i have a strange fault with out 59 plate 2.0 diesel mondeo that i need some opinions on.

every now and again we get a hesitation that feels like a misfire and engine malfunction appears on the screen - an intermittent problem

the first thing i did was change the fuel filter which has not sorted the problem

i took the car to a diesel specialist and discovered that the car is throwing up a P120f fault which relates to fuel pressure being outside specified range. As this was a suspected pressure regulation valve the whole pump was replaced and the car felt ok for a short while but the problem is still there.

i have since discovered that the issue is worse if the ac compressor is running? when looking at live readings on ford IDS the fuel pressure at idle is nice an consistent without the AC on but starts to fluctuate when the compressor clutch engages? again this is intermittent.

the problem has also there without the ac running but is far less noticeable

does anyone have any thoughts on how the ac compressor could effect the fuel pressure? sounds like madness but it's happening and we are out of ideas.

we have checked the voltage to the pressure regulation valve with the ac on and off and it seems fine.

the only part of the fuel system that we haven't checked is the injectors. however i cant see there being an issue there as it just doesn't feel like an injector fault. All four are reading ok on IDS

anyone got any suggestions?