The T4 with the 102 ACV is normally no slouch, hp don’t matter it’s the 250 Nm torque that does the work,.
We live near the Alps and our 2000 330,000 km Cali is fully loaded (overloaded if our 2 well fed daughter are in the back) although I can’t be compared to more powerful T5 & 6’s it never fails to deliver, apart from hairpins it climbs all hills in the Alps in 3rd gear, highway hills in France in 4th.
I’m not going to get too technical, assuming the engine is in good condition and it’s maintained correctly, a few things that can cause a problem:
Blocked fuel filter or a crappy brand or air filter
EGR valve blocked
Cat blocked
loose intake to turbo, the plastic pipe stretch with age & heat, needs a thicker O-ring or a hose clip
Turbo wastegate disc fell off or linkage wrongly adjusted
Now a bit tech stuff
MAF air mass sensor near the air filter, disconnect, if it improves performance its junk.
Other sensors can be faulty, boost & temp sensor on the intercooler, is it an early or late ACV, early have a hose to the ECU for boost pressure which can be leaky & the plastic hose inside the ECU can fail.
Turbo wastegate control, remove hose from the actuator block the hose, if improved and may go to limp mode with over boost, its a vacuum control issue, hose or valve.
Engine timing, if it’s off it can only be adjusted with diagnostics, VCDS.
Ideally you need access to VCDS to check fault codes if any ignore clear and recheck with a run and data logging to compare real time sensor data, sensors in the fuel pump can only be checked this way.
This may seem a lot but anyone with an old T4 Tdi or any pre 2003 VAG group with Tdi has to deal with these issues. Wait till todays get a bit age related issues.
I had many of these issues when I got my T4 some years ago, I have the advantage of being retired having spent 30 years at the heavy end of the worldwide VAG group.
My T4 is decated theres a big hole through it, EGR is clean blanked coded off, has better performance and fuel economy than it left the factory, turbo winds up from 1,600 rpm and is already pulling hard from 1,200 rpm.
You gotta love the old Audi 5 cyl lump, lasts forever, If you‘r a diesel junki sounds good too, that 5 cyl growl and scream at full chat.
note to self, maybe too much