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318i idles too fast

Dear BMW Gurus:

Please share your wisdom on the idle problem I'm having on my '84 
318i.  The car has about 155k miles on it and idles around 1600 rpm 
both cold and warm.  The car has long history with this problem and 
the previous owner finally gave up any hope of resolving it. 

The service records indicate the following:

8k miles   - Poor idle when cold - 500 to 1800 rpm.  Replace plugs, 
             reset timing, adj. milliamps, cleaned idle valve.
23k miles  - Engine cycles while idling (since last adjust 3 weeks ago) 
             Especially when cold - poss. needs decarbonizing.
30k miles  - Engine cycles when cold - Decarbonize valves.
             With warmer weather, eng. stalls - Replace idle valve & 
             Box(?), Adjust CO mixture & milliamps.
69k miles  - Idle cycles when car is first started - set CO.
71k miles  - When cold, car idles at 1400 rpm - set engine specs, 
             adjust milliamps EFI.
79k miles  - Idle surges about 300 rpm - Troubleshoot idle system, 
             defective oxygen sensor, replaced sensor, set milliamps. 
86k miles  - Rough idle when cold thru warm up phase - set engine to 
             specs, clean and lubricate idle valve.
92k miles  - Engine idles 1200-1400 rpm when engine is warm - reset 
             milliamps to specs, EFI.
131k miles - Idles too fast (No condition was noticed by mechanic) -
             Cleaned linkage & made visual checks.
138k miles - Sold car

It seems to me that the car's idle started acting up on me when the 
whether started getting cooler a couple of months ago.  I checked the 
coolant temperature sensor - 1360 ohms at 56 degrees F, idles around 
1600 rpm - 291 ohms with engine warm, idles around 1600 rpm.  The 
applied voltage to the sensor is about 1.7V.  The Haynes manual states 
the sensor's thermistor should produce 2100-2900 ohms when cold and 
270-400 ohms when warm.  Although the resistance reading is too low 
when cold, it is within the correct range when warm.  Therefore, when 
the engine is warm, I cannot attribute the fast idle to the sensor. 

I also checked the idle control valve per directions from the Haynes 
manual: 
1. The valve should be "vibrating and humming slightly" with 
   the engine running.  The valve doesn't vibrate or hum at all. 
2. Removing the electrical connector from the valve should produce an 
   idle of 2000 rpm.  It does. 
3. Resistance across the valve terminals should be between 9 and 10 
   ohms.  I received a reading of 10.6 ohms which seems close enough. 
4. Battery voltage applied to the valve should close it tightly.
   It does closes tightly.
5. An ammeter placed in series with the valve should read 400-500mA at 
   idle. It reads 560mA at 1600 rpm but will drop to within specs when 
   I reduce the idle by pinching closed the hose from the valve to the 
   intake manifold.  It seems that the control current is correct. 

I believe the problem stems from a bad idle control valve because;
1.  The valve does not seem to vibrate or hum.
2.  I can reduce the idle by pinching off the hose from the valve to 
    the intake manifold.  (I can even make the engine stall this way.) 

Before I spend $175 to replace the idle control valve, I wanted to ask 
if anyone on the list has experience with this reoccurring problem.  
Is it typical for this valve to cause such a maintenance problem over 
the lifetime of the car?  Jeez, its been replaced once (that I know 
of) already.  Any other ideas?

By the way, the oxygen sensor is putting out the correct voltage, I've 
found no vacuum leaks, and the timing centrifugal advance is operating 
normally. 

One more thing.  How do I adjust the C.O. on this car?  Is there an 
ajustment screw I can twist?  I haven't found anything in the Haynes 
manual about this adjustment. 

Please forgive me for the length of this posting.  I wanted to give 
as much information as possible.                                

Sincerely,

Jay Baudendistel
75212.2155@domain.elided