2000 Mercury Cougar 2.5L V6 Intermittent Crank/No Start / Low Fuel Pressure

Have been working on 2000 Mercury Cougar 2.5L V6 Auto w/ Return-less fuel system when cold ambient weather car will crank but not start. If car sits for several days or if it warms up (over 40F) car usually starts. Ignition Has good Spark, put in new plugs and wires. Good Cam and crank sensors.
Replaced fuel pump and fuel rail pressure sensor. Fuel Pump Primes but no pressure builds unless I cycle key on 5-10x. When car happens to start it shows steady 40+psi. but Fuel pressure ALWAYS drops to zero as soon as car is turned off (true if crank no start and when it runs). On warm days when car starts it is running really rich (LT Fuel trims -20+). Before diving into electronics I thought I was getting stuck open injector. I verified injectors are good, no leaks and all fire as expected when rail pressure is > 30psi

Electrical Check:
Fuel rail Pressure transducer Sensor - 5V ref is good, ground is good, signal line voltage is 1.2V when connector unplugged. When connector plugged in signal voltage goes to 0.48V (gnd and 5V ref still good)

At Fuel Pump Driver Module: (comparing values to Scanner danner video): Inertia Switch voltage supply is good 12V, Grounds are good.
The issue are 2 signal wires values I believe. Pin 1 (Ground/Switched) - KOEO (0V) Cranking (2.7v) when turn key off goes up to 3.3V then to 0v. Video shows it should go to 5v. Pin 7(Voltage , Ignition switch in start or run (not overload protected), switched) - expected square wave 0-5V, getting square wave 0.4 - 2.3 volts)

I am new to most of this but learning from videos and forum. Trying to understand these signal voltage as they seem low as compared to all videos where they show 5v ref and 5v signal when unplugged and 3.5v or so when connector plugged in.

Hoping for some guidance. I only have forscan, fuel gauge and voltmeter for diagnostic tools.
Thanks
Nathan

Thanks for the reply. I did take some more readings from rail pressure sensor, driver module and pump. All looks good to pump, good power and good grounds. The sensor signals from rail pressure sensor to driver to pump also now make sense as they are low because I have no pressure (duh !?! by me). I did not replace fuel filter when I replaced fuel pump (another ugh moment) , wondering if fuel filter was problem all along and when I replaced pump but left old filter in if I damaged the new pump. Yesterday I put in a new filter but no change, pump runs, no pressure no start. . This is a returnless fuel system, so no return line. I do have one question on a part that is on the fuel line ahead of the pressure sensor. There is no mention of this part in any manual. I will attach pic.

Some data I had from the last time car ran (5 days ago) before it got cold here and car would not start. I was monitoring Fuel Pump %, Fuel Pump Monitor %,Fuel pulse width FRP (v) and FRP (psi)

At crank, 0 FRP (psi), FRP (v) 0.52, FP% 75, FPM%,- 98-100%
As engine started RPM went to 1900 FRP - 39psi, FRP(v)=2.79, FP% -19, FPM-96%

From there I noticed that the FPM% bounced between 90-114%, over 100 most time, pressure stayed constant at 30-40psi and FP% was 19 and FRP(v) was 2.74

Thanks for any guidance or ideas

j'essaie de ne pas deviner.....mais parfois je le fais (Google translate )

That little disk thing, to the best of my knowledge, is a pulse dampener. If there is a vacuum line on it, it is only there as a fail safe so that if the internal diaphragm were to rupture, there will be no external fuel leak.
That said, if you pull the vacuum line, there should be no fuel.
When you changed the pump, did you change the entire fuel tank sending unit, or just the pump part of the assembly?
I have seen them wired backwards which will allow the pump to run and create no pressure, and I have seen the fuel line fall off in the tank so that the pump just sprays fuel around in the tank.

I would go old school
Connect a pressure gauge directly at the pump and measure what happens to the dead head pressure.
I don't know these cars but a lot of vehicles with returnless systems have plumbing inside the tank to scavenge and transfer fuel from the far side of the tank and a problem inside the tank is a possible suspect.

The purpose of the pulse damper on the fuel rail is simply to smooth out the fuel pressure when the throttle is floored from fully close and manifold pressure goes from high vacuum to atmospheric. The function a bit like an accelerator pump on a carb engine

Thanks for replies, went to go try the cougar this morning before heading out of town and of course IT STARTED... never fails as it warms up and sits for a couple days this happens.UGH!!! If we get cold again it will eventually not start.

So on the in line device I showed in pic, it only has in and out fuel line, no vacuum line attached. Could it be a pressure regulator? Haven't seen any documentation in manuals that returnless system has one.

I have to head out of town for week but will continue this when I get home Friday. I saw the scannerdanner video (funny it was a 2002 cougar) that they had wiring wrong on pump which caused no start. I changed the whole pump out and it was a direct fit, no wiring mods needed, . I'm not sure how to check pump pressure directly, with what I have access to.

I think this week I will work on confirming what that inline device is (if pulse dampener, could it cause a pressure change inline?) Would like to understand what it does and how. Then confirm where the pressure regulator is (on pump in tank?). I have the old pump I took out to play with as well. Thanks for ideas everyone,