Go BottomJD 318 has oil in the breather
 | Greg franklin
Join Date: Posts: 1 
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2001-04-19 26942
Sitting beside the 870 is my 318 with an 18 hp Onan engine. The oil level is correct, 95 psi compression on each cylinder and oil is pooling in the breather. this is being ingested into the carb and burning as white smoke. From the breather is a tube going to one of the covers over the lifter cavity. At this cover is a reed type check valve that appears completely normal and clean (except for oil!). I take it that this is part of the crankcase ventilation but I don't see how. There isn't an obvious vent hole going into the block that I can see. Another possible clue is when I check the oil (after a period of non-use), the level is extremely high, about 3-4 inches on the dipstick. Wipe it clean, redip and it is at the correct level. As I type I am now wondering if as the engine cools, some pressure differential is causing oil migration??? I am stumped, any ideas? thanks, Greg
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley 
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2001-04-21 27035
I don't know the tractor, but I'm guessing it's a gas engine with a closed crankcase system. I think I'd sort out the oil level issue first. I believe that dipsticks can read high in a cold engine unless the stick is wiped first. I don't know if it's an automotive type dipstick, or a screw-in small engine type. If it's a screw-in type, maybe a manual says if it's supposed to be screwed in to take a reading. If the level mark assumes the dipstick is screwed in, and the reading is taken without screwing it in, then the crankcase may be over-filled. Regarding pressure differentials: If it's a conventional crankcase/oil pan & pump system, then oil should stay on in the pan when the engine if off, irrespective of any pressure in the crankcase. Wiping the dipstick immediately after shutting down the engine, and again later could check the condition. I'd also make sure the fuel shut-off valve is off when the tractor sits awhile. Regarding oil pooling (in the carburetor intake I guess): A line going from the rocker-arm cover to the carburetor could be the crankcase breather, and the only one. I guess, excessive oil could be introduced into the air-intake by excessive oil at the engine end of the breather, excessive blow-by, excessive vacuum on the breather line, or any combination. The valve's purpose probably is to meter vacuum on the breather line. It seems like the valve would have to be open for oil to get to the air intake. If it was stuck open, engine performance should be affected. Comments in these forums shouldn’t be taken as recommendations, but I think I'd disconnect the breather hose on the crankcase side and run the tractor. The amount of blow-by as well as the presence of vacuum on the line could be verified. If the breather line comes off the top of the rocker-arm cover, you wouldn't expect a lot of oil in that area. It's unlikely that oil returns passages around the push rods would be blocked, but perhaps the oil feed to the rocker arm is open (such a condition could produce low oil pressure throughout the system that could damage an engine). The engine could be run with the rocker arm cover removed. You don't see a lot of oil there with most engines. The engine oil pressure also could be checked. Good luck sorting it out. Hope these are some ideas here. ....
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| Norm
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2001-04-21 27050
As a suggestion to try to isolate the problem, I would drain the oil, replace the filter (just because it is always a good idea) and fill with the specified volume of oil. The whole situation you describe could very well be caused by an over-full crankcase. This, as a starting place, will at the very least rule that out so that you can begin trouble shooting from a known baseline. ....
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| Greg franklin
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2001-04-21 27062
Norm, as an experienced oil-overfiller, I made sure this wasn't the case. I still don't have any logical explanation for an extrordinarily high reading on the stick on the initial pull-out. Maybe it is breaking the vacuum in the tube itself. I don't know. Subsequent readings are fine. And TomG, thanks for your fine troubleshooting blueprint. Here is what I have learned so far. The lifter cover does indeed enclose a big port going to the block interior. The reed valve in this path operates to let pressure out into the carbeurator intake. I assume that this is when the two horizontally opposed cylinders are travelling towards each other (crank throws are 180 degrees apart) and pressure in the crankcase builds. I further assume that when the pistons travel out, the reed valve shuts and a strong vacuum is created in the case but I don't know where the air is scavenged from if it even is. The oil is definately appearing during engine operation. I talked to a JD mechanic today and he shook his head and said the crankcase is becoming pressurized from ring or valve blowby. Further the 95 PSI compression should be closer to 125 PSI. This says to me that in this case the normally pulsating crankcase in now a more or less constant air pump and oil is carrying with it! He said he could bring his water tubes (manometer?) and tell me if there is vacuum on the crankcase. I don't have a trailer or I would already have had this tested. All this is exactly as you outlined. I guess I will run it like this until winter and then overhaul the engine. Sure wish it was an overfill problem! I'll keep looking and hoping but I think we know....Thanks again. ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2001-04-22 27069
Every now and then I get some pretty far-fetched ideas. I hadn't thought about horizontal opposed pistons pressurizing the crankcase. That idea led me to wonder about the function of the reed valve. I can't reason through how an engine would work very well if the intake manifold was fed an alternating pressure cycle. I guess that led me to wonder about the valve and what it looks like. I wonder if there's any possibility that it has another passage that's obstructed or could be installed backwards. I don't know the age of the tractor, or even if new tractors have automotive type emission control stuff-- like closed crankcase systems. If not, then trying to interpret the system as performing emission control may lead to some wrong conclusions. My impression was that 95-lbs. compression might be a little low. However, the good thing is that both cylinders are the same. Compression loss doesn't often affect all cylinders equally. ....
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| Greg franklin
Join Date: Posts: 1
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2001-04-22 27075
The machine is around 1985 vintage. The reed valve looks exactly like a piece of flat shim stock about .035 to .040" thick. There is a light coil spring to hold it flat against the port so I don't believe that the reed itself does much flexing. I can't find or see any place that air is introduce into the block. In thinking about the pulsing (and this is all unsupported conjecture) I would think that as the pistons create a vacuum, a small amount of air is scavenged from various clearances, some from vacuum assisted blowby (I think a lot in my case) and then an increasing negative pressure condition for the rest of the expanding volume. At this time the reed is drawn against the port and no significant air should be leaking past it. This reed valve system looks in perfect working condition. Now as the spark ignites the one cylinder with a compressed charge, the pistons travel towards each other and create a positive pressure within the block which is directed past the reed valve and into the region enclosed by the air filter. This positive pressure wouldn't occur until the existing vacuum is compensated for. I guess at this time the non-combusting cylinder is performing an intake stroke and hence via the reed valve is receiving a positive pressure blast from the crankcase? Granted the pressure blast is released within the filter region where it can escape and equalize with time so maybe there is enough time and filter area that this pulse is of minimal contribution. Oh well, very interesting conversation but kind of hard to dance around what is considered to be low compression.... ....
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