Re: Idle Stabilization Valve correct operation

[Phil Rose <pjrose@servtech.com>]

>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.
>

Then I chimed in:

>Steve, are you saying that the ISV was a little open ("dime's worth") when
>you initially examined it, but then--after doing the 9V "defib"--the
>resting (or "off") position was able to come to fully closed? In which
>case I gather that early stages of gunk can have the effect of allowing in
>a little unwanted air "leak"  in the nominally off position. Does this
>tend to happen because the ECU often doesn't reduce power in a
>discontinuous manner (i.e., a "step-function" to zero volts) like the 9V
>defibrillation does?
>
>As some ISV gunk begins to accumulate, there must be an increasing chance
>that--on startup--the ISV will cause the idle to be a bit (nickle or
>dime's worth) too lean. I will definitely do an ISV cleaning today as I'm
>sure I'll be finding the same "not-quite-OK" operation.
>
>BTW, no 200Q20V people have responded about my claim that I can detect a
>significant increase in idle speed if I _re_connect my ISV while the
>engine is running. Idle drops when I pull the plug, increases when I
>connect, etc. I thought the ISV would act "dead" unless the engine was
>switched off and then restarted. I recall my '89 100 needed to be
>restarted for the ISV to work after reconnecting. I guess the 3B Motronic
>is "different" in this respect?

**********
Now I've had a chance to peer into the bowels if my '91 200's ISV, and I
revise my earlier suppositions. I, too, see an opening about the width of a
nickel (uh, about 1.25 mm--for the numismatically challenged). There was
quite a bit of black crud showing on the cylinder surface, but application
of 9V could move the mechanism easily to and fro. Many on/off cycles failed
to diminish the size of the ISV opening at the "rest" (off) position. I
seemed to hear the rotation stop with a resounding "clack" as the shutter
hit the limit screw.

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.

Damn, I think the car runs better. Maybe I oughta rename it "Placebo". But
that sounds a little too Spanish for a German car, dontcha think?

Phil

Phil Rose				Rochester, NY
'89 100 (For Sale, 111K, automatic)
'91 200q				mailto:pjrose@servtech.com