My dads friend has a 98 SSEi (Silver) that suddenly is not buildling boost. The needle goes from -10 to 0, but not over and has diminished performance.

So far I have told him to check that the belt is there and what I Beleive is the BCA, the round thing that has the arm on it in the front of the */C. What else should we look at?

Also, what is the BCS vs the BCA?

Jay

You found the BCA. The arm moves in and out to rotate the shaft the bypass valve is on to open and close it. The BCS is the little box connected to the other end of the vac line that comes out the bottom of the BCA. It dumps boost when the PCM sees fit.

Can you pull the two halves of the BCA apart? What happens if you pull the vacuum line off the bottom of the BCA and run it? And check over vacuum lines running to and from the BCS, BCA, and MAP sensor for cracks. Is it possible the SC outlet gasket is blown?

John'* got you covered on this one.

If you pull that vac line off the bottom of the actuator, the PCM can't command the BCA to dump boost via the BCS.

In a nutshell, the boost control SOLONOID controls the ACTUATOR. Lots of factors are involved in what and when and why the PCM tells the solonoid to do something with the actuator. The actuator is DUMB, and only does what it'* told by the solonoid. So if your solonoid is bad, you can prevent it from telling the actuator to do the wrong thing by disconnecting that line (don't leave it that way as a fix, as it could be a bad thing later).

Disconnect that line, if the boost returns to normal, either fix a vacuum leak or replace the solonoid.

If the line being disconnect makes no difference, your problem is most likely a vacuum leak or a faulty actuator.

Or as John mentioned, a leaky SC outlet gasket.