What could cause high fuel pressure at idle in an electric fuel injection system?

High fuel pressure at idle in an electric fuel injection system can be attributed to low manifold vacuum. When manifold vacuum is low, especially at idle, it does not allow the fuel pressure regulator to function properly. The fuel pressure regulator operates by using manifold vacuum to help maintain the optimal fuel pressure in the system. If the vacuum is insufficient, the regulator cannot effectively lower the fuel pressure, leading to higher than normal fuel pressure.

This situation often results in excessive fuel being delivered to the engine, causing rich fuel mixtures and potential drivability issues, such as rough idling or stalling.

In contrast, other factors like a leaking fuel pump check valve or a plugged fuel injection valve could also lead to fuel delivery issues, but they typically would not singularly cause high fuel pressure at idle. A leaking check valve could lead to fuel loss when the engine is off and might result in hard starts, while a plugged fuel injection valve would hinder fuel delivery altogether. High manifold vacuum would, in fact, help lower fuel pressure, not raise it. Thus, low manifold vacuum is the most accurate cause of high fuel pressure in this context.