Weak Hydraulics

[PBettis]

Overall loss of engine power...hmmm. I would drain the current fuel and replace the fuel filter. I would also treat the fuel system a few times with "Seafoam" additive. This should help clear any carbon on the injectors and valves. Key words..."few times". Myself and others I've told have had great results with this particular product. Runs around $10.00 at the auto parts store. You also may have an injector pump issue. Try 2 fresh fuel and Seafoam treatments to start. As far as the nuts on the FEL pins, they may be used to pull a sticking pin out with some type of slide hammer or something. Sounds like the previous owner may have installed those possibly. The pins can usually be driven out from one side instead of pulled out but it sounds like yours will fall out on their own. Not sure on that model. Patience will win in the end! Keep us posted!!

 

[StevenDyche]

@ DFB The weakness in the hydraulics was both FEL and PTO. It was weak at both low and PTO speeds. The loader can lift the front of the tractor. I am still working on the flow and pressure checks. I will do them as soon as I can. Lastly, I didn't notice a loss in power and the John Deere Rep said that is common with the lower HP tractors. Thanks again.

 

[StevenDyche]

@PBettis - I have a bunch of medical appointments coming up, but I will start on the fuel system in a few weeks. The FEL pins are going to get replaced in a month or so when I save up the money. I will surely let you know what I find out during the procedure. Thanks!

 

[Muxster]

The diagnosis of loss of engine power is bogus IHO. If engine power were weak and the hydraulics were activated, the engine would stall. The hydraulic system uses positive displacement pumps and the loader, 3PH lift, and power beyond implements (backhoe) hydraulics are in series. first in line is a relief valve that lifts when the pressure is too high caused by cylinders reaching end of travel or load is too high. If the engine is running the hydraulic pump is turning. Its gear driven off the engine. When you operate the loader and the loader extends to the limits, the relief valve will lift. You should hear the engine lug at that point. If its not lugging at that point or you never get to the limits, it probably means the pump seals may be shot and the pressure is not building high enough for the relief to lift. It could also mean the relief valve is lifting (somewhat) all the time and not allowing the pressure to build. Since its weak in all loader and PTO lift operations its not likely a cylinder problem. So my guess is its either the pump seals or relief valve. You can easily remove the cover for the relief valve and examine the shims and spring. But IHO its more likely a pump issue.

The 790 is natively relatively weaker than most other JD models because of its low relief valve setting. Many 790 owners shim the relief to bring the lift pressure up to around 2500 psi. I did that with mine and I can lift just about anything I can get in the bucket.