Question 20 DDE01 - Designated Duty Engineer - Unlimited HP

The Correct Answer is B *** ### 2. Explanation of why Option B is correct: **B) No. 5 cylinder had been misfiring. You have successfully purged the air from the No. 5 cylinder high pressure fuel line, and the engine is no longer misfiring.** This option accurately describes the sequence of events and the final result: 1. **Initial State (Rough Running):** The engine was running roughly because No. 5 cylinder was not contributing power (it was misfiring). 2. **The Test (Slacken):** When the fuel line was slackened, the engine continued to run roughly with "no real change." In standard diagnosis, this observation confirms that the cylinder whose fuel supply was interrupted (No. 5) was the one that was already failing to contribute power. 3. **The Fix (Re-tighten):** Upon re-tightening, the engine immediately began and continued to run *smoothly*. If a misfire is caused by a mechanical failure (e.g., failed pump plunger, bent pushrod, bad injector spring, low compression), simply slackening and re-tightening the fuel line will **not** fix the issue. 4. **The Conclusion:** The only common scenario where this action permanently fixes a rough-running engine is when an air pocket has become trapped in the high-pressure line. The act of slackening the fitting allows the extremely high pressure to force the trapped air (which disrupts the hydraulic function of the injection pulse) out of the line, effectively "bleeding" the system. Once the fitting is re-tightened, the restored hydraulic integrity allows No. 5 injector to spray fuel correctly, and the engine runs smoothly. *** ### 3. Explanation of why the other options are incorrect: **A) No. 5 cylinder was and continues to be misfiring. You have successfully located the misfiring cylinder.** * **Incorrect:** While the initial test (no change in rough running when slackened) indicates that No. 5 was the misfiring cylinder, this option fails because the final observation contradicts it. If No. 5 *continued* to misfire, the engine would still be running roughly after the line was re-tightened. Since the engine began running smoothly, the problem was solved. **C) No. 5 cylinder was and continues to be firing properly. No. 1, No. 2, No. 3, or No. 4 cylinder is misfiring. No. 6 cylinder is firing properly.** * **Incorrect:** The initial diagnostic observation refutes this. If No. 5 cylinder were "firing properly," interrupting its fuel supply (by slackening the line) would cause the engine speed to drop and the roughness to increase significantly (running noticeably worse). The observation states there was "no real change," proving that No. 5 was *not* firing properly initially. Furthermore, if a different cylinder were misfiring, fixing the No. 5 line would not cure the overall rough running. **D) No. 5 cylinder was and continues to be firing properly. No. 6 cylinder is misfiring. No. 1, No. 2, No. 3, and No. 4 cylinders are all firing properly.** * **Incorrect:** Similar to Option C, if No. 5 were firing properly, interrupting its fuel would make the engine run substantially worse than before. The observation that there was "no real change" confirms No. 5 was the cylinder failing to contribute power. Additionally, if No. 6 were misfiring, the engine would still be running roughly even after the No. 5 line was re-tightened and secured.