Question 18

The Correct Answer is B ### Explanation of Why Option B is Correct The process described—slackening the high-pressure fuel line to an injector nozzle—is a standard diagnostic technique (often called "fuel cut-out" or "line slacking test") used to isolate a potential misfiring cylinder in a diesel engine. 1. **Engine Condition:** The engine is initially running roughly due to a suspected misfire. 2. **Test Execution:** You slacken the fuel line at #4 injector. This effectively stops fuel delivery to cylinder #4, causing the cylinder to stop contributing power. 3. **Result:** The engine runs **even rougher** than before. 4. **Interpretation:** If a cylinder is healthy and contributing power, removing its fuel supply will always make the engine run noticeably worse (rougher, lower RPM). If the engine was running roughly *before* the test, and runs *even rougher* when #4 is disabled, it confirms that cylinder #4 was functioning and contributing power (i.e., firing properly) to the engine's operation. 5. **Conclusion:** Since the overall roughness has worsened, #4 cylinder is firing properly. Therefore, the original misfire must be located in one of the other five cylinders. ### Explanation of Why Other Options are Incorrect **A) #4 cylinder is misfiring. You have successfully located the misfiring cylinder.** * **Incorrect:** If #4 cylinder were already misfiring (not contributing power), slackening its fuel line would make little or no difference to the engine's running quality. Since the engine ran *even rougher*, #4 was contributing power and is not the source of the original roughness. **C) #4 cylinder is misfiring. Either #1, #2, or #3 cylinder is also misfiring.** * **Incorrect:** This option incorrectly assumes #4 is misfiring. Regardless of whether other cylinders are misfiring, the test result (engine runs much rougher) indicates that #4 was firing properly. **D) #4 cylinder is misfiring. Either #5 or #6 cylinder is also misfiring.** * **Incorrect:** Similar to Option C, this option is based on the false premise that #4 cylinder is misfiring. The diagnostic test clearly shows that #4 cylinder is healthy.