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Old 02-08-2016, 03:50 PM   #1
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Onan Diesel Generator Blue Smoke at start

My Onan Diesel Generator gives blue smoke for about a minute when it starts. I don't recall this in the summer, but it is very noticeable with the winter weather (freezing and a bit below).

I know that blue smoke (called ash in the literature) comes from burning engine oil. And, on cars, if it smokes all the time, it is bad piston rings, but if only at the start, it is worn valve guides.

Is this line of reasoning the same with a diesel generator? My RV is new and the generator should be under warranty, so I want to know whether this is unusual and I should talk to Freightliner. The generator has almost 50 hours on it, and I'll change the oil this spring. I'm using it to keep my batteries up for the winter.


Gordon Sick, Calgary
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Old 02-08-2016, 03:59 PM   #2
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We have a generac diesel generator that does the same when it's cold, it's done it since new and doesn't use any oil.

2015 Columbus 320RS
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Old 02-09-2016, 07:23 PM   #3
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We had an Onan 5500 Quiet Diesel with Kubota engine on our Allegro Bay MH. It smoked at startup when very cold. I guess this is because the engine is too cold to properly burn the fuel even with the glow plugs working. It never used a drop of oil the whole 10 years we had it. With the battery monitor I could watch the current the glow plugs were drawing before and after startup. Below freezing it took a full 3-5 minutes for ours to stop pumping out blue-white smoke. Once in a snow storm I almost filled the north end of Mammoth campground at Yellowstone before it stopped.
Don't know why it does this when the Cummins main engine didn't do it and even our Kubota diesel tractor doesn't do it although in Fla. it doesn't get much opportunity. But it does have basically the same engine as the Onan only slightly larger. In cold weather all you get is a small puff of black smoke when it starts.
Plan on getting a lot of strange looks in cold weather.
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Old 02-09-2016, 09:38 PM   #4
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I'd say normal. IMO, It's not oil, it's a rich fuel mixture from cold combustion temp.
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Old 02-10-2016, 12:01 PM   #5
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I think it is just the nature of diesels I have been fooling with diesels for decades snd for the most part a little blue smoke when they first start up I have one in my dynamax dx3 and it puffs,a little blue when first started
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Old 02-10-2016, 05:34 PM   #6
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That is normal. nothing to worry about. I have owned many diesels. Big trucks, pickups, tractors, etc. They all do that on cold starts. The colder it is the worse it is. ( I live in MN)
Even with engine block heaters and oil pan heaters on bigger diesels they still do it on start up for a few minutes.
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Old 02-10-2016, 06:11 PM   #7
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Diesels Have a high compression ratio and because of that they also need to get warm to decrease the clearances between the piston and cylinder walls. That is why the truck drivers like to keep their engines running while the have their sleep breaks. the critical time for any diesel is just after start up and putting a load on it. The colder they are the more smoke you will see. Have been a diesel mechanic for over 50 years. ENCM RET

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diesel, generator

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