Most likely due to pump overtemp during low load (high bypass) operation. Classic problem with fuel pump sized to deliver required fuel @ max power being asked to operate in low load conditions where most fuel is bypassed and returned to tank.
Best solution: Vapor Works or equivalent where pump is only operating at enough load to deliver required fuel (no bypass). This mimics what OEM's are doing for very same reason (along with reduced parasitic loss for increased FE). Carl's system works awesome!
Next best: Staged pumps where only operate on one pump under low flow conditions with secondary pump coming online during increased load conditions. BTW, Vapor Works can also be used with staged pumps for those with greatly varied fuel delivery requirements from idle to max power.
I've fought this very issue so many times and it is extremely fustrating to be stuck on side of road due to such a seemingly simple issue.
This is another reason OEM's mount fuel pumps in tank - for cooling effect (as well as noise supression). But even that is not enough for most aftermarket high flow pumps operating at the now nominal 400 kPa (~60 psig) fuel pressure under high bypass conditions for extended periods of time.
Easy check can be after low speed cruising around in your car (conditions where you noticed fuel pressure fall off), can you even touch your external regulator without buring several layers of skin off your fingers?
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