Fuel Pressure Specs?

We do have access to and Ford websites for such information but problem is the description of the system is rather vague and principles of operations are not very comprehensive.

Without any specific criteria it would be almost impossible to get information as the amount is vast so whatever information that you require, let me know so I can search for them. I will see what I can get about the principles of operations.

You mentioned the first mechanic lost it so I do not know if the trouble codes then was the same P0191 or it could have been others.

P0191 - Fuel Rail Pressure (FRP) sensor circuit performance. The test procedures are below.

NOTE:
On CNG models, if vehicle is a no-start, go to TEST HB

16) KOEO/KOER DTC P0191

DTC P0191 indicates FRP voltage signal is more than self-test maximum.
Possible causes for this fault are:
High Fuel Pressure
Low Fuel Pressure
Excessive Resistance In Circuit
Faulty FRP Sensor
Low Or No Fuel

Turn ignition switch to ON position. Ensure vehicle has at 1/8 tank of fuel. Turn
ignition switch to OFF position. Release fuel pressure. Connect pressure gauge
to Schrader valve. With engine running, check fuel pressure. Fuel pressure
should be within specification. If fuel pressure is within specification, go to next step. If fuel pressure is not within specification, go to TEST HB (natural gas vehicles) or TEST HC (all other models).

17) DTC P0191, P1168 & P1169: Check FRP PID Fuel Pressure
Turn ignition switch to ON position. Using scan tool, select FRP PID from PID/DATA MONITOR & RECORD menu. If PID psi reading is not within 10 psi of fuel pressure gauge psi reading in step 16), go to next step (natural gas vehicles) or step 19) (all other models). If PID psi reading is within 10 psi, proceed as follows:

If DTC P1168 or DTC P1169 is not present, repeat QUICK TEST.
If DTC P1168 or DTC P1169 is present, clear DTCs. Road test vehicle 3-5 minutes at a steady speed. Stop vehicle and check for Continuous Memory DTCs. If DTC P1168 or DTC P1169 is present, go to next step.

18) Check Fuel Rail Solenoid

Using scan tool, access OUTPUT TEST MODE under ADDITIONAL SYSTEM FUNCTIONS. While observing fuel rail solenoid, cycle output on and off several times. If solenoid clicking can be heard or felt, exit output test mode and go to next step. If no solenoid clicking can be heard or felt, go to step 22.

19) Measure VREF Voltage

Disconnect FRP sensor connector. Turn ignition switch to ON position. Measure
voltage between VREF terminal and SIG RTN terminal at FRP sensor harness
connector. If voltage is 4-6 volts, go to next step. If voltage is not 4-6 volts,
reconnect sensor and go to TEST C.

20) Check FRP Circuit For Excessive Resistance

Turn ignition switch to OFF position. Disconnect PCM connector(s). Inspect
connector for loose, damaged or corroded terminals. Repair as necessary.
Measure resistance between FRP terminal at FRP sensor harness connector and
PCM connector terminal C40 (LS) or terminal No. 63 (all other models). Measure resistance between SIG RTN terminal at FRP sensor harness connector
and PCM connector terminal No. 91 (SIG RTN). Measure resistance between
VREF terminal at FRP sensor harness connector and PCM connector terminal
C20 (LS) or terminal No. 90 (all other models). If all resistance readings are less
than 5 ohms, go to next step. If any resistance reading is 5 ohms or more, repair
open circuit.

21) Monitor FRP Circuit With Scan Tool

Turn ignition switch to ON position. Using scan tool, select FRP V PID from PID/DATA MONITOR & RECORD menu. If PID voltage is less than.2 volt (natural gas vehicles) or more than 4.8 volts (all other models), replace FRP sensor. If PID voltage is not as specified, replace PCM. Program PCM.

NOTE:
When in output test mode, voltage measurement must be made within 7 seconds of activating test mode.

22) Check Voltage At Fuel Rail Solenoid

Disconnect fuel rail solenoid connector. Using scan tool, access OUTPUT TEST MODE under ADDITIONAL SYSTEM FUNCTIONS. Turn all outputs ON. Measure voltage between VPWR circuit terminal (Pink/Black wire) at fuel rail solenoid harness connector and negative battery terminal. If voltage is more than 10.5 volts, go to next step. If voltage is 10.5 volts or less, repair open in VPWR circuit.

23) Check Ground Circuit

Measure resistance between ground circuit terminals at fuel rail solenoid harness
connector and negative battery terminal. If resistance is less than 5 ohms, replace
fuel rail solenoid. If resistance is 5 ohms or more, repair open circuit.

24) Check VREF Voltage To FRP Sensor

Disconnect FRP sensor connector. Turn ignition switch to ON position. Measure voltage between VREF and SIG RTN circuit terminals at FRP sensor harness connector. If voltage is 4-6 volts, go to next step. If voltage is not 4-6 volts, VREF is out of range. Go to TEST C.

25) Check For Shorted FRP Signal

Disconnect scan tool from Data Link Connector (DLC). Disconnect PCM connector(s). Inspect connector for loose, damaged or corroded terminals. Repair as necessary. Measure resistance between PCM harness connector FRP and SIG RTN terminals, between PCM harness connector FRP and VREF terminals, and between PCM harness connector FRP terminal and battery negative terminal. If all resistance measurements are more than 10 k/ohms, replace PCM. Program PCM. If any resistance measurement is 10 k/ohms or less, repair short in affected circuit.

I need you engine size to look up specs, 3.0? 3.0 DOHC?
Flex fuel? Or 3.4?
With the Key on engine off reading should be 30-45 psi and test the system like this:
4) Check Fuel Pressure Release fuel system pressure. Turn ignition off. Install fuel pressure gauge. Turn ignition on. Using scan tool, access OUTPUT TEST MODE. Command fuel pump on. Note fuel pressure. For fuel pressure specifications, see FUEL PRESSURE SPECIFICATIONS article. If fuel pressure is as specified, go to next step. If fuel pressure is not as specified, go to step 10). 5) Check System Ability To Hold Fuel Pressure With fuel pressure gauge installed, turn ignition off. Exit OUTPUT TEST MODE. If fuel pressure remains within 5 psi (34 kPa) of specification for 60 seconds, go to step 7). If fuel pressure does not remain within 5 psi (34 kPa) of specification for 60 seconds, go to next step. 6) Check Pressure Regulator Diaphragm With fuel pressure gauge installed, start engine and operate for 10 seconds. Turn ignition off and wait 10 seconds. Start engine again and allow to idle for 10 seconds. Turn ignition off. Disconnect and inspect hose from fuel pressure regulator. If hose is wet with fuel, replace fuel pressure regulator. If hose is dry, go to step 12). 7) Check Fuel Pressure; Test Drive Vehicle With fuel pressure gauge installed, disconnect and plug fuel pressure regulator hose. Ensure fuel pressure gauge can be seen by vehicle operator. Drive vehicle while noting gauge reading during heavy acceleration. If fuel pressure reading stays within 3 psi (21 kPa) of original pressure reading, go to next step. If fuel gauge reading does not stay within 3 psi (21 kPa) of original pressure reading, go to step 13). 8) Check Fuel Pressure Regulator; Test Drive Vehicle With fuel pressure gauge installed, reconnect fuel pressure regulator hose. Install vacuum gauge to intake manifold. Ensure both gauges can be seen by vehicle operator. Drive vehicle while noting gauges during heavy acceleration. Gauge readings should be as follows: Ã Â Â Fuel pressure gauge reading increases and vacuum gauge reading decreases. Ã Â Â Fuel pressure gauge reading decreases and vacuum gauge reading increases. If gauge readings are as specified, fuel system is functioning properly and testing is complete. If gauge readings are not as specified, go to next step. 9) Check Vacuum Supply Turn ignition off. Disconnect and plug fuel pressure regulator hose. Install vacuum pump to fuel pressure regulator. Start engine and operate at idle. Observe fuel pressure gauge while applying vacuum to regulator. If fuel pressure changes as vacuum changes, repair restricted vacuum source. If fuel pressure does not change as vacuum changes, replace fuel pressure regulator. 10) Check Fuel Pressure Regulator Leave ignition off and scan tool connected to DLC. Release fuel system pressure. Disconnect fuel return hose at fuel rail. Connect a separate hose to fuel rail and put opposite end of hose in clean, one quart container. Turn ignition on. Enter OUTPUT TEST MODE. Command fuel pump on. Note fuel pressure and fuel returning to container. Exit OUTPUT TEST MODE. Command fuel pump off. If fuel pressure is 35-40 psi (240-280 kPa) 35-40 psi with fuel returning to container, go to next step. If fuel pressure is not 35-40 psi (240-280 kPa) or fuel is not returning to container, replace fuel pressure regulator. 11) Check Fuel Return Ensure fuel return line is disconnected at fuel rail. Disconnect fuel return hose at fuel pump. Check return hose for restrictions. Apply 3-5 psi (21-34 kPa) compressed air to return hose. If air flows freely, replace fuel pump. If air does not flow freely, repair or replace fuel return hose.
If there is any gas in the vacuum line to the regulator, replace the regulator.

It sounds like the pump itself is getting tired. Takes too long to get up to initial pressure specs. Or the filter is clogged.

Ok We will be watching for any more on this!

It should be 37 to 43 psi according to the shop manual.

Judging by the discription you have given, it is not the relay. Sounds like a pump problem.

10 psi low is quite a drop and needs to be check out immediately.

Change your fuel filter and check the fuel lines for kinks and bends that dont belong there. Make sure you put the filter on the right way.

Let me know.