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Discussion Starter #1
Ok, So I have to admit, I'm not well versed on Ford. I typically work on other brands. With that being said, here's where I'm at with this one.

2002 Taurus SE. 3.0 OHV.
Initial Problem was no heat. Did some troubleshooting, found a radiator leak which the previous owner tried to fix with Stop leak. Replaced the radiator and heater core. Still no heat, Found the bypass tube and hoses to the heater core also plugged.
Removed the bypass system and ran hoses from the water pump to heater core and heater core to thermostat housing.
Car fixed and out the door.

Two days later the customer calls, the car died on her. Stumbling and rough idle then wouldn't start.
Picked it up with a trailer the next day. Ran great to load on trailer. Rev engine after loading, Rough idle starts again and it dies.

Got it in the shop. Ran smooth for a minute or 2. same BS. Connected scan tool, Found P0191, Fuel Rail Pressure Sensor Performance. Bank 1 lean, Bank 2 lean, and misfire at startup codes. Ran system self test KOER. Same codes still active.

Replaced Fuel filter. Started engine. ( Took a few cycles of key and running for a minute or two to purge air from the system before it would idle)

Idled engine for maybe 4.5-5 minutes and it started stumbling. Opened live data on scan tool. Fuel Rail pressure at ~13 PSI. (at this point, I know it's low but don't have anything to back that up) Monitor FRP. Barely increases at WOT. Tops out at 20.21 and then begins falling. As the pressure drops so does the RPM. Engine starts stumbling as pressure drops. Play with the throttle a little and get the pressure back to ~12psi. engine idles smoothly for a moment then the pressure falls to ~8 and the engine starts to stumble again. A few moments later, 3psi and the engine dies.

Due to pressure change matching engine performance, assumed the sensor was correct. Double checked readings and did fuel pump testing with scan tool. Fuel pressure would never build over 15 per the sensor. Fuel Pump Monitor reading on Scan tool showed 100-114% usage. Determined (in error) that the Fuel pump was defective. Dropped the tank and replaced the fuel pump. Gained 2 PSI, Overall performance not changed

Did some Google Training, Confirmed the Fuel rail pressure should be ~40 as I suspected. On initial startup for the day, The engine will idle smoothly for the first 5-6 minutes. Even at ~15-16 PSI FRP. As it warms up, PSI drops to ~12, then 10, then 6 and it dies.

Installed gauge on Fuel Rail to confirm sensor readings. Sensor read ~3 PSI higher than the gauge until removing the vacuum line from the sensor. Once the vacuum line was removed the sensor and Mechanical Gauge read within 1 psi. Both at idle and at WOT.

Replaced Fuel Pump Driver Module. No change at all.

I'm now officially lost.

New Fuel Pump Driver Module, Fuel Pump, Filter. FRP Sensor matches mechanical gauge, Fuel lines are clear( Blew them out to ensure when I replaced the pump) I don't have a scope to measure duty cycle on signal wires from PCM to FPDM. Wiring harness ohms good. At initial startup, FRP is at ~15psi then as it warms up, it falls. Anyone have any suggestions to test or check? Going bypass the harness with new wiring to test in the morning just to make sure but I'm not confident that I'm going to find an issue with the harness.
 

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Check the inertia switch in the trunk it's on the passenger side just remove the trunk liner...you will see a ground wire take the ground off and clean the paint off where the ground is....the bad ground has been known to cause issues. Worth a try and it cost no $$$ takes about 15 min
 

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If a vacuum is forming in the tank, the problem will be most severe when the tank is close to full. Maybe remove fuel tank cap and see what happens. Or, after the car has stopped, stumbled, etc., slowly remove cap with your ear next to it to so as to hear an inrush of air.
 

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Discussion Starter #6
Ok. just got back into it after a long Holiday weekend. I checked and even bypassed the inertia switch. New Fresh gas. Initially it was around 1/8th tank. I added ~2 gallons, The fuel pressure improved but the engine still wouldn't run smooth. Added 5 more gallons. Fuel pressure now reading around 20 psi. Car ran smoothly for a 3 mile road test. Pulled back in front of the shop, ran KOER self test. P0191 still present, and after 10 minutes of idle, fuel pressure dropped and engine died. I had the same results with or without the fuel cap on.

Ran fuel pump test via scan tool, Built pressure back to 21 psi, I noticed that sitting there without the pump running the pressure was climbing. Made a cup of coffee, went back to the car, sensor reading 32 psi. Started the engine. It ran poorly for a minute and then died. It currently won't start to drive itself back into the bay.

Starting to believe there is a restriction in the fuel rail crossover. This would account for the sensor and manual gauge reading the same as they are on the same rail. Once I get it back in the air, I'm going to rig a pressure gauge at the fuel filter.
 

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I am thinking some kind of leak between the pump output and pump flange such as damaged hose, loose clamp at the pump or the flange, or maybe even a bad out of the box pump. If it were mine, I would drop the tank and have a close look at the pump, hose from pump to flange and flange.

Had a hose develop a small split in the line from the tank to hose years ago on an OBDI Ford that had a relatively new pump. After exhausting every other possibility, I dropped the tank and the small split was obvious. Replaced the hose with a new piece of submersible fuel line ($$) and the car ran perfectly for many years after the repair.
 

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Discussion Starter #8
Thanks Jeff. That's where I'm heading now. Put a gauge in the line at the filter, same pressure on the rail and before the filter. Of course, this time it has nearly 3/4 of a tank.
 

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What I have done several times when I had to drop a tank that had a fair amount of gas in it was to disconnect the fuel filter outlet and attach a long piece of fuel line to the filter outlet and run the hose to gas cans or to the filler neck of another car that was low on gas and hotwire the pump to pump out the gas into the gas cans or another car. Takes a while especially with a weak pump, but, IMO, better than trying to wrestle a nearly full tank out of the car.
 
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