Someone straighten me out if I'm wrong, I'm an amateur trying to figure this stuff out. It looks to me that during low demand, there is greater vacuum and the regulator allows excess fuel to return to the tank. When the throttle is opened for higher demand, the regulator closes thus increasing the fuel supply available. If there is a vacuum leak it would operate as if there were high demand for fuel. The increased pressure in the rail causes excess fuel to force it's way through the injectors creating an excessively rich mixture. Could ethanol in the fuel cause the regulator to fail?
Assuming all components are in OK condition ,what actually occurs is this:
The ECU, on the basis of a RPM X Throttle Position co-ordinate map, instructs the injectors to open for a certain duration. This controls the volume of fuel metered by the injectors. (The ECU is continually reading the RPMs and the Throttle Positioner % of opening.)
The ECU relies upon the basic map, and then continually adjusts the map on the basis of continuous sampling from the related sensors. The ECU makes these map adjustments on the basis of Absolute Air Pressure, Intake Air Temperature, and Coolant Temperature. The map adjustments that the ECU makes will vary from richer to leaner overall, and does this by simply changing the duration that the injectors are instructed to be open. Fuel Rail pressure is not manipulated with this system, and is intended to be a constant.
Ethanol within Honda's allowable limit in gasoline, 10% to be exact, should not cause any problems to any of the components. Methanol is restricted to 5 % re blended into pump gasoline. Methanol needs co-solvents to stay in solution with the gasoline, and the co-solvents used can be bad for the synthetic rubber ("elastomer material") parts. Methanol blended gasoline also needs corrosion inhibitors, so if they are lacking, metal parts can be affected.