I have recently pulled the engine and transmission from my
'87 560SL due to a leaking rear oil seal and seeing oil fouling
uniformly on all the stock spark plugs. On disassembly of the
intake manifolds I found that the lower plenum was very crusty
with oil soaked carbon and there was even a puddle of oil!
I have removed both cylinder heads and disassembled the
engine down to a shortblock assembly at this point. All the
cylinder bores look good and have good compression. The
valve stem seals appear to be hardened and leaking somewhat.
I am asking where is the oil in the lower plenum coming from?
When I removed the tube that runs from the valve assembly
on the left front of the exhaust manifold around the left cylinder head then through a two bolt flange in the front of the lower intake plenum, I found that the end of the tube that went inside the lower plenum was almost sealed up with carbonized oil...
What is the cause of this oil in the intake plenum?

Don't think I can help you (sorry), but what kind of oil was this motor run on-synthetic or conventional oil?

Best of luck with your repair

__________________
"If God had meant for us to walk, why did he give us feet that fit car pedals?" Sir Sterling Moss

Michael
2014 E63S Estate
2006 SLK55
1995 E500
1986 Porsche 944 turbo

I have used Mobil 1 since 1994....