Date: Thu, 14 Dec 2000 15:07:44 -0600
Reply-To: Darrell Boehler <midwesty@MIDWEST.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Darrell Boehler <midwesty@MIDWEST.NET>
Subject: Re: O2 sensor voltage test
Content-Type: text/plain; charset="iso-8859-1"
Hi James,
It seems the afm main spring gets weak with age and can cause a rich mixture (high o2 voltage reading, poor mileage). To adjust
the afm main spring connect the meter up as you suggest with the sensor unplugged and engine warmed up . All tune up items should be
proper, no exhaust leaks before the o2. Adjust the afm main spring tension for an o2 voltage of 0.6 - 0.7 volts with a reasonably
accurate lcd meter, also the rpm should be held at about 3K. Then use the idle bypass screw on the afm to set the mixture at idle.
The idle bypass screw has little effect on the mixture at higher rpm but if the afm main spring tension is not proper the mixture
cannot be set at idle with the bypass screw.
This method works on all styles of afm used on our vans and busses in the usa. If you have one with no o2 sensor use an exhaust
sniffer or install an o2 sensor in one of the j pipes. If you use a heated o2 sensor you can install it in the tail pipe if you
have no cat like most busses.
There should be plentiful info in the archives about adjusting the afm main spring tension, clockwise is more tension / leaner/
lower o2 voltage. It often takes about 3/4 revolution of the big black plastic afm gear to bring an old afm back to the proper
mixture. Mark the location of the gear before you begin to adjust it.
Darrell
----- Original Message -----
From: "James Creech" <jacreech@VISUALLINK.COM>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Thursday, December 14, 2000 12:11 PM
Subject: O2 sensor voltage test
> The Bosch FI book outlines a test for adjusting fuel enrichment and
> determining if the O2 sensor is faulty, for those not having access to a CO
> meter: Run the engine up to normal operating temp. so the sensor is hot;
> disconnect the wiring to the idle stabilizer control valve; disconnect the
> single wire (near the coil) which runs to the sensor (leave the double wire
> connected which heats the sensor) and connect your voltmeter's pos. probe to
> that lead (the female connector which goes to the sensor), and the negative
> probe to ground. Then, while observing voltage readings, adjust the hex
> screw on the AFM in both directions to determine the lowest and highest DC
> voltage. Numbers should be range between 100 or 200 mV (equals higher
> lambda value, equals leaner) and 1000 mV (lower lambda, thus richer). You
> want to set the adjustment at the midpoint between the low and high value,
> which should be roughly around 500 mV - for older sensors it can be a
> different value, but still you want the mid-value, which will produce a good
> balance between fuel economy and power.
>
> Now, my question for anyone who has done this or otherwise knows (Darrell's
> Digitool measures this voltage while the engine is running): I am getting
> values much higher, in the 700-900 mV range, with no variation beyond that
> no matter how far I turn the screw in or out. The Bosch book does not
> indicate this is a problem, but they do show a graph with a much larger
> range of values. Do I have a faulty O2 sensor? Or could I have a leaky
> AFM, such that the enrichment screw does not have as much effect as it
> should?
>
> Thanks,
> Jim Creech
> 87 Westy GL, 2.1
>
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