GM 5.7litre rich codes P0172, P0175

Hi guys, it's been a while The images are attached are from my Launch CRP123X
The vehicle is a 5.7litre Holden VZ. I can access it through my scan tool by going to GM and using the VIN details.
It' coming up with codes P0172 and P0175 which is system too rich on bank 1 and 2.
It has two fuel tanks, LPG and Petrol and a switch inside to change it over. It brings up the fault on both systems.
At idle LTFT is at around -10% on both banks, and climbs to 0% if I remove the oil cap.
I blanked off the purge valve and there is no change.
The MAP sensor reads at 28kpa at idle - I'm used to seeing around 34kpa on most vehicles.
MAP drops further to 24 kpa if I hold the revs at 2200 rpm though fuel trims return to 'normal'.
Could the 24kpa MAP sensor reading suggest something like valve adjustment and excess manifold suction causing the
bad fuel trims? Removing the oil cap seems to get the fuel trims right so would that further reinforce the idea that there is
too much suction in the manifold?
It does it with the purge valve blanked off and with the air filter removed.

I imagine this breather explains the difference when the oil cap is removed.

I found a fuel pressure gauge and have 430 kpa - specs say 400 kpa.