2008 XC70 owner mrscullini is troubleshooting long cranking on his wagon. “When the engine is shut off, it pretty quickly bleeds down to 0. trying to diagnose a hard cold start with excessive cranking.”
“Can somebody with vida/dice please provide observed fuel pressure numbers/specs at both the rail and overall system? I’m interested in readings both at idle and after engine shut down.”
Jimmy57 provides the answers:
The fuel pressure will be about 56psi on start when hot and then drop to 45psi ish after 45 seconds or so. The fuel pressure should not drop below 29 psi within 20 minutes post shutdown. On that system it can only leak back through fuel pump check valve, leak through overpressure valve, or drop out of a leaking injector. Injector causes it to be flooded if you attempt restart after it has run and shutoff for a bit. If it is the FP check valve or overpressure valve then no black smoke when it starts. Overpressure not serviced and the FP check valve is part of pump. Best to put the whole assembly. When fuel drains out of lines on that system then it will take lots of starter time. That is a dead end system with no return line so to purge air to allow fuel to reach injectors takes injector cycling during cranking.
Fuel pressure after sitting overnight on a car that starts immediately will usually be zero but there will be fuel in the rail. There is a difference bwtween zero pressure due to the contraction of fuel as it cools and a seepage at FP check valve versus some large leak and the rail draining. That difference is long crank cold due to having to purge air out of injectors via multiple injector cycles.
The fact your pressure goes away so quickly is a huge problem. You have not listed black smoke on start with very rough running until the engine runs 15 seconds +/- so I will take injector leak off the list. That would leave fuel pump/sender assembly as the cause. As I said in another post, the cause could be fuel pump check valve which is a part of the pump or it is the PVV as Volvo calls the max pressure relief valve. Technically there are two valve serving this purpose. The one built into pump is there as an emergency overpressure and is there on that model pump for the dozens of different make that use that pump. The one I am referring to is not part of the pump but looks like a fuel pressure regulator on the pump module and is there as the maximum pressure controller in the event the FP sensor fails and the ECM goes to full pump current. Then the ECM knows the pressure will be the approx 600kpa of the PVV for “limp home” or failure mode effects management (FMEM). The PVV could have some piece of something stuck in it and there is no source of the part and the disassembly needed to change it is likely to break plastic housing it is inserted into.