Oven cleaner spray let it foam up inside leave for about a couple of hours, then jet wash it out on the most powerful jet wash you have, repeat if you have to a few times until all crud is removed, remember the MAP sensor must be removed.
Been doing these since 2011 some I have seen have been rally bad indeed.
You can see more or less what the manifold will look like inside if the EGR is like this below.
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Manifold can be removed along with the EGR at the same time.
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Remove the EGR once you have the manifold of the car.
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With this amount of crud you would need to do the process at least three time before it is fully cleaned out.
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I have at least 5 ready in the shed which were for the Nano meets three had been claimed but were not able to be fitted to the members cars due to C19 lock down.
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