Help with Kubota F2400, foaming/air hydrostatic fluid, slow forward/reverse drive

[shawn macananny]

I know it's not a skid steer, but its close enough haha. Anyway it's a Kubota F2400 mower, same thing as a B2400 tractor a believe. What i am having an issue with is once the mower has been running for a while the hydrostatic drive is slow in either speed. What i have also noticed when i remove the fill cap is that is it somewhat foamy and has alot of air in the fluid. If i let it idle for a couple minutes it will be fine, or if i let it sit but as soon as i throttle it up in a minute or two it will be slow again. It seems like maybe the hydrostaic pump is drawing air and churning it into the fluid? Does that sound possible? Is all new bobcat fluid (not kubota but i dont see that making much a difference) new filters, case screens cleaned etc. Any ideas what else it could be?

 

[Tazza]

It sounds like cavetation, it can be caused by a restriction in the suction line, have you checked the filters? and if there is a screen it must be free of junk. With a restriction, it will make the oil foam when it's being drawn up into the pump. It could be sucking air in through a bad connection too, but 'generally' there is an external leak at this point too but not always.
I think in the worst case, a failing pump can foam the oil up pretty good too, but stick with the simple ones first, suction line filter.

 

[shawn macananny]

It sounds like cavetation, it can be caused by a restriction in the suction line, have you checked the filters? and if there is a screen it must be free of junk. With a restriction, it will make the oil foam when it's being drawn up into the pump. It could be sucking air in through a bad connection too, but 'generally' there is an external leak at this point too but not always.
I think in the worst case, a failing pump can foam the oil up pretty good too, but stick with the simple ones first, suction line filter.

I did clean both screens when i drained the case and i did replace both of the hydraulic filters. I filled the case with bobcat hydrostatic fluid instead of kubota, could this be causing an issue? The dealer said it wouldnt matter and i had an extra 5 gallons from my 751 so i just used it. There are a couple rubber hoses that look like they should be replaced, but i dont see any dampness at all around them and there are new lines running to the cooler. The spin on filters are both tight and i know the gasket surfaces were clean and orings not damaged when i installed them. i guess i will start by replacing the two small hose pieces, any reason i cant use heater hose for these hydraulic suction lines? they are really only about 3 inches long and joining two hard lines

 

[shawn macananny]

I did clean both screens when i drained the case and i did replace both of the hydraulic filters. I filled the case with bobcat hydrostatic fluid instead of kubota, could this be causing an issue? The dealer said it wouldnt matter and i had an extra 5 gallons from my 751 so i just used it. There are a couple rubber hoses that look like they should be replaced, but i dont see any dampness at all around them and there are new lines running to the cooler. The spin on filters are both tight and i know the gasket surfaces were clean and orings not damaged when i installed them. i guess i will start by replacing the two small hose pieces, any reason i cant use heater hose for these hydraulic suction lines? they are really only about 3 inches long and joining two hard lines

I pulled out my kubota manual i bought for it and found a diagram of the hydrostatic system. It looks as though the "main hydraulic pump" sucks oil from the top of the case out of the pumps as it's marked as either case or suction oil pressure (since its not in color) and it goes through the cooler into the pump that powers the power steering. I went outside and looked where it is leaving the case and i can swivel the 90* degree fitting with my fingers so i am going to try to fix that tomorrow and see if it works. It just has a rubber line hose clamped onto it and being its at the top of the case i wouldnt expect to see any oil leaking out. It seems to make the most sense at this point. As loose as it is if this fitting were under any pressure i would expect it to be leaking so i am assuming it is vacuum though i have never seen a pump that pulled fluid all the way through a cooler, that is how the digram is in my book.

 

[shawn macananny]

I pulled out my kubota manual i bought for it and found a diagram of the hydrostatic system. It looks as though the "main hydraulic pump" sucks oil from the top of the case out of the pumps as it's marked as either case or suction oil pressure (since its not in color) and it goes through the cooler into the pump that powers the power steering. I went outside and looked where it is leaving the case and i can swivel the 90* degree fitting with my fingers so i am going to try to fix that tomorrow and see if it works. It just has a rubber line hose clamped onto it and being its at the top of the case i wouldnt expect to see any oil leaking out. It seems to make the most sense at this point. As loose as it is if this fitting were under any pressure i would expect it to be leaking so i am assuming it is vacuum though i have never seen a pump that pulled fluid all the way through a cooler, that is how the digram is in my book.

Uploaded with ImageShack.us But it looks like is it considered case pressure coming out of the top next to the filter (where i found the loose fitting) then it runs to the main hydraulic pump (this is the pump mounted behind my injection pump i believe and only controlls the power steering that i can see)

 

[Tazza]

Uploaded with ImageShack.us But it looks like is it considered case pressure coming out of the top next to the filter (where i found the loose fitting) then it runs to the main hydraulic pump (this is the pump mounted behind my injection pump i believe and only controlls the power steering that i can see)

I can't see the oil being an issue unless it's far thicker than it should be, which i doubt.
The hydraulic pump i believe provides charge pressure to the pistom pump. Just not sure what else could cause the foaming.

 

[Tazza]

I can't see the oil being an issue unless it's far thicker than it should be, which i doubt.
The hydraulic pump i believe provides charge pressure to the pistom pump. Just not sure what else could cause the foaming.

Scratch that, there is another pump there for the charge, i see the two strainers.
Are there any points for you to test pressure? and more to the point, do they tell you what the pressure should be?

 

[shawn macananny]

Scratch that, there is another pump there for the charge, i see the two strainers.
Are there any points for you to test pressure? and more to the point, do they tell you what the pressure should be?

Yeah there are quite a few points for measuring pressure and its very detailed. And port P4 is a vacuum test port which can be seen on the diagram. I didnt get around to fixing the loose connection i am really thinking that may be what it is. It is the 90* line coming out of the top of the filter in the picture being sucked into whats called the main pump. I believe the oil cooler is under vacuum.

 

[shawn macananny]

Yeah there are quite a few points for measuring pressure and its very detailed. And port P4 is a vacuum test port which can be seen on the diagram. I didnt get around to fixing the loose connection i am really thinking that may be what it is. It is the 90* line coming out of the top of the filter in the picture being sucked into whats called the main pump. I believe the oil cooler is under vacuum.

Well i tightened it down today and another hose clamp i found loose but it didnt make any difference. One thing i did notice is that it when i went to start it it took about 30 seconds to a minute of the engine running for the hydrostatic pump to prime. It wouldnt move at all until then. The lift for the deck and the the power steering worked fine though. So i am thinking that there is possibly a pump problem. Does this sound like a problem with my hydrostatic pump then?

 

[Tazza]

Well i tightened it down today and another hose clamp i found loose but it didnt make any difference. One thing i did notice is that it when i went to start it it took about 30 seconds to a minute of the engine running for the hydrostatic pump to prime. It wouldnt move at all until then. The lift for the deck and the the power steering worked fine though. So i am thinking that there is possibly a pump problem. Does this sound like a problem with my hydrostatic pump then?

It does sound like a pump issue, it shouldn't take long to prime the system, really it should never loose prime in the first place. I'd start with looking at the charge pump and charge relief. If you have it out, check the wear plates (if fitted) for scratches where the rotating groups run.