On Fri, 10 Sep 1999, Mark McCulley wrote:

> Right, tightening the AFM spring is an attempt to run leaner and produce
> more power. Seems to me though that in a properly functioning FI system the
> lean condition would be compensated for by the O2 sensor and ECU which try
> to maintain the "correct" air-fuel mixture.
>
> >From: Bulley <gmbulley@BULLEY-HEWLETT.COM>
> >
> >By tightening the AFM spring, aren't you forcing the injection to
> >miscalculate the intake airflow, yielding a "lean" state? I love simple
> >solutions, but this one doesn't sound like such a swift idea. Anyone?
> >

Well, the spring is a mechanical device that weakens over time.  As it
weakens, the air vane open more than it originally did in response to
engine demands.  This makes for an initial richer condition.

The O2 sensor and ECU will compensate to some extent for this "too rich"
mixture, but they can't compensate enough in some cases.  About 3 years
ago I couldn't pass smog because the mixture was too rich.  Even after
screwing out the idle mixture screw all the way, I still couldn't get the
mixture (as measured by the O2 sensor) lean enough.  I finalld did some
adjustment of the spring, and wverything came back into spec.

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Tom Young               young@sherlock.SIMS.Berkeley.EDU
Lafayette, CA  94549    '81 Vanagon
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