I have a john deere 4620 (no turbo). It was working perfectly and then when running while stopped with the plow in the field, the idle was completely maxxed out. The tractor starts perfect now, but the idle instantly maxxes out and it has to have the fuel supply cut off or be turned off. the fuel injector pump has small leaks, but I think it may have been a leak from some earlier as well. what is idling doing crazy and what could cause the pump to leak externally?
I have to admit that I don’t think I’ve ever seen a 4620 without a turbo. Most 4620s smoke more than expected with a turbo.
The most common cause of uncontrollable engine speed is a broken govner cage in the fuel injection pump. If the throttle connection is working properly, this is most likely the case.
The govner cage is made of a fiber / plastic material that breaks down over time. Most of the time, in the early stages of failure, it will cause the tractor to develop an unstable idle. By this I mean that it will go up and down a bit at idle. As the condition worsens you can allow the engine to run at or well above 2500 rpms.
The fact that your pump is leaking often indicates that the sealing o-rings have hardened and shrunk enough to allow fuel to leak out.
In short, it looks like your tractor needs an injection pump rebuild.
What you are controlling with the throttle lever on the pump is actually a valve that changes the amount of fuel delivered to the pump plungers. The valve itself is buried in the lower casing of the pump and requires several special tools to repair.
I do not recommend that you try to repair the fuel injection pump yourself. The pump contains many small parts that have to be assembled sequentially and extremely tight tollarances that have to be accurately checked.
Your pump should be a Stanadyne pump that would require the Stanadyne Fuel Pump Special Toolkit. You would have to contact a diesel fuel system repair shop to get the proper tools.