This sounds like a lack of fuel and could be a lack of fuel pressure from the electrical pump at the tank.
1st thing to try is to remove the fuel filler cap and then flick the ignition on whilst listening for the brief ‘buzz’ of the pump
If the connections to the pump are corroded or damaged then the pump could stop at any time or could not be running at full speed. Check that its relay switches in and out and the relay contacts are clean and bright – replace the relay if in any doubt and do the same for the fuse,
Ideally measure the pressure at the injector rail (should usually be around 3Bar / 45psi +) and if this is low check the pump as described and also consider changing the fuel filter as if this partially blocked this too will reduce pressure.
You can also get this issue with contaminated fuel , if there’s water or dirt in the tank then either the petrol floats on top of the water or the filter sock on the pump is choked up – so if the other things check out then I’d consider pulling out the fuel pump and cleaning the filter sock
And one other thing to try is to loosen off the fuel filler cap, as there’s a 1 way valve built into the cap to let air into the tank and so prevent a vacuum from being created as the fuel leaves the tank to the engine
If this helps then replace the cap with a new item
thanks for the extra info
still worth checking the filler cap as this can effect both petrol and diesel
check the connections to the EGR valve, if either the vacuum pipe of electrical connection are damaged / corroded then the EGR can be on all time which certainly will hurt performance. Its also possible that its a fault with the EGR valve so its worth removing the valve and cleaning it out with brake cleaner, if it looks particularly clogged then replace it.
If this is OK then check the small vacuum lines to the turbo and its connected control solenoid on the bulkhead, any cracks or leaks can give turbo issues and its best to have the boost pressure measured actually measured with a boost gauge to check that the turbo is healthy and that the sensor is reading correctly
If its a variable vane turbo (they’ll be an actuator on the turbo body - but not a wastegate) then check its vacuum pipework as above and check the connections to the diaphragm / solenoid valve
Also worth checking that the glow-plug relay is switching off as they can stick on and leave the glow plugs also on - on some cars this can force it into limp home
Also worth considering adding a bottle of injector cleaner into the tank as a clogged injector nozzle will reduce power and give poor combustion - the next stage on from this is to remove all the injectors and have them ultrasonically cleaned and flow checked
Also check all the high pressure hoses post turbo/s for any split hoses or cracked metal pipes, also check the ends of the intercooler as its not unheard of to pop the end caps off under high boost.
I'd say it would be worth doing a compression test across all cylinders, as they should all be 10-15 psi of each other
Ok in that case I'd say the next step is to plug in a scan tool thats capable of monitoring so you can see what the boost pressure and fuel pressure is doing under load
as for it to be so starved of power then its either a lack of fuel or low boost pressure
if the boost hoses all look OK and the turbo seems to spin freely with no float from side to side
then its likely to be a lack of fuel
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