110 degrees C should cover it.

 

[gizard]

surely the path of least resistance in a running scenario where the pressure increases would be the header tank for expansion - but there is no evidence of oil leaking at that end - looking through the Mercruiser Bravo Service Manual (it's a big old manual!) the only valve I can see is the dribble valve between the transom shield and the drive it self (this was also replaced recently). I assume this is open all the time in a running scenario.

 

[Matt]

It’s a sealed cap - the system does pressurise.