Sullair 250Q won't shut off

Hi Chad, Thank you for your question. I am not familiar with this make and type of compressor, but generally speaking I can say the following: diesel driven portable air compressors with older control system (I mean, not with an electronically controlled engine), usually have the following two components to control the diesel engine: 1) speed setting actuator 2) fuel-shutoff solenoid / shut down solenoid. I suspect your shut down solenoid is malfunctioning. This valve does exactly what you now do by hand: it shuts off the fuel supply. It's the only way to stop the diesel engine. Your 'start/shut down' switch is connected to this solenoid. When you start the compressor, or when the compressor is running, the valve is actuated. The electrical current creates a magnetic field, which opens the fuel valve. When you shut down the compressor, there is no electrical power going to the fuel valve anymore, so it will close and shut-off the engine. I suspect in your case, that this valve is dirty on the inside and needs to be cleaned or replaced. This is most probably a John Deere part and not a Sullair part. If you need a replacement, you can probably get it both from Sullair, or from John Deere. More general info on portable air compressors can be found here: portable air compressors. Good luck and let me know if you need any more info.