Onan Smoke

[chris piazza]

Last month whenever I start the Genny (15kw 1980 Onan 2800hrs) it would smoke horrible, cold or hot load or no load. I knew the Injectors needed some work and last week I sent them to Scrod (Derek) and he rebuilt them for me and did an excellent job! now when I start the genny its smokes when cold and no load, this I understand. when I apply a load and its over 75%, no smoke and runs perfect. If the load goes under approx 50% I get the blue smoke again. Also when I run it no load, to cool it down it smokes bad again. Its so bad I only start it away from the marina and make sure its off before I get back. Before the injectors where done I was getting some fuel in the water, that problem has stopped. Any Ideas on the smoke?

Chris
Superior Nights 53C

[Pascal]

"If the load goes under approx 50% I get the blue smoke again."

blue smoke is oil, not fuel/injector. rings ?

[MikeP]

Blue (oil) smoke under load is typically piston rings; under no load it is typically valve guides. Do these engines use valve seals? If so, it's possible the seals have become brittle and ineffective and new seals would reduce/eliminate the smoke.

[chris piazza]

I have no clue if they have valve seals. I didn't think it was a ring because it doesn't smoke all the time. If it is a valve seal can I continue to use it this season and than deal with it during the winter layup? Other than the smoke it runs perfect

[MikeP]

I would assume it has valve seals, at least on the intake valves. If it's valve seals, they are not hard to replace. It just requires removing the valve cover, the rocker arms and the valve springs. On a 4 cyl engine a mechanic should be able to do it in 2-3 hours (maybe somewhat less) if the access is reasonable.

Lots of engines run for years with bad seals/guides/rings, etc. As far as running it; no it won't hurt the engine per se though depending on severity it could cause a lot of carbon buildup around the valve stems/piston rings. It will also, of course, use oil so you need to check the level more often.
The longer you wait, the more buildup there will be on other components and them more subsequent work may be necessary.

Unfortunately, unless the seals are so bad they have crumbled/broken and pieces are laying in valve spring area of the head and visible, the only way to check it is to essentially perform the same operation that it takes to replace them. Essentially, I'd suggest they be replaced because the symptom is a common indicator of bad seals and if you remove the valve springs to check them, it's silly not to just go ahead and replace them. If they are the common umbrella seals, they don''t cost much at all.

[chris piazza]

Okay, sounds like something I can do this winter. Don't really want to get into it now because vacation is coming up in a couple of weeks and I don't want it down than. I guess I still don't understand why they stop smoking under load? seems like if the seals are bad it would still smoke all the time. Is it possible that the rear injector pump seal is bad? When I first got the boat it was bad (per the PO) and I changed it and it stopped burning oil, but I knew the injectors were bad so it still ran crappy. now it runs perfect under load but smokes like crazy when its not. Damn diesels drive me crazy!

[MikeP]

"seems like if the seals are bad it would still smoke all the time."

There is more engine vacuum on the valve stems/guides/seals at low throttle settings than high. This is more pronounced on a gas engine than a diesel but the symptoms are the same.

[Avenger]

Also, when the engine is under load the cylinder temps are high enough to burn the oil more completely.

[Brian Degulis]

Blue smoke oil, white smoke un burned fuel, black smoke to much fuel. All true but not always as clearly as it sounds. Are you using oil? If it's smoking as bad as you say you should be adding oil but you don't mention that. After it's running and hot when you take the load off does it start smoking right away or does it take a while? If it takes a while check your temperature under load and no load it could be just a thermostat stuck open that's alowing it to cool down to much. Just to be sure check your air intake a restriction should be much worse under load but I'd check it anyway.

Brian

[chris piazza]

Okay I guess I may be describing the smoke wrong, its definitely blue, light blue but not white, not black and no fuel in water. I changed the impeller this winter and I get a lot of water flow from the exhaust, so I don't think its a heating problem. its a very noticeable smoke when first started and cold, it seems like a lot to me but the admiral says its not that bad, when we are under way (@10kts) you can see a slight blue haze down the port side when I first start it. under load 50% - 100% no smoke at all, as soon as I shut everything off to let it cool it immediately starts smoking just like a cold start.

Chris