Go to
http://techcapri.com/then go to
http://techcapri.com/1991_Service_Manual.htmlthen open: Section 09 - By-Pass Air Control System (564 KB)
look at page 9-2 for location of ISC/BPA valve.
It sounds like that is what you want to try replacing.... They can be hard to come by, and super expensive if you can find them NOS...
It's related, but I'm not sure that is causing the problem. The turbo boost sensor can be problematic and not register boost properly. On one car that part quit working, but I'm pretty sure the turbo was still operating fine. I was able to remove, then tap, blow, etc in the thing and it worked for a while longer, then died again.
from the manual:
The By-Pass Air (BPA) Control System maintains the engine idle speed quality throughout the engine's operating modes by means of the By-Pass Air Control Valve. The valve responds to engine coolant temperature only. As the engine warms up, the valve mechanically reduces the by-pass airflow from a higher cold start setting to a lower flow level at coolant temperatures of 60° Celsius (140°F) and higher, in accordance with engine demand. This action results in a smoother idle and faster engine warm-up during cold weather operation. Engine idle speed (hot engine) is adjusted at this lower flow level by means of turning the adjusting screw in the throttle body so as to admit more or less by-pass air.
The BPA Control Valve, similar to the engine thermostat, is packaged with the Idle Speed Control (ISC) as a combination unit. The BPA valve is not serviceable and must be replaced if a mulfunction occurs.
Idle speed control other than the above manual adjustments, is controlled by the ECA, acting through the Inlet Air Control (lAC) System, described in Section 10 of this manual.
