[Gmecm] Closed loop

[David Cooley]

your O2 readings are perfectly normal.  The ECM get's to it's "target" range
by going lean then rich over and over... those are called O2 crosscounts.
Believe it or not, a standard narrow band O2, .07 volts can be 16:1 and .9
can be 13:1 A/F not a wide range.
Look at any stock vehicle and you'll see the exact same thing.
 


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From: gmecm-bounces at diy-efi.org [mailto:gmecm-bounces at diy-efi.org] On Behalf
Of David Haggard
Sent: Saturday, May 14, 2005 7:10 PM
To: gmecm at diy-efi.org
Subject: [Gmecm] Closed loop


I've got my 1227747 real close to "perfect" after a few hundred chips. But I
have two questions. The car has a moderately-cammed 305 with 650 cfm TBI
throttle body and Holley/Weiand TBI intake. 
 
1. The one problem I have left is unstable idle and widely varying O2
readings.
    In closed loop, it will idle nicely for up to half a minute, then either
surge or stall. Then it will correct itself, idle nice for a few seconds,
then surge or stall.
    If I force it to idle in open loop (by setting the closed loop idle
thresholds higher than target idle), it idles smooth as glass. However, in
open loop idle it can be stalled out and die easily by turning the wheels
hard to the side (power steering load), or when coming to a stop after
highway speeds.
    In closed loop, idle or cruise, the O2 sensor voltage will vary from
0.07 volts to 0.97 volts. This seems like an awfully wide range to me. I
think it's this wide O2 range that causes the unstable idle in closed loop.
    When I force it to open loop idle, the O2 sensor reading stays
absolutely steady.
    I think the ECM is making injector pulse corrections in closed loop that
are too large / too fast, causing the unstable closed loop idle. Probably
not helping fuel economy, either.
 
    Is there a set of parameters for the 1227747 that adjust how much and
how quickly the ECM makes injector pulse corrections?
 
 
2. I understand that the '747 has a set of parameters for "highway mode
fuel" that are disabled by default. Can anyone tell me where these settings
are, and how to enable them?
 
Thanks. You guys are always great help. My flat spot problem was a stripped
TBI mounting bolt that was allowing a vacuum leak.
 
David Haggard


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