Originally Posted by glorkar
I always wondered what caused that. I friend of mine said it was worn bushings (I laughed at him). But the carbon thing makes sense. Is it just me, or does this seem to be a gm thing?
The oily residue inside the intake is perfectly normal. It does not need to be cleaned out...little point in it as it will come back pretty quickly.
It is a mix of some oily residue from pullover thru the PCV system and some fuel residue from the "heavy ends" of the fuel that don't vaporize sufficiently to be pulled thru the system.
When intake manifolds had carburetors and throttle body injection on them there was a lot of raw gasoline sluicing thru the manifold so they stayed pretty clean and there was no buildup like that. With modern intakes with highly tuned runners and port fuel injection the buildup is common in all engines as there is no gasoline sluicing thru the manifold anymore to clean it out and the tuned runners tend to trap the material in the plenum as it would have to run up hill to escape thru the runners....
Along with this the EGR deposits exhaust gasses into the manifold. When the engine is shut off those gasses remain. One of the first things to cool down is the TB. Much like watching moisture condense on a cold window, the gasses that remain in the manifold condense on the backside of the cooler throttle plate. The moisture evaporates but the residue and carbon don't and build up over years.