Question 14 MODE01 - Chief MODU Engineer

The Correct Answer is A **Why Option A (improperly seating intake valves) is correct:** Compression pressure is built by trapping air inside the cylinder during the compression stroke. If the intake valve does not close completely or seats improperly (due to damage, wear, or incorrect tappet clearance), it creates a leakage path for the compressed air to escape back into the intake manifold. This loss of air directly reduces the maximum pressure achieved during the compression stroke, leading to low compression pressure. **Why the other options are incorrect:** * **B) leaking cylinder liner seal rings:** Cylinder liner seal rings (O-rings) are designed to seal the coolant jacket from the crankcase or the combustion space. If these rings leak, it results in coolant contamination or leaks, but they do not directly seal the combustion pressure itself. Compression pressure leakage paths involve the valves, piston rings/cylinder walls, or the cylinder head gasket. * **C) late fuel injection timing:** Fuel injection timing affects when combustion begins relative to the piston position. While incorrect timing severely impacts engine power, efficiency, and starting ability, the injection timing itself occurs after the compression stroke is complete. Therefore, it does not influence the mechanical ability of the cylinder to build maximum compression pressure. * **D) carbon deposits on the piston:** Carbon deposits increase the effective volume of the piston head (reducing the clearance volume) or increase the temperature of the air, which generally leads to a *higher* compression ratio and thus *higher* compression pressure, not lower. (Though excessive deposits can potentially interfere with ring movement, leading to leakage, the primary and direct effect of deposits is usually increased pressure.)