I likewise had failed pump(s) issues. This is my learned understanding: there is an orifice (not a regulator per se) built into mechanical pump. In fact, after new engine pump installed, part of check out process is checking fuel pressure and adjusting orifice on it if necessary, with the bleed orifice being the "regulator" of fuel pressure. It appears the electric boost pump on high provides flow for 75% power (not idle) "blowing through" a failed engine driven pump via a bypass valve in the event of a failure. At idle, high boost electric pump is thus flooding the engine providing flow for 75% power (bypassing the "regulating orifice") rather than only providing the fraction of that fuel flow necessary sitting on the ground.
check this out:
http://www.kellyaerospace.com/articl...inuousFlow.pdf