[Gmecm] Adjusting 7730/$8D for larger cam

[Mike V]

I suck at explanations, but give me credit for effort.
Sounds like you have a good idea of what's happening.
The ECM is seeing less vacuum at idle speed than expected.
With a MAP system, that is usually interperated as a load
on the engine, and fuel is added.  The pig-rich problem
you describe is fairly common after a larger cam is installed.
If the fuel table (with its RPM vs MAP values) is
suggesting a given amount of fuel, and the O2 sensor is
seeing the over-rich condition, the block learn value
will be lower than 128.  Mine was below 100 after a
cam change in my Syclone V6..  As you take fuel
out by reducing the values at the lower end of the
table, you will see the block learn numbers go back up.
If the value goes above 128 by very much, you went
too far on the lean-quest and need to add
some fuel back in.  Some folks have band-aided this
cam-swap situation by using a micro switch to determine
when the throttle is close, and then jury-rig the MAP value
with a pot or resistor, thus lieing to the ECM about the
 idle vacuum.   I prefer the insanity that comes with
trying to tweak the tables.  I hope I didn't confuse the issue.
BTW, I have not worked with the $8D code, so others
may have better suggestions.
MV

> Now that my engine is running pretty reasonably, it's apparent that it's 
> PIG
> RICH at idle. I'm about 99% sure this is due to the lower-than-I-hoped 
> idle
> vacuum reading. When running with a carb, this engine managed about 13-14"
> of vacuum (44-47kPa if I'm doing the conversion right), now it's 10-12"
> (34-40kPa). The tune I received from my chip supplier supposedly wanted 
> 15"
> at idle (51kPa). Since I'm not reaching this level, even after advancing 
> the
> cam 2*, I think I need to change my idle vacuum level.
>
> Going through TPRT, it doesn't appear that this is any kind of one-point
> setting, and is probably in the tables. Is it reasonable to adjust the 
> table
> points on the idle end down to where I need them? It might be that I get
> better vacuum if the mixture is leaner, but I'm thinking that the rich mix
> is due to the apparent load on the engine, not a quirk in the VE table
> (yet).
>
> Any wisdom would be greatly appreciated...
>
> Clair
> _________________
> 67 Cuda Vert (?/833)
> 68 Sport Fury Vert (NOM 440/727)
> 69 Valiant 100 (340/833/7730EFI)
> 74 Dart Swinger (440/727)
>
>
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