Sniff Tube (exhaust pipe)

[scott.john.mockry@bangate1.tek.com]

The "sniff tube" is actually used by service technicians who want/need to 
adjust the CIS mixture and measure the upstream  exhaust. (before going through
 the Cat convertor). This tube has been used since the early 80's on VW's and
Audis to adjust the idle mixture to somewhere between 0.6 to 1.2 % CO 
(upstream measurement) with  the O2 sensor  disconnected and appropriate
breather hoses disconnected and or plugged etc. 

When I worked at VW I used to set most vehicles to the upper limit of 
1.2 % which would give better running  when the car is first started cold before
 the O2 heats up and the system goes into closed loop control. The newer cars 
with Heated O2 sensors go into closed loop rather quickly (usually under a minute).

Assuming your engine has no vacuum leaks you can also look at the duty cycle of
the signal controlling the frequency valve and adjust the mixture until the duty
cycle is around 35-45 degrees to get about 1.2% upstream. The factory recommended
setting is around 45-55 percent , the duty cycle varies back and forth in response to
the rich/lean signal from the O2 sensor. yada, yada, yada.  

>Yes, that's the 'sniff tube'. I believe that it is used for verifying 
>emissions compliance at the factory and serves no other purpose 
>once the car ships. Plugging/welding shut is a typical fix for 
>leaky sniff tubes, should cause you no problems. The way the sniff 
>tube attaches to the exh is different on different models.