Re: Idle Stabilization Valve correct operation

[Brett Dikeman <brett@pdikeman.ne.mediaone.net>]

>>Steve manning wrote:
>>>>While waiting for the oil to drain, I decided to pull my ISv and take a
>>>>gander.  It too, is opened just a crack; maybe a dime's width (not a
>>>>nickel) - can't really get the calipers in there<g> but it is opened.
>>>>De-fibbing with a 9V pops it open smoothly to almost completely open (just
>>>>that dime's worth closed).  Removing power snaps it shut again.  So, it
>>>>seems to be opening/closing the "full amount" with the initial and ending
>>>>positions being "off".  BUT, doesn't there need to be some air going
>>>>through?  I couldn't find any other bypass so I'm confused now.

If a 3B ISV closes "all the way" with no power, then _yes_, the valve has
failed or needs more cleaning.


>I applied several squirts of carburetor cleaner. I repeated the 9 volt
>jolts some more, and convinced myself that things were moving freely and
>weren't gonna change. The opening _could_ in theory be closed up by use of
>the potted adjustment screw. But why? Could the original adjustment
>actually have "worn" away to the extent of a 1.5 mm opening? I think not.
>The adjustment screw has a broad, flat end, and I doubt Bosch would be dumb
>enough to have designed a "pointy ended" screw for this application. So I
>think the ISV opening is the way Bosch set it to begin with. Also it's
>cemented in place and virtually impossible to turn (as reported by Brett).
>If the screw _could_ be turned, its fine thread would require at least 3
>full turns to eliminate the "gap". I left it alone and reinstalled the ISV.

Excellent move.  My determination is that this set screw is used to
calibrate the opening to a set width.


3B ISV's _must_ be open a slight amount with no power to them.  This is the
mode Chris referred to when he said that the valve is designed to hold a
fixed RPM if disconnected.

If anyone who is curious takes a q-tip and tries to open it, you'll find it
impossible until you try to _close_ it.  Now, keep on turning/pushing
it(this takes a little coordination.)  Presto...it opens up from the other
side.

During normal operation, the ECU moves the valve so that at first startup,
the valve goes from open to closed to open again; the first few degrees of
turn close the valve and then further turning opens it again.

This allows the ECU to retain the ability to close the valve completely
along with the failsafe "disconnected" opening.

BTW, I don't buy the lean argument after thinking a bit.  Airflow is
measured before the throttle, before the turbo; the ISV is really an
extension of the main throttle.  Air going through the ISV is just like air
going through the throttle.  Both cause an increase in airflow past the
mass airflow sensor.


Brett
PS:People have said this valve is "fragile"...I just don't buy that...it's
one very solid sucker...

------
Brett Dikeman
brett@pdikeman.ne.mediaone.net
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