During a ground check, if the engine runs rough and manifold pressure is high for a specific RPM, what could the cause be?

When an engine runs rough and exhibits high manifold pressure at a specific RPM during a ground check, this can be indicative of a dead cylinder. A dead cylinder occurs when one of the engine's cylinders fails to contribute to the power produced due to a variety of reasons such as a failure in the ignition system, a broken valve, or mechanical issues.

The high manifold pressure indicates that the remaining cylinders are still producing power, but the absence of combustion in one cylinder leads to an imbalance. This results in rough running because the engine is not operating smoothly across all cylinders, which is essential for proper performance. Maintaining correct balance across the cylinders is critical for smooth operation and efficient power generation.

While other options, such as several fouled spark plugs or poor fuel quality, can also lead to rough engine operation, they typically would not cause the high manifold pressure observed in this scenario. Intact performance of other cylinders typically compensates when minor issues arise, but the impact of a dead cylinder is pronounced and leads to a distinct roughness in engine operation.