Question 34 UFIV01 - Chief Engineer - UFIV

The Correct Answer is B **Explanation for Option B (Correct Answer):** 1. **Understanding the Problem (Black Smoke):** Black smoke from a diesel engine generally indicates incomplete combustion. This is usually due to an excess of fuel relative to the available oxygen, which can be caused by various issues, including a defective fuel injector (e.g., dripping, poor spray pattern) leading to misfiring or delayed combustion in one cylinder. This "bad" cylinder is the source of the excessive soot (black smoke). 2. **The Troubleshooting Method:** The procedure involves disabling the fuel flow to one cylinder at a time by loosening the high-pressure fuel line fitting. When this is done, that specific cylinder stops contributing power and stops injecting fuel. 3. **Interpreting the Result:** If the engine was previously producing black smoke, and disabling the injector on a specific cylinder immediately results in the exhaust clearing (producing a "clear stack"), it means that the cylinder you just disabled was the *source* of the black smoke. By removing the faulty combustion contribution (the misfiring or poorly fueled combustion) from that specific cylinder, the overall combustion quality of the engine improves dramatically, and the symptoms (black smoke) disappear. Therefore, the cylinder associated with the disabled injector was the misfiring cylinder responsible for the incomplete combustion. **Explanation for Why Other Options are Incorrect:** * **A) 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 disabling a cylinder causes the remaining engine to produce *denser* black smoke, it suggests that the remaining cylinders are now overloaded or running less efficiently due to the loss of power, but it does not specifically pinpoint the disabled cylinder as the sole source of the original smoke. When the misfiring cylinder is disabled, the smoke should decrease (as in B), not increase in density. * **C) 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 a cylinder has no effect on the smoke level, it strongly suggests that the cylinder you disabled was operating normally (or was not the primary source of the problem). The faulty cylinder must be one of the other remaining active cylinders. * **D) 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:** The premise states the engine is already emitting black smoke. More importantly, if the engine had a clear exhaust and disabling a cylinder caused black smoke, it would indicate that the sudden imbalance or loss of power introduced a new problem, but it doesn't indicate the disabled cylinder was misfiring *when the engine was previously running normally*. The test is designed to find the source of *existing* black smoke.