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How black and stinky and thin is the gear oil when you do drain it?
What we've found working with various marinas, and dealing with various volvo tech reps is that on the DP drives behind diesels, especially behind bigger boats, it really heat ups the gear oil, thins it out, and cause all sorts of shifting problems.
What happens is the oil gets burnt and gets to thin, and the clutch starts to slip when you shift it, and after a while it wears the clutch just enough that it either will not shift at low rpms, or slips at low rpms.
Some things that will help;
Think about changing the service interval on the gear lube to every 50 hours instead of every 100, that will help tremendously. Especially if you operate the boat on long hauls where you steam out for a couple of hours at a time on plane.
Drive showers do help, as it helps keep the oil cool. Changing the lube over to Mercury Marines High Performance gear lube really helps as well, as its quite a bit better than the Mobil 1 synthetic. I have a friend of mine that is a retired Volvo Diesel Master Tech, and he worked for years up in Ketchikan AK where the seawater is ice cold all year round. And he has seen this same thing since the 290DP drives behind diesels, and especially on bigger boats. If the boat is either semi-planing, or full planing. When the back of the boat comes up, the top of the sterndrive is out of the water, and it really does not cool all that well. So if you do run for extended periods at higher speed. Drop the boat back down for 5 to 10 minutes and let the oil cool a bit before pushing the throttles forward. That and everything else above will help keep you from having to do it a 3rd time over.
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