2008 ford ranger 2.5, what could be the problem if engine starts normal but after second the engine fluctuate?

no country told at all, the diesel is not here USA>
so the books are by country, our book has zero, diesel there.
wiki says

During the 2000s, Rangers produced by Ford Argentina shared a common chassis with North American-produced vehicles, while offering a diesel engine option to meet local demand. 2011 last one.
You live Argentina?

A diesel engine will not crank if it has air in the fuel system. If it does not have a self purging system, you will have manually bleed each injector line at the engine.

we dont have this engine here, USA.
but i have the WG jeep book,
starting page 12 , chapter 14


FUEL INJECTION PUMP BLEEDING
(1) If the fuel injection pump has been replaced,
air should be bled at the overflow valve before
attempting to start engine.
(a) Loosen the overflow valve (Fig. 18) at the
rear of the injection pump.
(b) Place a towel below the valve.
WARNING: WHEN CRANKING THE ENGINE TO
BLEED AIR FROM THE INJECTION PUMP, THE
ENGINE MAY START. PLACE THE TRANSMISSION
IN NEUTRAL OR PARK AND SET PARKING BRAKE
BEFORE ENGAGING THE STARTER MOTOR.
CAUTION: Do not engage the starter motor for
more than 30 seconds at a time. Allow 2 minutes
between cranking intervals.
(2) Crank the engine for 30 seconds at a time to
allow air trapped in the injection pump to vent out
the fuel injector drain tubes. Continue this procedure
until the engine starts. Observe the previous WARNING
and CAUTION.
(3) Tighten overflow valve.
HIGH-PRESSURE FUEL LINE BLEEDING
WARNING: THE INJECTION PUMP SUPPLIES HIGH-
-PRESSURE FUEL OF APPROXIMATELY 45,000 KPA
(6,526 PSI) TO EACH INDIVIDUAL INJECTOR
THROUGH THE HIGH-PRESSURE LINES. FUEL
UNDER THIS AMOUNT OF PRESSURE CAN PENETRATE
THE SKIN AND CAUSE PERSONAL INJURY.
WEAR SAFETY GOGGLES AND ADEQUATE PROTECTIVE
CLOTHING AND AVOID CONTACT WITH
FUEL SPRAY WHEN BLEEDING HIGH-PRESSURE
FUEL LINES.
WARNING: DO NOT BLEED AIR FROM THE FUEL
SYSTEM OF A HOT ENGINE. DO NOT ALLOW FUEL
TO SPRAY ONTO THE EXHAUST MANIFOLD WHEN
BLEEDING AIR FROM THE FUEL SYSTEM.
Bleed air from one injector at time.
(1) Loosen the high-pressure fuel line fitting at
the injector (Fig. 19).
(2) Crank the engine until all air has been bled
from the line. Do not operate the starter motor
for longer than 30 seconds. Wait 2 minutes
between cranking intervals.
(3) Start the engine and bleed one injector at a
time until the engine runs smoothly.

end quote.
pump timing is on next page
or tank filter line priming before. above.

Refresh my memory, What have you done to the car beside replacing fuel filter? Do you have a fully charged battery? Are you getting spark to all cylinders? Are you getting fuel? I can help you if you tell me what is wrong. Don't assume it's the fuel pump cause there is so much in between but we can see if your Fuel Pump is working. Open your gas tank and have someone turn the car on but without starting it. You should hear a hum for about 2 to 3 seconds.
This means your pump is working. Now you need to find out what is your fuel pressure? A good number is between 65Lbs. and 40 lbs.
Let me know how you make out I'll be on line all day today.

It is not normal for them to make loud noises. Sounds like it is strained, therfore making a whining noise. Not normal.

sounds like fuel pressure bleeding off. Bad fuel pump or bad pressure regulator on fuel rail. Can you hear pump run for 5 seconds when you turn key on? do this twice, 30 deconds apart and see if it starts ok.

Hi,

Just pump up the system with the pump on top of the fuel filter then when it is hard to pump then just start it up.

If it don't start just keep winding it over until it fires up.

Hope this helps.

Thanks

Jason

It sounds like your thermostat is stuck open, not allowing the engine to warm up enough.

When was the last time you had the fuel filter changed?

Fluctuating an high RPMs sounds like a leaking or missing vaccume hose to me, I had simmilar problem found open hole with hose off, replaced it now the RPMs are under 1000 again.
As for the stalling I also am having a simmilar problem, and am finding air in the fuel rail, I suspect the fuel pump is going.
I cant go more than a mile without problems.