Blow-off Valves

[Robert Deis]

On Sat, 5 Apr 2003, Brady Moffatt wrote:

> I just bought an 83 UrQ (woohoo!) with a BOV, pssshht and all, and no black
> puffs of smoke occur.

As others have said, if it uses a MAP system then this is expected.  If it
uses a PAF system (CIS) then something's funny..

> The conditions in the combustion chamber should be very similar behind a
> closed throttle, BOV or not. The fuel metering takes place at the beginning
> of the system, ahead of the turbo and throttle plate and it doesn't know
> that the throttle has been slammed shut until after the fact, so it should
> cause a rich condition regardless of BOV.

Keep in mind the ECU knows when you close throttle, and it knows the RPM.
So it's got several inputs that tell it how fast you're consuming the air
in the intake path as well as the MAF telling it how much air is present
in the intake path.

The reason venting thr BOV to atmosphere confuses the ECU it that air it
thought was in the intake path "dissapears," leading to a rich condition
regardless of throttle setting and RPM, becasue air that the ECU thought
it was using is gone.

--
------------------------------------------------------------------------------
  Rob Deis                  "Let the people know beforehand what the law
  MiB3347                      is and what they are to expect."
  rdeis at io.com                              -- 18th Congress, Rec. 75