Hello all, I have read postings from this forum several times to solve general questions. Now I have a more specific problem I was hoping one of you might help with. I have a '99 Miller Bobcat with a 16 hp Onan (580 hrs). It was blowing oil out the exhaust to the tune of 1 pint/hour. Compression check read 127 psi on both cylinders. I figured it had to be valve seals and maybe valve guides. I tear it down, and the valve stem guides check within the middle of Onan specs, as do the valve stems. The seals seemed to be as pliable as the new ones I bought.
So, I'm wondering if there is something I am overlooking that one of you may have encountered before. I have new valve seals and valve guides which I am going to install. Then lap the valves, adjust valve lash and build my way back out. I'm just a little worried that I didn't find obvious wear. I am anxious to see what you all come up with. The people on this forum have always impressed me. Thanks in advance from the newbie!
Hello John_Jackson, forgive me if this seems obvious, but are you possibly overfilling the crankcase? It's been quite some time since I've had much to do with Onan engines, but if your valve stems, guides, and valve seals are all in good shape the only other possibilities for oil issues would come from the oil control rings on the pistons themselves or the PCV valve. Check the intake tract to see if there appears to be excessive oil present in the intake manifold, if you are seeing an excessive amount of oil there then it is likely that the PCV valve is acting up and the motor vacuum is drawing oil into the intake manifold, on through the cylinder, and out the exhaust. If you aren't finding excessive oil there it is possible the oil control rings are having an issue. Just my $.02. Best regards, aevald
Thanks for the reply aevald, I don't mind anyone pointing out the obvious. I got this machine used, and I even ordered a replacment dipstick to be double sure myself. I also tried switching from variable wt to 30wt with no apparent change. I didn't think it would be possible for that much oil to get by the rings and still maintain such good compression. Or, could it be that the oil around the rings is what is causing the good compression? No luck with the PVC valve, I had checked that too. The intake manifold was dry as a bone. The cylinder walls are bright and smooth without a hint of a ridge at the top. I'll wait to see what other ideas come up before I pop a piston out of the bore. Onan seems a little proud of their parts.
Hello again John_Jackson, rlitman had a good point about the oil filter bracket. It very well could be something like this, especially if you're finding oil everywhere and not able to really pin it down. As far as relating the oil to the compression and such. Take a compression test with the cylinders dry, meaning simply attach the gauge and spin the motor over and take note of the compression(sounds as if this is what you've already done), next, squirt a little bit of oil into the cylinder and spin it over again and see what you get for compression. Generally, if you have a burned or warped valve, adding oil to the compression test will not change the compression values, adding oil and then checking the compression and noticing an increase after adding the oil is an indicator of having bad rings or rings that haven't seated. If this is what you notice then the rings could actually be the issue as much as you don't want to have to deal with this. This is a bit off topic but some of the Onans had an issue with the plunger that operates the points, there is a seal on this plunger and when it gets old it will allow oil from the crankcase to pass into the box where the points are housed and fill with oil and cause the engine to run poorly or not start at all. I realize this isn't the issue you asked about, but for your future information you should keep this in mind. Good luck and regards, aevald
Sorry to ask (especially as I am sure I'm more of a newbie than you), but is it blowing out the exhaust, or around the exhaust? My Miller AEAD-200le with an 20hp Onan engine, was blowing out at least a quart an hour, which turned out to be coming from the oil filter bracket, but at that rate, everything was bathed in oil.
Seems that Onan used a rectangular plate which has two bolts and a gasket to connect it to the block (instead of having the filter thread onto a nipple directly in the block), and these brackets are known for coming loose. One bolt was almost impossible to get to without removing the starter. So much oil was getting ejected that finding the source drove me crazy.
Don't know your particular engine too well (unless its a B48M), but could it be something as simple as a clogged crank case vent valve, or bypass valve?
Also, if you're looking for an engine repair manual, I have found that while Miller does not provide the engine repair manuals (just the manuals for the welding half of the unit, with a few preventative maintenance tips for the engine half), you can get the engine repair manuals on the web for Gravely tractors (these Onan engines are the same as used in those tractors, with the exception that the welders have speed controls added).
Best of luck to you, and if you happen to have a B43M engine (which is 16hp), its the same manual as the B48M, so I can send you a pdf copy.
-- Robert
So far I'm leaning towards the oil control rings. Thanks for the hint about the bracket rlitman, I'll try to remember that one down the line. This engine is clean and dry everywhere. Just blowing oil out the exhaust. This Onan is a P216 just for reference. aevald, I don't know if a "wet" compression test here would apply since there is already so much oil getting in the cylinders. That's why I was wondering if that may prove the problem in a round-about way. By default, my compression test was already "wet".
So, if nothing else comes up in the next day or so the pistons will come out this weekend. Thanks to you both for your input.
Hello John_Jackson, that very well could be a big part of the answer. One other thing that you could possibly try, would in a sense be similar to a leak-down test. If you can position one of the cylinders so that it is on it's compression stroke with both of the valves closed and then apply some compressed air to the cylinder through the spark plug hole(make sure this air isn't too high in pressure, possibly 30 to 40 psi.) with an adapter and a pressure regulator and then listen through the oil fill spout for bleed-through it might give you an idea as to the condition of the compression rings and such. Do this with each cylinder and compare the results. Kind of flintstonish, but might just tell the tale. Regards, aevald
Sounds good, but I have already yanked the heads. Honestly I don't know what else it could be but the rings and possibly a little leakage from those valve seals. I just checked and they have a couple ring sets in OKC on the shelf. So it looks like I'll be picking those up tomorrow and re-ringing over the weekend. Thanks again aevald.
Gotcha. I don't have the Service Manual for that, but I found a parts catalog with complete exploded diagrams if its of any help to you.
You should be able to get it at:
http://www.propane-products.com/1ENGINES/96500265.pdfor, if you're having trouble with their site (its been a bit weird), I downloaded it and can send it to you.
-- Robert
Downloaded just fine. A very good parts book/pdf. Thanks for the help rlitman.
Was this unit working fine and this happened all the sudden? Or had the engin been in storage for a while and this is the way it has been since You started using it again? If the engine sat for a while, there is a chance that the oil rings are "all gunked up" and not scraping the oil from the cylinder walls. There are some products available in auto stores like "Pep Boys" that are supposed to solve this problem, I never needed them, so I cant vouch for any of them.
I have had three Bobcat 225's.with the 16hp Onans,the only oil related problem that I had with either of them was if the air filter became the least bit dirty ,they would start collecting oil in the air cleaner.
My .02 cents worth
gawelder
You may also want to check your carb. Any excessive unspent gas will end up in the oil. Check the plugs.
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