A vehicle with a return type fuel system has failed an emissions test with certain readings at idle. Which of the following could cause these readings?

In a return type fuel system, the fuel pressure regulator plays a critical role in maintaining the appropriate fuel pressure for the engine. If the fuel pressure regulator is leaking, it can introduce excessive fuel into the intake manifold, which can lead to a rich fuel mixture. A rich fuel mixture causes incomplete combustion, resulting in higher levels of unburnt hydrocarbons, carbon monoxide, and possibly increased levels of nitrogen oxides during emissions testing, especially at idle when the engine is operating under stoichiometric conditions.

The other options, while they could impact engine performance, are less directly associated with high emissions readings. An intake manifold vacuum leak typically leads to a lean condition, which may reduce hydrocarbons but can increase NOx emissions. A plugged fuel filter restricts fuel flow, which could cause performance issues but would not necessarily lead to failed emissions readings in a consistent manner. A plugged EGR passage, while it could lead to increased NOx emissions by not allowing exhaust gas recirculation, would not cause the same rich condition that leaking fuel pressure regulators can produce.

Thus, the leaking fuel pressure regulator is the most likely cause of the emissions failures due to its direct effect on the fuel mixture in the combustion chamber.