Question 8 DDE01 - Designated Duty Engineer - Unlimited HP

The Correct Answer is B ### Explanation for why Option B is correct: **Option B: After disabling the injector of a given cylinder, if the engine previously producing black smoke now produces a clear stack, this indicates that the cylinder associated with the disabled injector is misfiring.** 1. **Black Smoke Indication:** Black smoke (soot) from a diesel engine stack typically indicates incomplete combustion due to an air-fuel ratio imbalance (too much fuel or insufficient air). In the context of a misfiring cylinder, this imbalance is often caused by an issue in that specific cylinder—most commonly a faulty injector (e.g., leaking, dribbling, or improperly atomizing fuel), poor compression, or timing issues—which causes unburned or partially burned fuel to exit the exhaust. 2. **Troubleshooting Procedure:** Disabling an injector (by loosening the high-pressure fuel line) effectively stops the cylinder from contributing power (or fuel) to the engine. 3. **Diagnosis:** If the engine is running rough and producing black smoke (indicating one cylinder is overloading the combustion system with poorly burned fuel), and you disable the injector for the *faulty* cylinder, you remove the source of the excess unburned fuel. When this primary source of the black smoke is removed, the remaining cylinders (which are assumed to be operating correctly) take over, and the overall combustion efficiency improves dramatically. The result is a transition from black smoke to a clear or light exhaust stack, confirming that the disabled cylinder was the source of the combustion problem (misfiring). ### Explanation for why other options are incorrect: **A) After disabling the injector of a given cylinder, if the engine previously producing a clear exhaust now produces black smoke, this indicates that the cylinder associated with the disabled injector is misfiring.** * **Incorrect:** If the engine was previously producing a clear exhaust, it means all cylinders were operating properly. Disabling a functioning cylinder simply reduces the overall power output, but it does not introduce the poor combustion required to suddenly create black smoke (which signifies a chronic over-fueling/poor combustion problem in a specific cylinder). This scenario describes intentionally disabling a *good* cylinder. **C) After disabling the injector of a given cylinder, if the engine previously producing black smoke now produces even denser black smoke, this indicates that the cylinder associated with the disabled injector is misfiring.** * **Incorrect:** If the original black smoke was caused by a fault in a single cylinder, disabling that cylinder should alleviate the problem (making the smoke clear, as in B). Producing even denser black smoke after disabling an injector suggests that the engine is now even more overloaded or that the remaining good cylinders are struggling more severely, but it does not confirm that the disabled cylinder was the *cause* of the original black smoke. **D) After disabling the injector of a given cylinder, if the engine previously producing black smoke continues to produce equally dense black smoke, this indicates that the cylinder associated with the disabled injector is misfiring.** * **Incorrect:** If disabling the injector makes no change to the black smoke, it means the disabled cylinder was *not* the source of the problem. The cause of the black smoke (a poorly functioning injector, turbocharger issue, general overloading, or a fault in a *different* cylinder) still remains, and further troubleshooting on other components or cylinders would be necessary.