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After removing the air-filter assembly, the manifold vacuum source on the rear throttle body will need to be capped. Unlike port EFI that places the injector in a low-pressure area, Cross-Fire utilizes no vacuum signal to the fuel-pressure regulator. Identified by GM as the Model TBI-400, it’s two four-cylinder throttle bodies attached to an intake manifold via a plenum. It has two idle-speed controls and one TPS.