My Stilo has 2 problems. First one - engine is difficult to start when its cold. I have to hold starter on for 10 or more seconds.
The second one is Engine Fault message and corresponding power loss when accelerating. When I'm trying to accelerate harder, engine sometimes is even completely switched off.

Here what FatECUScan reports at iddle speed:

Errors
P0191 - Diesel preasure control / Invalid signal / Fatal / Light ON

Parameters
(I can not post links to images here )
...
Engine Speed 852 rpm
Fuel Metering 8,77 mg/i
...
Fuel Pressure 298,5 bar
Fuel Pressure Signal 1,290 V
Desired Fule Pressure 300,0 bar
Fuel Pressure Regulator 18,74 %
...
Boost Control/Sol. 75,00 %
Boost Control/Sol. Not active
Actual EGR 1,00 %
Capacitor 1 Voltage 80,097 V
Pedal Pos. Track 1 0,679 V
Pedal Pos. Track 1 0,342 V
Vehicle speed 0,00 km/h
Air Temperature 5,4 C
Air temprature signal 3,548 V
Baro Pressure 1.005 mBar
Baro Pressure signal 3,979 V
Intake Air Quantity 52,7 mg/i
Desired Intake Air Quantity 295,6 mg/i
Air Flow Signal 2,331 V
Glow plug heater module Not active
CAN data line OK
Engine Temperature 63,1 C
Engine Temperature Signal 2,053 V
Engine/ECU stop Not active


What is the main suspect? Overboost valve? MAP?
Thank you.

P0191 = Fuel Rail Pressure Sensor Circuit Range/Performance.

Incorrect fuel pressure or an incorrect pressure signal to the engine management ECU could cause various problems, so if that's the fault you're getting, I'd be inclined to start there.

The fault could be with the actual sensor or it's wiring or connectors.

A common fault caused by some tuning boxes that are set too high.

I had this fault code with my Punto, turned out to be the high pressure fuel pump.

It can be that. I am not the first owner of this car.
My problem appeared when mechanics replaced the old regulator of fuel pressure to the new.

Today I've made a test drive with laptop connected to ECU. Here is data from FiatECUscan:
Engine was switched off at 14,2 sec.
View attachment FESExp_1102061351_FIAT STILO 1.9 JTD_File1.txt

"Time" "Engine Speed" "Fuel Pressure" "Desired Fuel Pressure" "Fuel Pressure Regulator" "Boost Control/Sol." "Boost Pressure" "Vehicle speed" "Intake Air Quantity" "Desired Intake Air Quantity" "TAG"
"sec" "rpm" "bar" "bar" "%" "%" "mBar" "km/h" "mg/i" "mg/i" " "
0,0 853,0000 298,5000 300,0000 19,2000 75,0000 1025,0000 0,0000 52,6000 289,1000 ""
2,0 852,0000 307,6000 300,0000 19,2000 75,0000 1025,0000 0,0000 53,1000 288,7000 ""
4,0 846,0000 300,3000 300,0000 19,2600 75,0000 1025,0000 0,0000 53,3000 288,4000 ""
6,0 849,0000 304,0000 300,0000 19,3700 75,0000 1025,0000 0,0000 49,3000 334,4000 ""
8,1 766,0000 344,3000 390,5000 24,1500 75,0000 1052,0000 7,7800 60,1000 696,1000 ""
10,1 1431,0000 410,2000 806,5000 76,8200 37,7400 1668,0000 19,5000 130,2000 444,9000 ""
12,1 1488,0000 331,5000 623,2000 53,3900 74,2800 1499,0000 27,3300 195,2000 1200,1000 ""
14,2 1795,0000 62,2000 642,6000 1,2300 74,2000 1308,0000 27,2000 75,9000 1200,0000 ""
16,2 224,0000 42,1000 300,0000 1,2300 75,0000 1020,0000 25,3600 26,9000 407,3000 ""
18,2 0,0000 27,4000 300,0000 1,2300 75,0000 1001,0000 23,4400 8,6000 405,8000 ""
20,2 0,0000 18,3000 300,0000 1,2300 75,0000 1001,0000 21,4400 2,0000 405,0000 ""
22,2 0,0000 14,6000 300,0000 1,2300 75,0000 1001,0000 18,8400 0,3000 403,8000 ""
24,3 0,0000 10,9000 300,0000 1,2300 75,0000 1001,0000 15,8600 0,0000 403,0000 ""
26,3 0,0000 10,9000 300,0000 1,2300 75,0000 1001,0000 12,8200 0,0000 402,6000 ""
28,3 0,0000 9,1000 300,0000 1,2300 75,0000 1001,0000 10,6800 0,0000 402,2000 ""
30,3 0,0000 9,1000 300,0000 1,2300 75,0000 999,0000 8,9800 0,0000 401,8000 ""
32,4 0,0000 7,3000 300,0000 1,2300 75,0000 999,0000 7,3700 0,0000 401,7000 ""
34,4 0,0000 7,3000 300,0000 1,2300 75,0000 999,0000 5,1800 0,0000 401,7000 ""
36,4 0,0000 7,3000 300,0000 1,2300 75,0000 999,0000 2,5700 0,0000 401,9000 ""
38,4 0,0000 7,3000 300,0000 1,2300 75,0000 999,0000 0,0000 0,0000 402,4000 ""
40,5 0,0000 7,3000 300,0000 1,2300 75,0000 999,0000 0,0000 0,0000 402,4000 ""
42,5 0,0000 5,4000 300,0000 1,2300 75,0000 1001,0000 0,0000 0,0000 402,4000 ""


Here another try. "Engine fault" and off at 11,7 sec.
View attachment FESExp_1102061412_FIAT STILO 1.9 JTD_File2.txt

"Time" "Engine Speed" "Fuel Pressure" "Fuel Pressure Regulator" "Vehicle speed" "TAG"
"sec" "rpm" "bar" "%" "km/h" " "
0,0 852,0000 294,8000 19,4300 0,0000 ""
1,0 844,0000 302,1000 19,4300 0,0000 ""
2,1 850,0000 302,1000 19,4500 0,0000 ""
3,1 847,0000 304,0000 19,5000 0,0000 ""
4,1 773,0000 305,8000 19,9600 0,0000 ""
5,2 896,0000 307,6000 20,0200 3,0400 ""
6,2 933,0000 320,5000 20,3500 6,4300 ""
7,3 933,0000 413,9000 29,8000 11,6800 ""
8,3 1685,0000 410,2000 80,0000 19,4200 ""
9,3 2040,0000 335,1000 18,6400 18,7500 ""
10,3 1656,0000 357,1000 47,3900 23,5600 ""
11,7 1665,0000 150,1000 80,0000 30,4600 ""
12,6 1917,0000 64,1000 1,2300 28,5100 ""
13,6 922,0000 54,9000 1,2300 27,2700 ""
14,7 26,0000 43,9000 1,2300 26,4000 ""
15,7 0,0000 31,1000 1,2300 24,9700 ""
16,7 0,0000 23,8000 1,2300 20,9900 ""
17,8 0,0000 20,1000 1,2300 19,0000 ""
18,8 0,0000 16,4000 1,2300 17,8100 ""
19,8 0,0000 14,6000 1,2300 16,2800 ""
20,9 0,0000 12,8000 1,2300 14,5000 ""
21,9 0,0000 12,8000 1,2300 13,2400 ""
23,0 0,0000 10,9000 1,2300 11,7800 ""
23,9 0,0000 10,9000 1,2300 10,5600 ""
25,0 0,0000 9,1000 1,2300 9,5600 ""
26,0 0,0000 9,1000 1,2300 8,8100 ""
27,1 0,0000 9,1000 1,2300 7,7100 ""
28,1 0,0000 9,1000 1,2300 6,8100 ""
29,1 0,0000 7,3000 1,2300 5,8700 ""
30,2 0,0000 7,3000 1,2300 5,1200 ""
31,2 0,0000 7,3000 1,2300 4,3100 ""
32,2 0,0000 7,3000 1,2300 0,4500 ""
33,3 0,0000 7,3000 1,2300 0,0000 ""
34,3 0,0000 7,3000 1,2300 0,0000 ""
35,5 0,0000 5,4000 1,2300 0,0000 ""
36,4 0,0000 7,3000 1,2300 0,0000 ""
37,4 0,0000 5,4000 1,2300 0,0000 ""
38,5 0,0000 5,4000 1,2300 0,0000 ""
39,5 0,0000 5,4000 1,2300 0,0000 ""
40,5 0,0000 5,4000 1,2300 0,0000 ""
41,6 0,0000 5,4000 1,2300 0,0000 ""

I have this error code come up on my stilo . The car starts first time every time. Drive for one or two days fine. Then out of the blue it cuts out dead . I pull the key out and put it back in starts again first time. . I've removed the pump in the tank and cleaned left it running for a short while and fine . Any suggestions world be truly helpful ?

as the above posts
when I had my tuning box and especially on hot days for some reason I had it a few times , I think its the hp pump fuel regulation passing out of its expected % range to meet the actual fuel pressure .
I see the original poster had only 18% on the regulator for only 300bar at idle.
im not sure on the 115jtd but compared to mine(jtdm 150hp) that sounds low, I think mines in the 30-40% range(need other jtd 115hp owners to check theirs and confirm whats normal)
and another poster at point of fault regulator hit 80%(accelerating not idling) this might be a limit.

but first step maybe a fuel injector cleaner, this will clean the injectors may help with leak off volume and also will help shift any build ups in the fuel lines, hp pump and regulator.
once that tank of fuels gone id be throwing a new fuel filter on it , as the old one will catch any crap the cleaner has flushed out.
if you have anything(software) to read the parameters you can track if they improve step by step.