3000 sometime lite on diesel some time heavy , At a 100 km new injection what can I do

lots of KM there,

that feature is called limphome mode
it is for 3 reasons, to save damage to engine or trans
or to save you driving a sick car. all 3 is truth.
or saves the cat from meltdown.
or damage to transmission using wrong high gear going too slow this would wreck transmission clutch packs
the engine can do the same thing even back to 1988 year first EFI
injections.
Limphome can happen if any MAJOR sensor goes DEAD>
The EFI then uses Virtual created sensor. (software)
but is crude and burns lots of fuel and CAT overheats.
and loss of power.
or is low fuel pressure.

no engine told by you.(nor what fuel it burns)
no DTC errors scanned, did you scan it ?

the owners manual lost, telling what the real name if lamp is.

no dyna model told.
U300 to U400 , 3 tons "?
nor engine there.
diesel engine? LPG or Petrol? engine?
if D4D , fuel filters clean? diesel?


quote 2006
Even though they were built on the U300 and U400 platforms, for marketing purpose these models still used the Dyna 100, 150, 200, 250, 300, and 350 names, indicating payload. The Dyna 100 was available with a 2.5 L (2,494 cc) D-4D common rail turbo diesel engine offering 88 PS (65 kW). The Dyna 150 is a chassis/cab version of the Dyna truck, with twin rear wheels, and an uprated 102 PS (75 kW) engine, shared with the HiLux and HiAce.

Its sounds like you're priming pump has a leaking oil in to fuel line

Diesel !
cam ? timing? or the injector pump... ??????
ask Isuzu for a book.
wiki says, what is a space cab.>?? a 4,000 lb truck./van, sure.

The 4BE1 is a 3.6L direct injection engine. It replaced the 4BC2 in Isuzu NPR's from 1987- non turbo, VE rotary injection pump or Zexel/Diesel Kiki A-type injection pump. Bore and stroke are both 105mm. Power ratings in the last versions (NKR300)are 74 kW (100 SAE HP) at 3,500rpm and 242 Nm at 2000rpm.

check for fuel leak at the pump. pull and fit new tips to the injectors. Have the injection timing checked by an accredited diesel shop ( same one that is replacing the injector tips)

There are many factors that can cause this condition. Since I have no direct familiarity with the "Citi Rox" engine (I do not believe we actually have that engine available in the U.S.), I will assume it is a GASOLINE (PETROL) engine, not a diesel engine.
If the vehicle has high mileage (over 100,000 miles), check the following:

1. Is the CHECK ENGINE light on or flashing? If so, take the vehicle to a local auto parts store and have them scan for codes. Look up the codes they find on the Internet, and that will help in your diagnosis.
2. Of course, replace the air filter if it is dirty, and make sure there are no obstructions in the air intake.
3. Inspect the throttle body. If it is dirty, clean it with an approved throttle body cleaner for your vehicle, or have it cleaned.
4. Inspect and clean the IAC (Idle Air Controller). Also inspect the EGR valve (Evaporative Gas Recirculation valve) to be sure it isn't leaking or otherwise compromised. The IAC is often the culprit in this condition but sometimes a leak in the EGR valve or its mounting gasket will throw the computer off far enough to where the car will not run correctly.
5. Did the vehicle sit for more than three months recently? If so, either replace all fuel in the tank or, at least, treat the extant fuel with Fuel System Water Remover, then fill the tank with new fuel after driving about 100 miles (150-ish km) with the water remover in the tank. Once you have filled the tank with new fuel, add Fuel System and Injection Cleaner to the tank and drive for another 100 miles (150-ish km). If your performance improves, you'll know you had old or compromised fuel in the tank.

I hope this gives you a starting point!

better chec the wiring or connections 1st--pull the plug from the pump and inspect for flaws and corrosion then plug back in--pump itself can be tested--the fuel injection pump in engine area may have issues

Audi 1.9 TDI and 2.0 TDI engines with pump-nozzle injection system, (year 2005 included), don't have permission to run with Bio fuel !!!
First you must replace fuel filter, interflow a quality Diesel additive (for 60 liters Diesel fuel dosage) fill up tank with Diesel fuel and run at least 300 km on highway at 3000-3500 rpm in 4-th gear.
I recommend you Foerch Diesel Reiniger System additive! I use this additive from 3 years ago and is certified on Audi TDI engines!

Don't know the USA system but here in Europe the sister car with the same engine does 13 liters per 100 km on city, 9 l / 100 km on road and 12 l / 100 km on highway.

Has the car been serviced recently?
If not I would suggest getting it done.Not sure if you have diesel or petrol, but oil can thicken causing more load on the motor and airfilters get blocked and can cause a rich fuel mixture.

If petrol you can take out the plugs to check if they are a light brown colour, if very dark or black you need to check and adjust carb/injection settings/filter.Need to know what engine it is really.