Re: startup rpm

[ti@amb.org (Ti Kan)]

geordie writes:
> I have been wondering about this for a while, when I start up my '82
> coupe, it only revs at idle speed when cold, around 800-900 rpm, and it
> doesn't keep itself at 1500rpm like most cars do when they are cold.
> What would cause this? I would like to know if my car is supposed to
> keep itseld above 1000rpm when initially started like most other cars, I
> can't imagine I should need to keep my foot on the gas.

Yes, with the CIS system as found on the early 80s cars, there are
three things that contribute to increased cold idle speed:

1. Auxiliary air regulator
   This is an air valve that bypasses the throttle body.  It is
   open when cold to allow extra air to enter the engine, increasing
   the idle speed.  It is heated electrically as well as via heat
   conducted via the engine.  When warming up, the valve gradually
   closes to drop the idle speed.

2. Control pressure regulator (aka "warm up regulator")
   This is the thing on the side of the block with two braided fuel
   lines connected to it.  It alters the control pressure 
   within the fuel distributor, enriching the fuel-air mixture
   when cold.  It is also heated electrically as well as via heat
   conducted via the engine.  When warming up, the enrichment is
   gradually reduced.  The cold enrichment has the side effect
   of increasing the idle speed somewhat.

3. OXS thermoswitch
   On CIS cars with oxygen sensor, an additional "OXS thermoswitch"
   is plumbed into the coolant pipe.  This switch is closed when the
   coolant is below a certain temperature, signalling the OXS control
   unit (ECU) to run open-loop with cold-enrichment.  As the coolant
   warms up, the thermoswitch opens, and the ECU goes closed loop to
   maintain stoichiometric mixture (14.7:1).  The cold enrichment
   causes the idle to increase a bit.

CIS-E cars don't have a control pressure regulator.  Its function is
supplanted by the differential pressure regulator on the side of the
fuel distributor, and controlled via the ECU.

If your CIS car doesn't run slightly higher idle when cold, check
the above items.  Note, though, that the car's basic fuel-air mixture
should be set correctly when hot, as well as the ignition timing and
idle speed should be adjusted to be correct before testing cold operation.
Also check to ensure that there are no vacuum leaks.

-Ti
96 A4 2.8 quattro
84 5000S 2.1 turbo
80 4000 2.0
-- 
    ///  Ti Kan                Vorsprung durch Technik
   ///   AMB Research Laboratories, Sunnyvale, CA. USA
  ///    ti@amb.org
 //////  http://metalab.unc.edu/tkan/
///