I might mention that a vacuum is not uncommon when your free revving the engine. When you say you have hooked a boost gauge to the ALDA line I would tend to think that you are not measuring boost while on the road. The engine really needs to be under load to make boost. You can have someone stand on the brakes of the car and wind the engine up with it in gear to get some boost.

To me it sounds like its an air leak some place. Check for leaks around where the compressor housing mates with the intake manifold.

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green 85 300SD 200K miles "Das Schlepper Frog" With a OM603 TBO360 turbo ( To be intercooled someday )( Kalifornistani emissons )
white 79 300SD 200K'ish miles "Farfegnugen" (RIP - cracked crank)
desert storm primer 63 T-bird "The Undead" (long term hibernation)

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