Stumped: High Idle 4ksq

[Ti Kan]

Shayne writes:
> Problem:  4ksq idles high when warm or cold.  1200-1700.
>
> Checked:  For air leaks, set timing, checked differential pressure
> regulator, pressure temperature sensor, CO levels, ISV cleaned, resistance
> of ISV to ECU checked, duty cycle, WOT, closed throttle position sensor, all
> ground wires, fuses and throttle plate adjustment.
>
> Replaced:  cap, rotor, plugs, fuel filters, air filter.
>
> Found:  The only possible problems I found is low resistance between the ISV
> and the ECU.  I came up with 15 ohms.  Spec is 20.  Not that far off.
>
> I am totally stumped.  Could it be I need to replace the ECU??  I have heard
> of the independent ISV control modules on 5000 going bad, but this one is
> built into the Jetronic ICU.  So, I must replace the whole ECU.

It's rare for the ECU to go bad and cause the problem you describe,
but not impossible, I guess.

Have you checked the idle adjustment screw on the throttle body?
If that is cranked way-open then the idle may become high enough
that the ISV won't be able to bring it down sufficiently when fully
closed.

It's possible that the ISV is bad.  Try disconnecting the electrical
plug to the ISV while engine is idling and see if there is any change
in idle speed.  You may also want to check the duty cycle of the
ISV connection (I think you need an oscilloscope for that).

By "closed throttle position sensor" you mean the idle switch on the
throttle body?  Is it making good electrical contact when the throttle
is closed?  If not, the ECU won't know it's supposed to be idling and
will not operate the ISV.

Also, maybe there is a wiring break somewhere between the idle switch
and the ECU?  Ditto the ISV and the ECU?  Time to have a good look at
the Bentley manual wiring diagram and check some circuits for continuity.

Just some suggestions...

-Ti
2003 A4 1.8T multitronic
2001 S4 biturbo 6-sp
1984 5000S turbo
1980 4000 2.0 5-sp
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