Enough info to keep you busy reading until May Day.

ASSUMING:
head gasket problems without compression/leak down testing does WHAT?
(Answer: Mis-Informs)

'Could be a HG,Or could be "Wallered Out" Cylinder(S). Etc.,Etc.,Etc.

KNOW what's wrong with this 3.5 before trying to Kiss Pandora!

Smartest Plan would be to have a 3 Liter OM603 Lower End of Good Provenance
Waiting in the Wings,In Case Bad goes Worse.
(It Plugs Right In to the Old Head of the 3.5 liter)

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'84 300SD sold
124.128