Scirocco Central

Hi,
I posted on this topic way back when and it has never resolved no matter what I've tried.
Decided to tackle it again so seeking views or advice again.
The car is a 2.0TDI 170 (2010) which I have had from new and still only has 23000 miles on clock.
The car will either start first turn of the key or will turn over for a long period either without a start at all or mostly with a reluctant start. Once running it is fine and will restart straightaway over the next hour or so. It runs faultlessly, no misfires and no fault codes. The battery is excellent and the engine spins over easily.
The problem is that it is so unpredictable as to when this will happen (but more than 50% of the time).
VCDS shows no fault codes and neither does my Launch 123E diagnostic.
I have long suspected fuel drain back or no residual pressure in the fuel lines due to a leak but can find nothing visibly to evidence that.
I ran fuel pressure tests with VCDS and they show:
Coolant Temp : 85.5 deg
High fuel pressure (actual) 288 bar
High fuel pressure (specified) 248 bar
High fuel pressure control status 15.0
High fuel pressure control valve 18.9%
High fuel pressure control deviation -5.10

Most of the above seems ok to me but not an expert so any thoughts very welcome.
In VCDS if the coolant is over 80 deg and you switch off and then back on the fuel pressure is supposed to rise to above the norm and then very slowly drop eventually arriving at 10 bar as residual pressure (this is held by the check valve). On my car the pressure immediately drops to zero when this test is conducted and yet I can find no fuel leaks at any of the injectors, leak off junctions or the check valve.
I now feel at a total loss on this matter and no one seems to be able to provide me with any possible solutions but I hope maybe someone on here might this time have some answers or thoughts for me on this infuriating problem.

John

I don't have any experience of modern fuel system pumps, but it sounds like the check valve isn't working, allowing pressure to drop to zero, and sometimes allowing fuel to syphon back into the tank

There's a load of information here, from your earlier post in 2021:

https://www.sciroccocentral.co.uk/forum ... 61#p217761

Did you manage to resolve the issue then?

Looking at the VW workshop manual linked to in that thread, it sounds like it would be worth running the procedure "Checking fuel system pressurisation pump", when the fault has occured. I assume there is an equivalent procedure in VCDS.

This would ensure the fuel lines leading to the high pressure pump are primed, before attempting again to restart. If the car then starts without issue, that would probably narrow the fault down to the low pressure side of the system, most likely the non-return valve of the lift pump within the fuel tank