Question 9 MODE01 - Chief MODU Engineer

The Correct Answer is B **Why option B is correct:** The spring has two critical failures: the spring constant ($k$) is lower than specified, and the length ($L$) is reduced (meaning the installed pre-load is too low). 1. **Low Spring Constant ($k$):** A low spring constant indicates the spring material has fatigued or yielded. This spring will not provide the necessary force at high speeds to ensure the valve closes properly and stays seated on the camshaft's base circle. 2. **Reduced Length ($L$):** Reduced length further lowers the installed tension, exacerbating the problem caused by the low $k$. 3. **Consequence (Valve Bounce):** Both factors lead to **valve float** or **valve bounce** at operating speeds. Valve bounce occurs when the spring is too weak to keep the follower in constant contact with the cam profile, causing the valve to slam back onto the seat and the follower to repeatedly strike the cam lobe. 4. **Course of Action:** Since the material properties (spring constant) are permanently degraded, the spring must be **replaced**. Furthermore, because valve bounce has likely occurred due to insufficient spring force, the system components that experience high-impact loading—specifically the **cam and follower** (or tappet)—must be inspected for damage (marring, pitting, or excessive wear). **Why the other options are incorrect:** * **A) Restore the spring to its original length by stretching.** Stretching a fatigued spring (which already has a low spring constant) is extremely dangerous. It compromises the material integrity further, making the spring even more likely to fail catastrophically (e.g., break) or permanently yield during operation, immediately leading to severe valve bounce and engine damage. * **C) Anneal spring to restore the spring constant to the original value.** Annealing involves heating the spring material, which would soften the metal, completely destroying the heat treatment and the desired spring properties (spring constant and yield strength). This would make the spring useless or worse than it currently is. Springs achieve their required constant through specialized heat treatment processes (quenching and tempering), not annealing. * **D) Remove shims to restore spring to its original length.** If the spring has *shrunk* (reduced length), removing shims would only reduce the installed length and tension even further, making the existing valve float problem worse. Shims are added to *increase* the installed tension/length (pre-load) if needed, not removed when the spring has already reduced in length. Furthermore, this action does nothing to address the critical failure of the low spring constant.