Патрубок интеркулера лопнул или соскочил

A charge pipe between the turbocharger, the intercooler and the engine split or blown off, so pressurised air escapes before reaching the cylinders. It is the single commonest cause of a sudden, dramatic boost loss, and it is usually audible — a distinct whoosh under acceleration.

Hoses come off rather than failing, which is the useful distinction. Boost pressure acts to push a hose off its stub, so a clamp that was not retorqued after work, or a stub with an oily film on it, releases under load and often reseats afterwards. A fault that appears only at full throttle and cannot be reproduced on a ramp is this, and it will pass every static inspection.

Pressure-test the charge system cold. That finds a split on the underside of a bend and a joint that will not hold, neither of which is visible. Oil in the pipework is worth noting while you are there — it softens the rubber and lubricates the joint, so it both causes the release and points at turbocharger seal wear upstream.