Whilst modern diesel CDI,TDI engines sure have lots of power and great fuel economy we are seeing a lot of intake system carbon buildup problems.
They have two anti-pollution measures that cause this problem. Firstly the oil vapor “blowby”, caused by the combustion pressure escaping past the rings and thus pressurizing the crankcase ( happens moreso in 18:1 plus high compression diesels ) is vented back into the inlet tract just before the turbo intake, to coat the innards of your inlet, intercooler, EGR / “throttle” flap and intake manifold. It gets burnt in the combustion process, which is the idea. Pain, but OK in itself.
Second anti-pollution measure is the recycling of exhaust gas. This is done to lower the combustion temps, and thus lower NOX emissions. Problem is this “sooty” exhaust mixes with the aforementioned oil, and makes a nice thick black sludge, which does indeed coat the EGR inlet, “throttle” flap, and gums up your inlet manifold.