During a manifold vacuum test, the vacuum gauge needle registers a steady 12 inches at idle. Which one of the following would cause this?

A steady reading of 12 inches of vacuum at idle indicates a specific condition within the engine's intake manifold. This lower-than-expected vacuum can be attributed to late ignition timing. When the timing is set too late, combustion occurs later than optimal, which results in a longer burn time and thus incomplete combustion. This situation creates additional pressure in the cylinders during the intake stroke, leading to reduced vacuum levels in the manifold.

Understanding how this affects engine performance is crucial. An engine with late timing does not operate efficiently, reducing power and potentially affecting emissions. Conversely, conditions like a rich air-fuel mixture could lead to incomplete combustion as well, but it typically does not manifest as low vacuum unless coupled with other factors.

An open throttle plate would result in lower manifold vacuum due to greater airflow being allowed in, but at idle, we generally expect a closed throttle position. A dirty air filter can restrict airflow, but that would typically cause the engine to struggle or stall rather than just consistently show a steady reading of vacuum.

Thus, late timing effectively explains the observed steady 12 inches of vacuum at idle, as it directly affects the combustion process and consequently the pressure conditions in the manifold.