My car is Volvo S40 1999 with 2.0 engine. The fuel pressure is on the idle at 2.2-2.3 bars (should be 2.5) and on higher rpm max. 2.6-2.7 bars (should be 3.0). When I unplug the hose that goes from fuel pressure regulator to intake manifold, pressure jumps to 2.9 bars. I have changed the fuel pump to OE walbro, fuel pressure regulator, fuel filter and injector seals. There is not really a fuel pump relay, but there is a system relay that controls the pump (I think). Also changed that. I have cleaned connectors and fuel lines seems to be ok. None of these part changes have affected anything to fuel pressure. When driving with petrol there isn't really a problem but with e85 injectors' duty cycles become too high on high rpm. On normal drive everything seems to be ok. Any ideas how to solve this?
Sounds like your FPR isn't doing its job correctly. This could be being caused by the Walbro pump may not be providing pressure as the FPR expects, thus providing lower than expected fuel pressure. Two things you might be able to do. First is to replace the pump back to factory stock. While Walbro makes great pumps, it may not be doing what the stock one was designed to do. Second, if you don't want to do that, you'd need to get an adjustable FPR, where you can turn it up a bit. This is a common "fix" in the aftermarket world when things like this happen.
Most if not all EFI fuel pumps output higher than regulated pressures as you found when disconnecting the vacuum line to the fpr. This shows the fpr and pump are operating as designed albeit a little less than spec. The difference of 0.2-0.3 bar at idle (approximately 5 psi) shouldn't be an issue as you haven't found any related to fuel while driving.
However, is your Volvo compatible for E85? As you may or may not know, E85 has up to 85% ethanol/15% gasoline and only recommended for vehicles specified as flexible fuel compatible for regular 87 octane and E85. E85 is not recommended for engines not designed to run high ethanol content fuel for several reasons. 87 octane gasoline contains no more than 10% ethanol.