I often say I'm in the 42nd year of my apprenticeship....
We had a customer's car in for rough running/cutting out/black smoke. Cold start was excellent.
A quick listen at the injection pump revealed constant suction at the auxillary air filter even when the engine was up to running temperature and removal of the auxillary air filter confirmed it.
https://youtu.be/TrsMgUlEUaUWe assumed, as is normally the case, that either the waxstat was defective or the piston seized but dissassembly revealed a perfectly working WRD.
We re-assembled and after some head scratching realised that the stat was not actually getting warm even though coolant temp was rising to normal.
Who can guess what we found next?