Usually on a PC300-6 komatsu excavator we find the komatsu SA6D114-1 engine. I recently encounterd the following problem: on load like traveling a steep hill the RPM decrease from 2100RM to 1750RPM and without smoke. I checked the fuel priming pump and fuel filter just to start with classic troubleshooting and couldn't see any failure. I removed the return line from fuel pump and found enough fuel coming out that means the fuel pump is fed enough with fuel. Anyway, what should be the cause of this low power, is it the fuel pump or the injector. have you seen this on this specific engine?
The memory cells in my head are a little fuzzy on this engine. I think it is the Cummins C8.3. In the USA they put the 8.3 engine in that model and it was the beginning of a steep drop in sales here for that model. Komatsu used to put their own designed engine in the 300-5 and it would only drop about 175 RPM on a full load. They went to the Cummins engines for the Dash 6 line in the US to meet EPA regulations. The Cummins engines were not designed with enough torque to maintain that RPM drop so Komatsu changed the hydraulic pump settings to be able to deliver about the same amount of oil at the lower full load speed. As I recall 1,750 was pretty normal for that model on full load. That is what you get when you put an engine designed for truck use in a hydraulic machine. On the machines that I had experience with, the fuel pumps were operating at maximum delivery and could not be turned up to the point of getting enough horse power or even make them smoke. Basically, if you have no fault codes and everything else is working properly, you are getting all that you are going to get.
the SA6D114-1 is the emissionized 505 cummins....treat is the same for trouble shooting
Why on new machine we don not see this behavior, like on PC300-7 that has the same engine with slight modification like the timing and the additional aftercooler
I never got to work on one of those as I moved most of my customers into Hitachi excavators. I know they changed some things on the later models. You might check the waste gate on the turbo charger to see if it is frozen open. I had a friend who was a Komatsu service rep at the time who told me that was what was going on. I had half a dozen 300 Dash 6 machines that all did exactly the same thing.