re.high idle after cleaning isv

Phil Payne quattro at
Thu Oct 25 08:52:48 EDT 2001

> >     When the ISV is unplugged in my 90q20v the idle drops to around
> >or so.  On my 80 the idle would go bonkers if the ISV was disconnected,
> >and down rapidly unless the DPR was disconnected as well.  Then when both
> >were disconnected the idle dropped on that car too.
> So basically, what the freaking h*ll is the ISV needed for???? :)

It adjusts idle according to requirements.

There is a specific nominal idle for each car, largely determined so as to
reduce engine vibration when sitting at traffic lights, etc.

On the solenoid type of ISV, this is set up using the current through the
ISV - a nominal 430mA (It's actually pulse-width modulated) at the nominal
idle with all the gases correct, engine hot, fan off, headlights and other
electrical loads off.  It's a bit like juggling three balls in a circus,
watching three dials and tweaking the adjustments. Exactly 470mA is a
message of distress from the controller - if you can't shift it, there's
something else wrong.  430mA is a 'must achieve' value - no tolerance.
440mA is WRONG.

Very good description on Scott Mockry's site.

The obvious point has been mentioned - once disconnected, the ISV will not
be operated again until the engine has been switched off and on.

The purpose of having it is to allow the controller to raise idle
deliberately.  This is why the controller has so many inputs, including road
speed and battery voltage.  Idle when moving at more than about 4mph is set
higher than stationary idle to provide more power for the steering and
brakes - similarly it's raised if the battery voltage falls or if the air
conditioning comes on.

> I notice sometimes on my '89 200 that the idle will be slightly less than
> 1,100RPM when I pull into the driveway and put the car in park.  I once
> unplugged the ISV when it did that and the idle dropped to about 750RPM.
> cleaned it and it does it once and while now... bad ISV?

Probably.  The book states that no atempt should be made to clean the ISV.
If you examine it carefully, you'll see it uses a surface-mounted seal
around the end of the plunger - this is very easy to damage.  Blow into the
end - if you hear nothing it's making a good seal. Apply 9V from a PP3-style
radio battery and blow again - should be no resistance.  If it fails either
test, pitch it over the fence.  Don't use 12V - you'll let the smoke out.

If the ISV servo system is working properly, you can get the car up to
reasonable speeds without touching the accelerator - put it into first and
let the clutch out slowly - the ISV will compensate for the falling engine
speed and you'll pull off.  Once you're moving, it will raise idle and you
can repeat the trick with second and third gears..

  Phil Payne
  +44 7785 302 803
  +49 173 6242039

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