How can the pump stall but not the motor? Aren't they directly connected?
Or do you mean the cylinders stall?
That's a monster.
The engine can only produce a set amount of hp and when the limit is met, the engine will stall. There are several different sets of circumstances when the hyd system will stall, but not stall the engine. For one, if the relief valve on splitter is set so low that all excess pressure is being bypassed over the relief, the engine would just keep chugging alone and the pump will just keep pumping oil and making heat as a byproduct of the oil flowing over the relief. That to me would be the obvious first thing to look at and check. The relief could be set to low, or simply be stuck allowing oil to bypass. Another problems could be as Kevin suggested, the coupling between the pump and the engine could be slipping. I have seen that happen a few times. After those two simple fixes, things can get a lot more complicated. Some of those vane pumps have a built in relief. All the vane pumps work by spinning fast enough to sling the vanes outwards to wipe oil from a oblong shaped bore inside the pump. These vanes can wear out and the bore can also wear out, which would let oil bypass internally and reduce the amount of oil produced, pressure as well as flow rates.
In any case, you cant properly check the problem without having a pressure gauge installed. I believe every vickers vane pump I have ever worked on had replacement vanes and cyl kits to rebuild the pump. Very simple rebuild, just remove the cap off the end of the pump, slip out the vane assemble and slip in the new one and bolt the cap back on. Just dont let the little vanes fall out and a caveman can do it. Of course if your going to go so far as to replace the vane assembly, might as well remove the pump shaft and put in new seals and bushings/bearings and you have a brand new pump.
Cost would be the big factor and if the problems with this splitter is the pump, I wouldnt even consider rebuilding the vane pump. I would buy a new 22gpm 2 stage pump and bolt it on. I suggest the 22gpm pump because his engine is a 16hp, and a 28gpm two stage is pushing the engine limits. Altho with his two 4in bore cyl, it is possible his splitter might not ever see the pressures that would stall his engine. He would see a increase in speed over the single stage vane pump as a added benefit.
Another thing to check on is whether or not the vane pump he has is rated to turn the 3600rpms he said he was running his engine. Most of the smaller vickers pumps are usually recommended to run at 2500rpms or less. Spinning them faster will produce a little bit more flow, but at great cost to the pump internals as well as the excess heat produced.
Edited to add, One more possible problem that hasnt been discussed. His splitter has two cyl. It is also very possible that one of those cyl could be bad allowing oil to bypass the piston seals. If only one cyl is bad, since the cyl are connected, oil would flow from both cyl around that bad seal and flow back thru the control valve back to tank. After checking the relief valve, and maybe even before looking at the relief, I would disconnect one cyl and plug it off and just see if the system could build pressure, using only one cyl. I would do this with both cyl, checking one at a time. Also make sure you remove the pin on the rod end of the cyl that you remove the hoses from, otherwise if left connected to the blade, the working cyl will push/pull the rod on the disconnected cyl, and squirt oil all over the place. You cant just plug the ports on the unused cyl, because it if is left connected to the blade, the rod has to travel in and out as the blade is raised and lowered. Pull pin and cap hoses or deal with a mess.
