OEM NB2 fuel system pressure creep
 

One of the race cars in our shop is an MSM with a 100% OEM fuel system. It's had a problem going lean under sustained WOT we haven't been able to diagnose.

Running N/A as a SuperMiata
Stock NB2 engine tuned for 140whp on E85.
MS3 pro with DIY harness

Logged voltage in race conditions, steady 14.0~14.1
Idled for 30 minutes and fuel pressure crept up from 60psi to about 67psi with gas cap removed.
Replaced OEM pump with 190 High Pressure Walbro, no change.

It's got us scratching our heads. We're guessing it has something to do with either the returnless fuel system or some mystery table in the MS enabled that we don't know about.

Any ideas?

Yep, the returnless system is a pain in the butt. Swap to return style and watch your afr's become buttery smooth and stable.

Has the regulator been replaced?

It has not. I guess that's next.

I always prefer to diagnose before replacing parts so we learn something. Sonny has DNF'd several races because of the issue though so we really want to understand it. If it's endemic, we want to know. Most of the cars here get significantly modified fuel systems so we sometimes have a blind spot for 100% OEM stuff, like the VTCS manifold.

Another reason we'd like to diagnose it instead of just converting/modifying is that we have another NA with a complete NB2 drivetrain we hope (wish/dream) to BAR (make emissions legal) using the OEM fuel system.

<p>In for results.</p>

To me, a fuel pressure related issue points to the regulator. Or a clogged return hose. Something that is messing with pressure over time.