The Case of the Missing Oil Leak

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An engine seldom “just gets better”—close to never—but our generator was leaking vast amounts of oil from the rear main oil seal prior to the cylinder head replacement. If the generator sound attenuation shield front cover was removed during operation, it would spray the engine room floor and the neighboring wing engine, creating a real mess. With the cover on the mess was more localized, but still a mess, spraying down anything in the vicinity of the generator cooling fan.

Above is a picture of the engine 179 hours since the cylinder head change, and 29 hours since it stopped leaking entirely.

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In the picture above, you can see what the engine looked like during its heavy leak phase. We wiped it off frequently and kept oil rags near parts of the cover where the oil would leak out. The leak didn’t affect engine operation, but it really made a mess.

When we replaced the cylinder head we also purchased a rear main oil seal, but elected not to change it until the exhaust valve problem was known to be fully corrected. We wanted to give it at least 50 engine hours before taking on the rear main oil seal replacement, since it is quite a big job and we really don’t want to start on it until we knew there were no other generator problems.

And then a strange thing happened. After replacing cylinder head, the rear main oil seal leak went from intolerable to moderate. We were relieved and decided to defer the job until we had more time this winter. And then, about 150 engine hours back, it stopped leaking entirely!

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Almost certainly the oil leak will return, but it’s nice that the big job of replacing the rear main seal does not need to be done immediately. The new cylinder head now has 289 hours, and the generator is running great and producing full power. The rear main oil seal has leaked very little during that period and not at all during the last 150 hours. We continue to enjoy the engine service reprieve and we’re testing a few hypotheses as to why the generator rear main oil seal is no longer leaking. 

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When we replaced the cylinder head, we also installed new injectors. The leading candidate explanation is the original problem was caused by a worn rear main seal combined with diesel oil dilution from poorly atomizing fuel injectors and operating with a failed cylinder (injected diesel not fully being burned). Normally diesel oil dilution will show up as a rising oil level, but it was leaking so much oil prior to the cylinder head replacement that this issue might have been hidden.

We’re far from confident in this conclusion, and we’re continuing to gather data and see what we can learn. We’re finding this super-interesting, and we’re glad that we didn’t change the rear main oil seal at the same time as the cylinder head because it gives us a chance to learn more about the problem.

The rear main oil seal shouldn’t leak even in the presence of fuel dilution, but without dilution it’s back to just barely able to do its job. So its clearly getting close to end of life. But, with the fuel dilution corrected, it appears to be able to do its job and has stopped leaking entirely for the last 150 hours.

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Above is a picture of the engine taken today, where you can see no evidence of oil leaks and there haven’t been any for the last 150 engine hours. If you look carefully, you can see we’ve also made a slight change to the engine crankcase breather system. We’ll blog that later, but it’s just a neater version of that red shop rag we had hose-clamped to the engine oil fill to ensure the crankcase was at atmospheric pressure. Positive crankcase pressure is one of the most common causes of engine oil leaks.

We expect we eventually will have to change that rear main oil seal that’s been in use for the last 7,000 hours. But, in the interim, we’ll enjoy the reprieve and the nice clean and shiny engine.

 


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10 comments on “The Case of the Missing Oil Leak
  1. Phil McClelland says:

    Hi James,

    I am vicariously loving your journey, both through the water and mechanically. (“One day, one day” I mutter to myself.)

    Must admit; the first thought I had when I read the oil had stopped leaking was it had run dry…
    It’s been a few years since I worked on marine diesels but can’t recall any that behaved like this.
    What are the fuel burn numbers like with the reconditioned injectors?

    • I suspect your thought is correct that poor injector atomization wouldn’t do this. I think what was happening is the cylinder with the damaged valve wasn’t firing and that fuel was leaking into the sump. The engine was leaking so badly I didn’t see a rise in oil level. With the new head (and new injectors), all three cylinders were firing and with the oil change, we had non-diluted engine oil in the sump and the leak stopped.

      The seal shouldn’t leak even with fuel diluted oil so I’m sure the leak will be back as the seal continues to age but, at this point, it has gone more than 500 hours without leaking.

  2. Don says:

    James,
    Any chance there was a small oil leak on the head gasket above the main seal and leaking down the back of the engine. If I remember the video, the gasket separated when the head was removed and may not have been obvious. I enjoy watching your videos and really enjoy the attention to detail and how neat and clean you keep everything.

    • MVDirona says:

      That’s a good point but on these engines, no pressure oil passes through the head gasket so it can’t leak oil. The oil is brought up to the rocker arms in an external pipe but the rocker carrier and the valve cover can leak oil so your guess is good and it’s an easy miss-diagnosis to make. But, in this case I checked very carefully and there was no oil leaking down from above the bell housing prior to do this job so it looks fairly certain that the oil was coming from the rear main oil seal.

      Thanks for the ideas and for the feedback on the videos.

  3. Mike Ukena says:

    Hi James,
    If you remember, I made a previous comment about the seal possible sealing itself after you fixed the head. The reason I thought of that was that I had an automatic transmission that had a recall and the dealer added an additive to the transmission without making allowances for the volume. Of course, it ended up overfilled and the seal leaked all over the garage floor. When I fixed the issue, it quit leaking…since, 50,000 more miles with no leak!
    Glad it worked.
    Mike

    • MVDirona says:

      At this point, anything is possible but in this case I’m sure the oil level was correct and, to avoid any possible crankcase pressure, I was running the crankcase open with only a shop rag over the oil filler so crankcase was definitely at atmospheric pressure. My working theory is the weak cylinder with the damaged valve seat was still getting normal injection volumes and when it’s not firing that diesel will wash down the cylinder walls into the crankcase and dilute the oil. Fuel thinned oil will have a greater tendency to leak at the rear main seal.

      I’m still collecting data on this one but I suspect that even diesel thinned oil should still get held back by the seal so, if this is the problem, the seal is still likely to be failing.

  4. RICHARD FABER says:

    Could it have been a case where the old cylinder head was somehow causing excessive crankcase pressure? If that were the case then you may very well be home free.

    • MVDirona says:

      I can’t think of any mechanism that would allow a cylinder head to cause pressure in the crankcase. And to avoid any possible crankcase pressure, I was running the crankcase open with only a shop rag over the oil filler so crankcase was definitely at atmospheric pressure. My working theory is the weak cylinder with the damaged valve seats was still getting normal injection volumes and when it’s not firing that diesel will wash down the cylinder walls and dilute the oil. Fuel thinned oil will have a greater tendency to leak at the rear main seal.

  5. Mal says:

    Well if it aint broke don’t fix it:) lucky you held off with that repair.

    • MVDirona says:

      I suspect the leak will be back but we’re loving the return of the clean and bright engine without needing to do any work.

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