I’m trying to troubleshoot a hard to start when hot issue. I have bench tested the VSV for the fuel pressure regulator and it checks out perfect per the factory service manual. The fuel pressure in my truck is about 34 psi and raises to about 38 psi when I pull the vacuum hose off of the fuel pressure regulator. The engine coolant temperature sensor checks out by bench testing.
The vacuum line going to the fuel pressure regulator from the vsv seems to be always on vacuum. When the truck is hot and I start the truck, there is still vacuum on the fuel pressure regulator, the vacuum is not switched off.

Can someone suggest to me how to check if the vacuum to the fuel pressure regulator is being switched off by the VSV during hot starts or some other condition?

 

There should be no vacuum on the FPR when it's started hot. It raises the pressure in the fuel rail to try and reduce the chance of vapor lock in the fuel rail. Once the truck is running, the vacuum is applied to the FPR to lower the fuel pressure to the normal, "running" pressure.
I am pretty sure the system only functions during a starting cycle, when the engine is fully up to operating temp. The may well be an atmospheric temp sensor input off the MAF, or maybe the TPS, I don't recall, as well. IOW: Atmospheric temp above X, and engine temp above Y, remove vacuum from FPR during the start cycle.

Is all this as clear as mud?
Pat☺

EDIT:

According to the schematic I found, yes, it takes into account bot atmospheric AND engine temperatures when determining the Fuel Pressure Up (Fpu) signal applied to the VSV. Both conditions must be met before the VSV will remove the vacuum from the FPR. So, not only the ECT sensor, but the Atmospheric Temp sensor in the MAF both must be checked. And the VSV, naturally.

Does this help at all?
Pat☺

 

Thank you Pat. It is relatively clear.
I also read that the vacuum to FPR would also be off for two minutes after a hot start. But when I pulled the vacuum line off FPR about 15 seconds after a hot start, there was vacuum on the line. My vsv switches on my bench with battery. So I am concerned why there is still vacuum to my FPR only seconds after a hot start. My vsv is not receiving voltage during/after a hot start?