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Old 09-10-2020, 09:50 AM   #1
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Question How Much Smoke and Oil From a Diesel Exhaust?

I was having a survey done on a boat with a Lehman 120 engine that I was considering purchasing. When they dropped the boat back in the water after the hull inspection I hitched a ride with the owner back to his slip. When the engine was first started (after hanging overnight at about 12 degrees C) there was a fair bit of black smoke and an oily slick extending back about 2-3 m from the stern of the boat. By the time we had arrived back at the slip (about a 10 minute run at cruising speed) the smoke had turned lighter and decreased somewhat, but the slick extending back from the exhaust remained.

I'm new to old trawlers and am trying to learn what is "normal". It seemed like a lot of oil in the water to me. My little 13 hp Westerbeke never left any slick in the water even when cold. The engine supposedly has about 2001 hours on the meter.

I was planning on having a mechanic do an engine inspection but decided not to bother after the survey. For future reference what should I be expecting to see from this type of engine if it is in decent working condition? What sort of things should I be looking for as a first pass inspection I can do myself before paying a mechanic several hundred dollars to confirm an engine is in good condition?

Obviously a steep learning curve here for someone moving over from a sailboat. Thanks in advance for any advice.
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Old 09-10-2020, 10:20 AM   #2
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Are you certain that the slick was oil and not fuel? A bad injector could result in unburned fuel coming out of the exhaust leaving a sheen behind. Also the rule of thumb is that white smoke is water, blue smoke is oil and black smoke is fuel.
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Old 09-10-2020, 10:25 AM   #3
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Old engines, whether they are Lehman, Perkins or whatever, smoke. Lube oil smoke which is usually blueish, is almost certainly related to rings and black smoke which is usually partially burned fuel can be rings, injectors, overloading (but not in neutral at idle). Black smoke can cause an oil sheen that is difficult to differentiate from a lube oil based smoke sheen.

Running hard for a few hours can reduce it even when later started up from cold.

I suspect that the smoke you are seeing will not affect engine performance and the engine may have many thousands of hours of life left. But only you can decide if it is too much for your sensibilities.

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Old 09-10-2020, 10:36 AM   #4
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A 120 Lehman will smoke some at a 12C start. A bit of oil sheen then is normal too. Completely normal. Mostly due to incomplete combustion at lower temps with engine cold. The way to check this is to take the boat out and run it under at least half load for like a half hour or more, then come back to the dock and with engine hot and idling there should be almost no smoke and almost no oil sheen.

There are things that can go wrong (gear oil cooler, engine oil cooler) that can put liquid oil out the exhaust. Checking those get technical.
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Old 09-10-2020, 10:48 AM   #5
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Are you certain that the slick was oil and not fuel? A bad injector could result in unburned fuel coming out of the exhaust leaving a sheen behind. Also the rule of thumb is that white smoke is water, blue smoke is oil and black smoke is fuel.
Sorry; my bad. I really couldn't tell if the slick was from oil or diesel. To me at least they would both look the same. I get that an older cold diesel will smoke some and maybe pump out a bit of "oil" at startup. I was just wondering when and if I should expect this to stop in an engine that had no real mechanical problems.

Thanks for the reply.
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Old 09-10-2020, 10:51 AM   #6
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Old engines, whether they are Lehman, Perkins or whatever, smoke. Lube oil smoke which is usually blueish, is almost certainly related to rings and black smoke which is usually partially burned fuel can be rings, injectors, overloading (but not in neutral at idle). Black smoke can cause an oil sheen that is difficult to differentiate from a lube oil based smoke sheen.

Running hard for a few hours can reduce it even when later started up from cold.

I suspect that the smoke you are seeing will not affect engine performance and the engine may have many thousands of hours of life left. But only you can decide if it is too much for your sensibilities.

David
Thanks for the reply. I will be making a little "smoke color" reference card for my wallet, and taking it along for the next boat I look at.
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Old 09-10-2020, 11:02 AM   #7
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Both David and Ski said to run it at cruising speed load for 1/2- several hours. Seems not exact but they're both telling you to take it out and run it at cruising speed roughly 1600-1900 rpm for well over an hour. It should stop this slick from showing up afterwards. Basically a lack of use issue. If you know for a fact the current owner has been running the boat under load for several hours on a regular basis the past few months, then this "test" won't make any difference as one is assuming the engine hasn't been run loaded much lately. Maybe you can convince owner/broker to allow this. Sounds like a great idea to me as well. But you did admit that evaluating this type of engine on your own is a steep learning cove. I personally, see the value of hiring a mechanic. Hope I cleared up why and when you'd run this type test. It's just basically an attempt at "blowing out the carbon" from a granny driven vessel. If it not been driven in a granny manner, no point in doing it. It requires you to examine the current owners back story and evaluate it for truth.
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Old 09-10-2020, 11:50 AM   #8
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Listen to Ski.
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Old 09-10-2020, 11:51 AM   #9
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Both David and Ski said to run it at cruising speed load for 1/2- several hours. Seems not exact but they're both telling you to take it out and run it at cruising speed roughly 1600-1900 rpm for well over an hour. It should stop this slick from showing up afterwards. Basically a lack of use issue. If you know for a fact the current owner has been running the boat under load for several hours on a regular basis the past few months, then this "test" won't make any difference as one is assuming the engine hasn't been run loaded much lately. Maybe you can convince owner/broker to allow this. Sounds like a great idea to me as well. But you did admit that evaluating this type of engine on your own is a steep learning cove. I personally, see the value of hiring a mechanic. Hope I cleared up why and when you'd run this type test. It's just basically an attempt at "blowing out the carbon" from a granny driven vessel. If it not been driven in a granny manner, no point in doing it. It requires you to examine the current owners back story and evaluate it for truth.
Based on a previous survey from 5 years ago the engine has been run 11 hours in the past 5 years. The owner claims they have just been too busy to use the boat, and that is the reason for selling. Based on other issues the surveyor has found I have walked away from this one. Surveys sure aren't cheap; but if you find a good one who is working for you and not the broker; they sure can pay for themselves in future money and grief saved.
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Old 09-10-2020, 12:44 PM   #10
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This is good news. You're listening to the surveyor and understanding that lack of use causes issues. Then you do what's right for you. You're bound to find a boat that's right for you. cheers
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Old 09-10-2020, 01:16 PM   #11
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In my limited experience (which is a heck of a lot more limited than Ski's) you need to run rather hard, about 200-300 rpm off of top for at least an hour for a Lehman to really blow out carbon build up, clean up the injectors which results in a semi-permanent lessening of smoke and sheen on startup as well as during normal runs.

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Old 09-10-2020, 01:44 PM   #12
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I had twin 120 Lehmans. One of them developed a problem and was pulled and completely overhauled by the Ford Lehman dealer in Dan Diego. Bored with new pistons, the whole nine yards. Both engines smoked at startup before the work was done and spit out a diesel sheen. Afterwards, well, not one bit of difference. This is what they do, hot weather or cold. about twenty years later, I had a know-it-all lock tender on the Caloosahatchee lean over the wall and look at my idling 120s and tell me I needed an engine overhaul - I laughed him off.
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Old 09-10-2020, 02:30 PM   #13
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A 2000 hour engine should not smoke beyond warm up.
Hold a paper towel or rag over the exhaust. If it's black and oily, then the rings or valve guides are probably worn. A rebuild is in the future.
Fuel won't be as black and will smell like diesel. Probably a clogged injector nozzle. They can be rebuilt or replaced at minor cost.
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Old 09-10-2020, 05:45 PM   #14
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A 2000 hour engine should not smoke beyond warm up.
Hold a paper towel or rag over the exhaust. If it's black and oily, then the rings or valve guides are probably worn. A rebuild is in the future.
Fuel won't be as black and will smell like diesel. Probably a clogged injector nozzle. They can be rebuilt or replaced at minor cost.
True enough, but for myself, I am walking from any boat with a diesel doing that. Lots of other stuff might be wrong, but the power plant is not allowed to show any sign of weakness or neglect. Yes, I will pay more, but the right boat for me is the wrong boat for somebody else. Diff strokes.
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Old 09-10-2020, 05:58 PM   #15
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I find that diesels, whether my DD's in the boat or the modern Cummins in the land yacht, wake up noticeably better with a few hours of engine heat.
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Old 09-10-2020, 06:43 PM   #16
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True enough, but for myself, I am walking from any boat with a diesel doing that. Lots of other stuff might be wrong, but the power plant is not allowed to show any sign of weakness or neglect. Yes, I will pay more, but the right boat for me is the wrong boat for somebody else. Diff strokes.
Rich:

Your engine, a Yanmar 6LP I believe, is a generation newer than the Lehman discussed here. The Lehman smokes. Your Yanmar 6LP smokes less. Newer common rail injected engines smoke even less. There is nothing wrong with an older engine smoking. It is the nature of the beast.

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Old 09-10-2020, 06:56 PM   #17
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Rich:

Your engine, a Yanmar 6LP I believe, is a generation newer than the Lehman discussed here. The Lehman smokes. Your Yanmar 6LP smokes less. Newer common rail injected engines smoke even less. There is nothing wrong with an older engine smoking. It is the nature of the beast.

David
+1.
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Old 09-10-2020, 06:58 PM   #18
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Rich:

Your engine, a Yanmar 6LP I believe, is a generation newer than the Lehman discussed here. The Lehman smokes. Your Yanmar 6LP smokes less. Newer common rail injected engines smoke even less. There is nothing wrong with an older engine smoking. It is the nature of the beast.

David
David!
I spent 29 years with two 120 Lehmans. I KNOW they smoke at idle and clean up well underway. I think I said that here.
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Old 09-10-2020, 09:27 PM   #19
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Mine used to smoke and leave a bit of a sheen. After a total rebuild there is very little smoke at start up and none after it comes up to temp.

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Old 09-10-2020, 09:58 PM   #20
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I think Ralph said he was moving on to the next one ... excellent decision - imho! Any internal combustion engine, kept near the water that has only seen 11 hours use in 5 years - is likely gonna be a money pit! (not to mention the rest of the boat)



Wonder when the oil was last changed??


Good luck Ralph, keep us posted on what you find.
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