Assuming inlet port 3 belongs to cylinder 3, the last one and always the one with most problems, the one furthest from from the oil pump, my answer is thus:
Probably EGR gone bad.
I know there is a network of holes round the back of the cylinder head which cool and serve to extract air from the exhaust to the EGR. One of them faces to the inlet manifold.
I'd blank off or replace the EGR as a first measure, depending how you feel about nitrous oxides.
Note: you cannot just disable the EGR. The ECU needs to see it working.
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