The Bypass Air Control (BAC) Valve is the primary idle control valve on the
engine. Without a properly functioning BAC valve, idle problems may appear.
On top of the BAC valve sticking, the electrical connector is prone to failure
due to the heat generated by the turbo; the plastic connector housing can crack and
the electrical pins can back out or short against each other. If the two
electrical leads short against each other, this could blow circuits in the stock
ECU and definitely cause idle problems.
For a dirty or sticking BAC valve, you can do the cleaning yourself. First,
remove the BAC valve from the engine. Shoot liberal amounts of lubricant
(i.e. WD-40, Liquid Wrench, P'Blaster, etc.) into the orifice. If possible,
actuate the BAC valve by connecting the two pins of the BAC valve with +12VDC/ground
from the battery terminals with some wires. If you can hear clicking, this is
a good sign. Drain as much of the lubricant as possible, and shoot down with
carb or brake cleaner. Replace on engine after cleaning.
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