Date:         Fri, 6 Jun 1997 22:02:55 -0700
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         mholser@Adobe.COM (Malcolm Holser)
Subject:      Re: O2 sensor question

> From: vw4x4@fyi.net
> To: vanagon@lenti.med.umn.edu
> Subject: Re: O2 sensor question
> 
>     What does the compuuter do if you just disconnect the wire?
>
It goes into "open loop" mode, where the fuel is metered according to
the maps in memory, based soley on the air flow meter, engine rpm and
temperature.  If you are perfectly in tune, and your meter is adjusted
really well, you won't notice much difference.  I would suspect many,
many cars out there have bad O2 sensors and never notice.

Typically, the maps are set a bit rich, and the sensors are not perfect.
So the result of a bad O2 sensor is bad mileage and poor performance.  On
the other side, racers and "enthusiasts" will disconnect the sensor and
run open-loop.  They carefully tune their systems for maximum power, which
is  *not* provided in closed-loop mode (but minimum emissions are) and
their cars are also not typical.

Sometimes things break -- like exhaust pipes getting holes, or burned valves,
or bad spark plug wire -- almost anything, and these can make your FI get
bad readings from the sensor.  Solution?  Unplug the sensor and drive home!
Actually, if your car ever starts running really awful, it is worthwhile
to try it with the sensor unplugged and see it it gets better.  This is NOT
an indication of a bad sensor -- rather the opposite.  If things improve,
your sensor is likely good, and something else is likely bad.

It is also good to tune the car with the sensor disconnected for best tune
in open-loop mode -- the transition from open to closed-loop operation will
be gentler on the car, and it will run nicely if you ever have the sensor
fail.  The sensor will correct quite a lot of problems, but you will really
notice it if it fails.  VW did not add sensors to most vans until the
waterboxers, but California cars had them several years before the others.
The others ran fine -- and their sensors were basically always disconnected.

malcolm