2011 A4 B8 cranks but won’t start....HPFP? Fuel pressure climbs with engine off?

[85rovah90]

Troubleshooting my 2011 A4. No CEL currently but has occasionally come on for a misfire on 2/3 but it clears on the next good start.

Symptoms- car will intermittently be hard to start, ie 9/10 cold starts no issues but after it’s warm it will crank for awhile but not turnover. In my diagnosis I checked fuel rail pressure and when it’s sitting around 600 psi/40 bar before start, it fires up without issue. For some reason though after the car is warm and I shut it off I watch the fuel rail pressure climb to 700-800 psi with the car off. When I go to start it, it spikes to 1100-1200 psi and it’s hard to start because it’s flooding itself out.

Anyone have any ideas why fuel pressure would climb after the car is shut off? Most of what I've read on the HPFP says it will fail by not producing enough pressure, whereas this is the opposite. Please help!

[ROH ECHT]

Pressure climbs in the rail because of heat. The Fuel pressure regulator valve, or FPRV, will release pressurized fuel back into the low side of the delivery portion and then the filter regulator will return fuel into the tank if necessary. The oem FPRV limits rail pressure at about 125 to 130 bar and the filter is likely 6.4 or 6.6 bar. If it were flooding itself out, then there would be an injector sticking open...but this is likely not the case given the rail pressure does increase when shut off. You haven't shown what the results of the low side are. There are two ways the low side can back bleed fuel into the tank. One is the LPFP/tank delivery pump has a built in check valve so fuel it delivers into the fuel line cannot return into the tank....or the fuel filter's built in pressure regulator is opening before it reaches 6.6 or 6.4 bar (depends on the engine) and returns fuel prematurely. One more thing that is found to restrict start-up when hot is the LPFP control module. When it is hot it will not allow a start up. It sits in the lid for the LPFP under the rear seat.

There is a bleed back test to do to determine if the bleed off is at the front or rear; https://www.youtube.com/watch?time_c...ature=emb_logo

[85rovah90]

Ok thanks for the info, I haven’t checked the low pressure side yet so I’ll look into that.

[ROH ECHT]

If you want to see this in action...shut it off when warm and watch the high pressure rise up to the limits of the FPRV.
You can watch the low side pressure at the same time with VCDS...as it should not drop the pressure.