I have been working on a 4230 JD that runs perfectly until it reaches operating temperature (approx. 185-190 degrees F.) tractor then starts losing power and pouring black smoke like it is straining. We have replaced the injection pump three times, we have replaced injectors, we have removed the pipe from the intake manifold(non turbo tractor), we have removed the muffler, adjusted valves, there is no binding in the drive train, engine, or hydraulics as it will roll easily to a stop and you can throttle back and engine runs perfectly, we can cut it off and will start right back. Let it cool a few minutes and it will run perfectly until temperature again reaches the 185-190 degree mark. This one has me stumped! We need help badly on this one! Thanks!