The correct answer is D) excessively early injection timing. High cylinder firing pressure, accompanied by low exhaust temperature, can result from excessively early injection timing. This is because early injection timing causes the fuel to ignite prematurely, leading to high in-cylinder pressures and temperatures. However, the early ignition also results in the expansion stroke starting earlier, leading to lower exhaust temperatures. The other options are incorrect because: A) Improper fuel rack positioning would affect fuel delivery, not necessarily result in the pressure/temperature symptoms described. B) Lengthy exhaust valve duration would affect the exhaust process, not the injection timing. C) Extended operation at light load would not directly cause the high pressure/low temperature condition.