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Old 07-13-2011, 12:31 PM   #1
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Question Onan 4K blowing gray smoke

My Onan 4BGEFA26100M (vintage 1995, unknown hours [but at least 286] cuz the PO cut the wire) quit running and spewed some oil out of the crankcase breather hose (I think)....

It appeared there were multiple exhaust leaks in the muffler, connections, connecting pipe and tail pipe on this genset. So I replaced it all. Fired the generator up to find it still exceptionally loud (now I'm thinking the the entire exhaust manifold is leaking). Nevertheless, I let it run. After about 10 minutes, it quit and spit about 1-2 ounces of oil out. The oil came out at the lower corner of the air preheater control cover. On the Onan diagram, it appears the crankcase breather hose is behind this.

Just to see if it would, I started it again this a.m. (about 12 hours after the incident) and the exhaust is definitely gray, like it's got oil in it, so I shut her down. What happened? What's the fix?

Thanks in advance, A.C.
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Old 07-13-2011, 01:04 PM   #2
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1-check compression,valves maybe stuck,or burned/ drain oil,have checked to see if gas is in it(rings)--is it tring to run backwards when shut off (timing). I have manuels for Genset III+part#book if you need something,will try to help
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Old 07-13-2011, 03:41 PM   #3
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Really sounds like broken rings, blow by into the crankcase. Follow Grumpy's advice and do a compression check, that will tell the most.
Bad compression could be valve, valve seats, valve guides, or rings. But if you are getting oil burn and oil out of the breather, probably rings. That's a pull out and rebuild. Can be pretty cheap if you can do it yourself, not so much if not.
Good luck, please let us know what you find!

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Old 07-20-2011, 10:17 AM   #4
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Thanks for the information. I pulled the muffler off and sure enough there's oil there too. So blowing out both ends.

Out of curiosity, I asked my automotive mechanic about the oil testing to discover the problem. He looked at me cross eyed and said, "I don't know anything about generators." And kept to that statement when I asked if oil testing was something that could be done to discover problems on any motor (referring to it as a general automotive principal). Further, I don't know where to have this done. The oil is very black. I changed it and the filter maybe 15 hours ago. I didn't check compression cuz there's no way to get to the rear plug without pulling the genny out. I haven't quite figured out how to muscle that 500 pound beast out of its hole yet and only want to do that once if I can fix it myself.

I called the local Cummins shop for a quote. They said $1100 for a rebuild. YIKES! That's all the guts, which I guess if it's opened up, is the way to go. This is a 15 year old engine with at least 300 hours on it. (estimating 15 that I put on it, above the 286 on the wire cut clock). Seems low for this problem, but I don't know anything about it's life for the last 15 years. Of course, the service guy said, we see these with 20-30,000 hours on them. (Don't know if I believe that).

Could a guy (me) with a service manual do this job? I've never rebuilt anything, just done periodic maintenance on cars and motorcycles (except valve adjustments).

Thanks again, A.C.
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Old 07-20-2011, 01:35 PM   #5
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We did quite a bit of engine oil testing as a predictive maintenance tool on critical equipement in a large industrial setting. Never really heard of it being used much elsewhere.
On rebuilding the generator engine yourself, can't really recommend it unless you have experience. Most repair manuals assume you have a certain level of knowledge. As such they generally do not cover all that you may need to know.
If the $1,100 is a full rebuild with a warranty, it's not that bad. Of course most your cost will be labor (probably 70%), but worth it if you get a quality rebuild.
I don't think there are any easy fixes on this, but then again it's been years since I've done any engine work myself.

Good luck, let us know how things turn out.
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Old 07-20-2011, 02:56 PM   #6
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Checking oil for metal should be a normal event for diesel repair shops. They use it to determine if blowers or turbocharger bearings need replacement.

Dave
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Old 07-21-2011, 04:48 AM   #7
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Quote:
Originally Posted by acglass View Post
Thanks for the information. I pulled the muffler off and sure enough there's oil there too. So blowing out both ends.

Out of curiosity, I asked my automotive mechanic about the oil testing to discover the problem. He looked at me cross eyed and said, "I don't know anything about generators." And kept to that statement when I asked if oil testing was something that could be done to discover problems on any motor (referring to it as a general automotive principal). Further, I don't know where to have this done. The oil is very black. I changed it and the filter maybe 15 hours ago. I didn't check compression cuz there's no way to get to the rear plug without pulling the genny out. I haven't quite figured out how to muscle that 500 pound beast out of its hole yet and only want to do that once if I can fix it myself.

I called the local Cummins shop for a quote. They said $1100 for a rebuild. YIKES! That's all the guts, which I guess if it's opened up, is the way to go. This is a 15 year old engine with at least 300 hours on it. (estimating 15 that I put on it, above the 286 on the wire cut clock). Seems low for this problem, but I don't know anything about it's life for the last 15 years. Of course, the service guy said, we see these with 20-30,000 hours on them. (Don't know if I believe that).

Could a guy (me) with a service manual do this job? I've never rebuilt anything, just done periodic maintenance on cars and motorcycles (except valve adjustments).

Thanks again, A.C.
try these folks for oil analysis. any large truck service shop probably does oil analysis in house or knows where to get it done locally.

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Old 07-21-2011, 07:21 AM   #8
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It sounds like Rings,like above blackstone is the one we used in the past , aNAPA could recamend someone in your area,one more thing that could be wrong,valve seals or guilds --GOOD LUCK
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Old 08-30-2011, 09:05 PM   #9
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Finally got it into the Rocky Mountain Cummins shop in Colorado. What a racket! And of course the only game in town for generator work. The mechanic drained the oil and found gas in the crankcase with it. Says one of the plugs fouled causing a cylinder to shut down filling the crankcase with fuel. This is what was coming out both ends. He said they call it, "making oil" when this happens. Wanted to charge me $450 to change the oil and plugs!!! I asked him twice if he did a compression check, and he said no. Does his explanation make sense?

A.C.
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Old 08-31-2011, 01:35 PM   #10
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Hmmm. I have some trouble with this. If your plug is fouled without ring/valve/valve stem or guide issues, then you have an ignition problem which I doubt in this case. IF (the big IF) you are not burning oil (again rings, valves, etc.) AND there is an ignition problem, the plug should come out wet with fuel but otherwise clean. Fuel that is not burned will be exhausted and not blown down into the crankcase unless there is a pathway (broken rings, or in extreme cases piston damage). Get the compression check done. Also, if it smokes you are burning oil!
Of course my experience is from 35 years ago as a service manager in an MC shop (found a different career path early on!), but the internal combustion engine has not changed that much.

Good luck, keep us posted.
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Old 08-31-2011, 05:47 PM   #11
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maybe...

If ell was ok inside, and a plug was not firing for a long time then the cyl wall could get wet with fuel and get by the rings into the crank case.

This would be after the fuel has wiped allof the oil off and maybe damaged the cyl walls.

Remove the plug and see how wet it is, put new one in and see.

While it is out check for spark, most 2 cyl engines fire both plugs at same time, one is on compression the other is on exaust, so it makes for simple less parts design.

If it wasonly firing on one hole it would have been rough and sounding, but since only 2 holes 180 apart not too bad, just not sounding right and no power.

ONANs are noisy anyway.

So for DIY steps, change the oil with standard 30 wt, if it has a filter then use detergent (automotive) type, if it does NOT have a filter then use NON-Detergent type, if no filter you want the crud to settle out into the bottom of the engine, if you have a filter you want it to stay in suspension so the filter can get it.

New plugs and make sure the fuel is good, air cleaner good and clean as a dirty one makes it rich and can do same.

If the wet plug is the hard one then the PO may not have EVER changed it.

Now test run it, let idle for a few minutes, shut it down and check both plugs, if they look good, re-start and turn on the AC for 30 minutes, then re-inspect.

If still smoke free and plugs are good, then you may be ok, if smoke or wet plugs then more work...

The engine is not much different than that on the lawn mower, bullet resistant and still work when trashed, but will blow up at some point.

The ONAN will toss a rod through the crank case if it is abused, if it is not a low speed (operates at 3600 RPM) then it is running at the upper end at all times, so it is pushing the comfort zone for the engine and if something is broken it will need to be repaired before the thing blows up.
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Old 09-06-2011, 04:11 PM   #12
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The mechanic (yes I left it with RM Cummins cuz, again, there's no other place in town), says he took the heads off to remove the carbon build up (not that much) and replaced the manifold gasket (some blow by) changed oil, filter, fuel filter, and plugs. He said valves and heads looked good. Still no compression check, but ran it on the load tester and it did fine. He says that the carb was out of adjustment (too rich). And since the plug was fouled in one cylinder, the fuel was not being detonated and passing through the valves and/or piston (I'm unsure of the path; maybe the exhaust valve?) while the engine was operating. Once the crankcase overflowed it cut the motor and spit this oil/fuel mixture out both ends (what I described happening in the original post).

What do you guys think about that?

Thanks again, A.C.
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Old 09-06-2011, 04:40 PM   #13
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onan blowing smoke

something i thought about is the fuel pump if your engine has a pulse fuel pump that uses changing crankcase presures to pump fuel i would look closley at the pump. the pump type i am talking about has a hose from the pump to the crankcase to deliver changing pressures to pump diafram if the diafram leaks into the pulse line it will fill the crankcase oil with fuel.lee
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Old 09-07-2011, 12:28 PM   #14
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My 89 Onan 4kv generator was blowing blue smoke when I bought the RV.

I'm a fit-it guy, so time to do some work.

Bit the bullet, pulled it out (used a motorcycle jack) and took it apart. When the heads came off, there was oil standing in the cyliders. Rings, rod bearings, head gaskets, oil pan gasket and a honer. Oil and filter, too.

Honed the cylinder walls, installled new rings and bearings, gaskets and heads back on and oil pan and gasket back on. Reinstalled and fired it up. Has run like a champ every since. Put about 45 hours on it since the "rebuild".

It's a shame they didn't re-ring it while they had it apart. Then it would almost be "new".
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