From: mucit@cs.rochester.edu (Bulent Murtezaoglu)
Subject: cold start problem solved.
Organization: Computer Science Department University of Rochester
Date: Sat, 19 Dec 1992 05:34:14 GMT


I had some trouble with this one, but it turned out to be very simple.
The symptoms:

-The car would start and work just fine in warmer (> ~35F) weather.
-It was hard to start in colder weather.  I'd need to press the 
 accelerator and crank it for 10s or so several times.
-After starting, the idle would be rough and the car would miss and
 sputter if I tried to drive it.
-After about 3-5 minutes of idling, the idle would abruptly
 become normal.  This would translate into the car just taking off
 from under me if I was attmepting to drive it.
-The cold start valve circuit had a problem but fixing that didn't help
 much.  The cold start valve was not leaking.
-The spark appeared yellowish, so I replaced the coil but that fixed neither
 the color of the spark nor the problem.


My immediate suspicion regarding the abrupt change for the better after
warming up was the O2 sensor system.  I figured such an abrupt change 
would be caused by the temperature sensor closing and the controller
fixing the mixture.  So I tested it thinking that the idle mixture was
excessively lean and the O2 sensor loop was fixing it for me in warm
weather.  But the duty cycle readings off the test connection proved me
wrong (50% warm, ~70% cold) -- and it was reasonable for the 50% figure
to be precisely that as _I_ had adjusted the mixture using the O2 sensor
output some time ago.  At that point "life" interfered and I just
made the mixture rich enough that the car would be drivable w/o 
warming up.  *Don't* do this.  It makes the car drivable, but it also
hurts your milage and makes the engine extremely sluggish.

A few days ago I had the time and the weather to play with the car again
and I found the problem.  It turns out that the O2 sensor system relay 
has 2 sets of contacts.  The controller gets power through one and the 
frequency valve through another.  *And* the test output isn't really
a convenient connection to the frequency valve.  It is possible for 
there to be the proper signal at the test output and the frequency
valve not to get any power.  That was exactly what was happening.
Either the second set of contacts were failing in cold weather or 
there was a temp. sensitive connection problem in the freq. valve
wiring.  A good look at the wiring diagram in Bentley would have saved 
me much bother.  As would not just blindly assuming the that it was the 
temp. sensor causing the duty cycle to return to 50%.
Now I just slap the relay when it doesn't seem to want to start,
and it works fine.  I'll inspect the wiring or replace the relay soon 
though.

The moral of the story:

- The frequency valve is essential for maintaining the proper system
pressure in CIS.

- If you have any kind of exhaust leak, you probably won't be able to hear the
frequency valve.  Feeling it vibrate works just as well.

- CIS on VW's is well engineered and durable.  The big ticket components
don't usually fail.  Look for obvious cheap problems first.

- Repeat the above when you're busy under the hood.

Hope this helps someone.

cheers,

-Mucit
Newsgroups: rec.autos.vw