So I just got my old Bobcat 520 running last weekend. Thanks to the tips from Tazza, I got the hydrostatic transmission reassembled and everything put back together. Filled it with lots of oil, fired it up and everything worked great for about 1.5 hours! Then, the engine started billowing lots of white smoke. Checked the engine oil level, and it was very high (2+ quarts high). So, it seemed like the smoke was due to the high engine oil getting sucked in the breather and forced into the carburetor (it's a gas engine). Being a gas engine, I checked the usual fuel suspects for leaking into the case and mixing with the oil and increasing the level. Didn't find anything obvious, but figured that must be it somehow. Drained the engine oil, refilled and got it to fire up again. After getting all of the oil out of the cylinders, and burning out what was left, it seemed to run fine for 3-4 minutes. But, after only that amount of time, it started bellowing smoke again. Checked the oil level again and it was already a good 1-2 qts high, after only running for a few minutes. No way fuel was getting into the oil that quickly. So, the only thing I can figure is the transmission blew the seal at the engine side and is forcing its oil into the engine? The hydraulic oil resevoir was also low after this, when it was above the minimum before it ran. This doesn't have a u joint or anything, just a straight coupling between the engine and transmission. There's an adapter housing that doesn't have any holes, so if the seal is blown on the transmission, seems like it could force oil straight through the bearing plate and into the engine. I haven't pulled it apart yet, but I'm suspecting this is what is going on. Anyone ever heard of something like this happening? For it to blow the seal and force a qt or two in a few minutes into the engine, seems like something must've let go inside the transmission to build that kind of pressure at the seal? Like a shoe popping off a piston or something and forcing the oil somewhere other than back through the valve plate? Looks like I'll be pulling the engine again and digging in, just curious if anyone has any good ideas on this one! Thanks
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I didn't think the pump bolted to the engine like that, a leaky seal should just allow oil to exit the pump and leak out around the machine, not into the engine......
The engine oil doesn't smell of fuel?
Your pump, how does it mount to the engine? are there any seals that may allow the oil to be pushed into the engine? or is it connected by a U-joint?
The engine oil doesn't smell of fuel?
Your pump, how does it mount to the engine? are there any seals that may allow the oil to be pushed into the engine? or is it connected by a U-joint?
Nope, no fuel smell in the engine oil. And it filled up so quick the 2nd time, I didn't see how it possibly could've dumped fuel in that quick. The only other thing I'm thinking is that maybe the oil level being high filled up some other areas in the engine, and it drained back down into the case after I got it started. The pump mounts to the engine via an adapter that looks kinda like a bell housing. It's about 6" long, 10" ish in diameter. Two bolts on the pump side, four bolts on the engine side. No u-joint, just a splined coupling on both the engine and pump side, with a coupler in between. I'm having trouble seeing how it could be moving oil that quickly from the pump into the engine also, but I can't think of anywhere else the additional fluid could be coming from... Still need to get some time to take some things apart and go exploring.
ancomcabs
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- Nov 3, 2011
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How is the engine cooled? Does the oil look like it was contaminated with water?
if the pump seal was bad, i can see it could pump it in very fast. I'd try cracking that coupler open and seeing if it is leaking. You did say your hydraulic level did drop......
Very odd
It's air cooled, so no coolant to get in there... Tazza, good call, that's my plan of attack. I'm almost positive it has to be leaking in through there and I must've blown that input shaft seal somehow. Why it blew is a different story though, I have no idea there yet. Of course now I get to go out of town for a week, so I won't get a chance to check it tear into it though. Thanks for the tips.
Do let us know how you go when you get to attack it.
Well, I think I figured it out. The short answer is that due to buying it as a box of parts, and not having the experience of taking it apart myself, I messed something up when I put it back together. I ordered my parts and put the pump back together using the diagram for the pump as provided by Vickers. I couldn't source any parts through Bobcat, so I didn't really pay any attention to the Bobcat parts diagram. The vickers seal kit came with two seals and the vickers diagram shows two seals on the input/output shafts - one on the engine side and one on the vane pump side. After the problem happened, I read through the maintenance manual about the hydraulic system. The manual states that internal leakage from the hydrostatic pump is returned to the vane pump through an internal passage. Well, there's no internal passage that I could think of, other than the drive shaft between the two, which I had put a seal on. Decided to check the bobcat parts diagram for the hydrostatic pump. Exactly the same as the vickers, but no seal on the vane pump side, just the engine side. So, I figure since I put that seal there, the fluid wasn't able to return to the vane pump, and eventually it blew the seal on the engine side, causing the problem. If I had gotten lucky, the seal on the vane pump would've failed, and I probably would've never known this was a problem! But, the other one did... So, I took the pump back apart, replaced the seal on the engine side (it had a nice section that was blown out) and removed the one from the vane pump side. Got the pump back together, but haven't put the engine back in to test it out. But, I'm reasonably certain this is what caused the engine side seal to fail..... Oh well, not too expensive of a fix, if this takes care of it. Pretty crazy that Bobcat relies on oil passing through the output shaft bearing on the vane pump side of the hydrostatic pump to return oil to the rest of the system. Based on the vickers diagrams I was using, I'd say that's not a "typical" application of their pump.
This is almost the same as what happened to a 753 i bought. It had a constant whine and the charge pressure was 400 PSI and let out a constant hum. The pump the previous 'professional' installed was wrong, there was no bleed hole or bearing for the excess fluid to go through. I drilled the required hole, the charge pressure was 120 and the noise was gone.
Hahaha, well good thing I'm nothing near a "professional" so I don't feel too bad that I messed it up. I do wish this thing had something more in the way of gauges for troubleshooting though, see what the pressures are running at would probably help. Hopefully when I get it back together it's no worse for the wear...
