I took the entire intake manifold off as part of a glow plug replacement job. It was also good to replace the intake manifold gasket at the head because it appeared to be leaking.
The best I could do was scrape all I could with a gasket removal scraper, and then use degreaser and a garden hose through the sections I could not reach. I'm sure I got the majority of the buildup out of there.
This is caused by the EGR valve porting exhaust soot into the intake manifold. The intake also gets oily crankcase blowby as it is ported into the intake near the turbo. The combination of soot and oily air creates a sludge buildup. If your EGR would happen to malfunction somehow, then the soot would not be present.....
Ken300D