This applies to 1992-1999 L67 motors. May work on newer L67 motors.

Turn on your car and let it idle, than pull the top vacuum line off the BCA (Boost Control Actuator).

Does the Boost Control Actuator shaft travel in both directions fully when you do this?

If not, turn off the car, and put some light oil in all of the BCA ports and the vacuum lines that feed it. Also on the shaft that extends and retracts, and a little bit on the two pivot points on the front and back of the supercharger for the bypass butterfly valve.

Next without turning on the car, work the BCA shaft in and out (up and down). This will draw in the oil and distribute it, freshening up the seals and lubricating the internals.

Reconnect the vacuum lines.

Ok now turn on the car and try again.

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If the BCA Shaft travels fully when you do this, you'll want to try the BCS Bypass (Boost Control *olenoid).

Do this as a troubleshooting step only, do not drive for an extended period in bypass.

Pull the little elbow off between the BCS and BCA. Pull it off the BCS (lower end coming from the small solenoid) and plug it. Go for a drive. Do you have full boost now?

If this fixes your problem, you'll need to replace the BCS.