So I've got an interesting problem for all of you tonight, about a month ago during the mother of all storms, my 2008 TD4 gave the generic P0299 (turbo underboost) and we've been trying to chase the issue down since.
The symptom is as follows: you're cruising along and when you command power, there isn't any until you put your foot flat to the floor and she slowly accelerates away. It's intermittent and there's no rhyme or reason to when it happens.
We have done the following diagnoses:
*Smoke tested the entire intake system
*Had all of the pipes off to check for tears
*The turbo was recored 4500km ago but was sent back and a clean bill of health was given. The crankshaft position sensor was also replaced at the same time. The air filter was also replaced when the problem started. The EGR cooler has also been blanked off and the cooling hose bypassed as we believed it to be leaking internally.

Out of desperation I went to my local parts store this afternoon and got the following:
*Manifold pressure sensor
*Air flow sensor (the one by the air filter)
*Throttle body
*Exhaust pressure sensor
*Camshaft position sensor

None of these sensors are showing error codes, but I've learned to trust your gut if something doesn't seem right.

If it's not a sensor could it be a cracked manifold that's only showing up under acceleration? Or even more bizarre could it be the accelerator pedal that's worn out?

On these engines, you can't just replace parts of the air intake path and expect it to function correctly without carrying out an air path calibration.
This calibration must be done when MAF, MAP, EGR, ATS, TB and turbo are replaced.
You need a suitable diagnostic machine to carry it out, but importantly the EGR must be functional, or the calibration is terminated, also the air filter needs to be new, and the engine needs to be at operating temperature.

This is the first step to hunting down causes for the p0299 code. 2009 Rimini Red SE TD4. Soon to be sold.
Hyundai Ioniq 5 Ultimate on order.
Gone. 2006 Tonga Green i6 HSE.