[ba] Stumped: High Idle 4ksq

Buchholz, Steven Steven.Buchholz at kla-tencor.com
Tue Apr 15 13:22:20 EDT 2003

Have you tried blocking the air path through the ISV?  I don't think there's
a line you can pinch to do this, so you will need to plug it somehow.  If
the engine still runs there is some auxiliary path for air you need to track
down ... start by checking the bypass port on the throttle body (the PO of
my former '85 used this to cover up a poorly functioning ISV).  It looks
like you already checked to ensure that the TB is completely closing.  Does
the 4kQ have an auxiliary path for air if the A/C is on?  I think not, but
if you find one try pinching it off to make sure air's not getting through

If the engine dies with the ISV plugged, then the problem is in the ISV or
the circuitry that drives it ... I'm pretty sure the ISV is supposed to be
closed when powered down, so you can disconnect the connector to the ISV ...
if the idle doesn't slow or the engine stall the ISV is the culprit, and if
the idle is affected by unplugging the controller may be the cause.  I
*think* I have a spare ECU in the garage, let me know if you think you need
it ... get the P/N on the unit you have and I can check to see if it matches
the one I have ...

Good luck!
Steve Buchholz
San Jose, CA (USA)
 >  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.
 >  Anyone have any clues??
 >  Thank you, in advance.
 >  Shayne P.
 >  Oakland, CA.
 >  Audis, MBZs & a black sheep BMW.
 >  BUY ME:  1988 BMW 528e
 >  PARTING:  1972 Mercedes Benz 280 SE 4.5
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