So I decided to take the GC8 out for a drive this afternoon, and noticed that before I started the engine, that the boost guage needle was sitting on -10Hg (vacuum). Nothing switched on, everything off.

I fired up the car, took it for a blatt around the block, the guage read correctly (-20Hg vacuum, and up to +14psi boost). Switched off the engine and the boost guage needle returned to 0.

What would cause a boost guage to be sitting in the vacuum with engine off? Why would use (a drive) fix it?

Car was last run 2 or 3 days ago.

This is interesting and I've experienced the same thing on our GC8.
With the engine off of course I was removing the intercooler. As I unseated the rubber joiner hose to the TB, it was like a vacuum being released.
Logically the I/C is open to atmospheric pressure through the turbo, intake and air filter.
Has me baffled.

My only explanation is a slight vacuum due to cylinder position in the intake runners prior to the TB butterfly plate.

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MY00 WRX. 89 Prelude. 99 CRV

Sounds like a crap gauge. What brand is it?

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Same thing happens every time I remove my intercooler.

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MY08 STI 422whp at 1.5bar on E85, built by Diversion Garage, assembled and tuned by EvoR

Update from new owner: 375whp at 1.4bar on 98, at Tokyo Motorsport