Tuning with high fuel pressure or lowering?

Ongoing tuning issues on Mitsubishi Outlander Turbo (same as Airtrek), with the 4G63 engine from the Evo but smaller turbo and top mounted IC.

Changed the MAF, BOV, fuel rail and regulator from Evo. Subaru Dark Blue injectors. 3-port EBC Grimmspeed from Evo X. High Pressure Walbro 255. Should ideally be able to reach 1.5-1.6bar boost.

Lots of issues with AFR and fuel cuts under boost.

Measured fuel pressure and it was 65psi (4.5 bar) with or without vacuum hose but it rose under boost.

Ran the fuel return line into a bucket and fuel pressure dropped to 3.7 bar.

Drilled out the fuel return siphon on the in-tank assembly, which immediately dropped from 4.5 bar to 3.7bar - same as with the fuel return into a bucket, so now I know it's the regulator.

The fuel pump being the High Pressure Walbro 255 which is probably way above my needs with only a stock turbo, but at least I have sufficient fuel for future needs (bigger turbo and FMIC).

To get to the point, I have considered tuning for the increased fuel pressure (increasing injector scaling) but I have a feeling it will just cause more issues elsewhere.

Stock Evo fuel pressure is 43.5psi (3 bar) with vacuum disconnected BTW.

Adjustable FPR on the way, and I hope it will resolve the issue!

Is this a well-known problem with the high pressure Walbro?

Did anyone successfully tune at higher than stock fuel pressure?

Hello ideally you are doing the correct thing by replacing the regulator this is a common issue I see, it is possible to tune around it but I wouldn't unless I was tight on funds to renew the regulator

As Ross said, if the pressure regulator - and the rest of the return plumbing* - isn't able to bleed of the pressure because there is too much restriction to flow, the only remedy there is to replace it with a higher flowing one.

Remember, pressure is basically just the resistance to flow, and the entire system should be considered - this is also why the position of the pressure gauge take-off is important.

*As illustrated by the drilled out component.