Question 25

The Correct Answer is B. ### 2. Why Option B ("adjust the fuel rack") is correct: The symptoms described—high firing pressure (Pmax) and high exhaust temperature (T_exh) in **one cylinder**—strongly indicate that this specific cylinder is carrying too much load relative to the others. This imbalance is caused by the cylinder receiving an excessive quantity of fuel. The fuel rack controls the stroke of the fuel pump plunger, thereby regulating the amount of fuel delivered to that individual cylinder. By adjusting the fuel rack to reduce the amount of fuel supplied to the affected cylinder, the engine operator directly reduces both the peak heat release (lowering Pmax) and the total heat energy (lowering T_exh), bringing the cylinder back into balance with the others. This is the primary method for balancing cylinder load in a diesel engine. ### 3. Why the other options are incorrect: **A) retard fuel injector timing:** Retarding the timing (injecting fuel later) primarily lowers the firing pressure (Pmax). However, it does not reduce the overall quantity of fuel delivered (which is the root cause of the high T_exh). Furthermore, late injection often leads to incomplete or inefficient combustion, which can actually increase the exhaust temperature. This adjustment would address only one symptom (Pmax) incorrectly and potentially worsen the other (T_exh). **C) increase scavenge air pressure:** Increasing scavenge air pressure affects the combustion process uniformly across **all** cylinders. While it generally improves efficiency, it does nothing to correct the specific fuel delivery imbalance occurring in just one cylinder. The problem requires a targeted adjustment, not a general engine setting change. **D) reduce fuel booster pump pressure:** The fuel booster (or supply) pump provides low-pressure fuel feed to all high-pressure injection pumps. Reducing this pressure would affect the fuel supply capability of the entire engine equally. This could lead to difficulty filling the injection pumps, potentially causing poor combustion or even starvation across all cylinders, rather than balancing the load in the single problematic cylinder.