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  1. #1

    Default 12V71 Twin-Disc Gear Water in the Gear

    Hi there,
    my name is Uwe. I live in Germany and I have an old 1977 "Versilcraft" with two Detroit Diesel 8V71 engines. Every year I change the oil and filter, check the cooling water and clean the air system. So I have had no problems with the Detroit Diesel over 25 years ,

    Here is my question. My friend wants to buy a boat "Baglietto 20M" with two Detroit Diesel 12V71. During a check I found water in the gear oil. Where can that come from?

    I also measured the temperatures at the cylinder outlet. All cylinders are around 52 degrees Celsius but one is only 20 degrees Celsius. What could be the cause?

    I would be very happy to receive an answer because there are hardly any Detroit Diesel spe******ts in Germany. All just Volvo.

    Greetings from Germany

  2. #2
    Join Date
    Nov 2009
    temecula, ca., usa

    Default Re: 12V71 Twin-Disc Gear Water in the Gear

    Hi Uwe,
    Good question!

    More questions:
    Water in both gear boxes?
    Lots of water?
    Might there be a shared cooler for the boxes?
    Closed or open propshafts?

    Just thinking about the "possibles" there.

    Almost forgot about the exhaust manifold question....

    That 20 runner likely indicates a dead cylinder.
    Could be a bad injector or exhaust valve issue.

    Is there a maintenance log?
    Last edited by jgmo; 02-17-2020 at 12:47 PM.

  3. #3

    Default Re: 12V71 Twin-Disc Gear Water in the Gear

    Hi jgmo,
    Thank you for your prompt reply.

    Water in both gear boxes?
    only in the starboard engine.

    Lots of water?

    The amount is difficult to estimate.
    I judged it by the color of the oli.
    In the port engine, the oil is golden yellow and clear.
    In the Starbord Engine, the oil is light brown, milky.
    The seller says "he changed the oil in 2017 and has driven about 100 hours to date"

    Might there be a shared cooler for the boxes?
    There is one transmission oil cooler and one engine oil cooler per engine.
    Since it is a closed system, could it be a crack in the oil cooler?

    Closed or open propshafts?
    Rigid shaft with shaft coupling, fixed to the gear flange.

    exhaust manifold.

    It's also my guess, I just wasn't sure. (I'm just a hobby engine mechanic, just for my engines)

    We always have discussions with VOLVO mechanics about the right oil. I only use SAE 40 for 2 strokes. The Volvo mechanics always say "nonsense"

  4. #4
    Join Date
    Nov 2009
    temecula, ca., usa

    Default Re: 12V71 Twin-Disc Gear Water in the Gear

    Ok...so you are saying that the engine crankcase is water contaminated? I'm not familiar with most boat drives as my Detroit experience is mostly with trucks, buses and off road construction equipment.

    The engine oil can be contaminated with coolant in several ways on a Detroit 2 cycle engine. Yes, the oil cooler is often the culprit but, depending on some other factors such as water above port engine block and whether or not you have aftercooling for turbocharged intake, it could be something besides the oil cooler for sure.

    Another possibility might be a cracked injector tube (or "cup" to some folks). The dead cylinder that you found might be a good place to start looking because pulling the valve cover is probably one of the easiest chores that you face if you decide to investigate. You say that it's a closed system so that makes pressure testing easy and should help you find the leak.

    As far as the Volvo guys pooh-poohing 40 wt. oil for the 2 cycle, they are simply wrong. That's not ME saying that, it's Detroit Diesel and the oil manufacturers. While it's true that vicosity "shear" characteristics of the latest multi viscosity oils is much improved, it is still a concern for operating the 2 cycle and those oils are still ONLY recommended for extreme cold environments.
    And, it's also true that operators use multiple viscosity oils in their 2 cycles and report no issues, I would not recommend the practice where reliability and longevity is of concern.

    Good luck to you and your friend.

  5. #5
    Join Date
    May 2015

    Default Re: 12V71 Twin-Disc Gear Water in the Gear

    Your friend should have the engines checked by a qualified mechanic with Detroit experience before buying the boat. Some Volvo or other brand mechanic isn't going to be useful. Rebuilding a 12v71 is a lot of work and money for your friend.
    The engine oil cooler is on the front of the engine and with some effort and plumbing, can be pressure tested in place. Otherwise take it out and to a radiator shop.
    You got a dead cylinder and compression test can rule out some of the causes. There are adapters that go in the injector cup to measure compression. Only someone that works on Detroits is going to have one. Sometimes one can be found on eBay. The injector can be pulled and taken to a shop that rebuilds injectors to be tested.
    Close inspection could reveal a injector cup leak into the valve area. A cup leak on the bottom of the head sometimes will put bubbles in the coolant. The cup can be replaced without removing the head, but if you can't find a real Detroit mechanic, they won't have the tools.

    There are many reasons for a dead cylinder, most have expensive fixes. Bad rings, piston, sleeve, valve, bent rod. Injector, cup or valve are probably the cheapest.

    I guess MTU is the Detroit dealer in Europe.

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