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Mystery Solved!
To close on this one, the attached picture shows the disconnected vacuum line from ??? to a solenoid valve that feeds directly into the RH Throttle Actuator of the V12. The yellow pencil points out the disconnection. The 'thumping' sound was air being drawn in whenever the solenoid valve opened. As I noted above, the 'thumping’ was very regular and not timed to engine speed. I have no idea what the purpose of this solenoid valve is, and would appreciate it if anyone could enlighten me.
The '96 SL600 ran well with the line disconnected, and only a little better (smoother idle, mainly) after reconnection of the vacuum line. Of course, this makes sense being connected to the intake manifold. I found this out of frustration... just let the engine idle and kept poking around until I found something - perseverance!
Cheers,
Jeff
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MBCA Member #B012089 (Lone Star Section)
OBK Member #47 (W123 Division)
'96 SL600 (105K) Triple Black - Mein über-Fräulein
'79 240D (292K) Yellow/Saddle - Mein Spielzeug
'01 ML430 (123K) Black/Saddle - Wife's Ride
'94 SL500 (164K) Green/Champagne - Daughter's Dream
'73 450SL - RIP
'86 300E - RIP
'88 420SEL - SOLD
'94 S320 - SOLD
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