[Gmecm] Two '747 problems

[David Haggard]

Thanks, Nick for the thoughts.

My manifold is Holley, but my TBI is a factory unit from a '93 454
Chevy. I also failed to mention that the injectors are flow-matched 350
Police injectors.

The funniest part of my first issue is that after I stop the engine and
restart, the idle is perfect from then on. 

David

> -----Original Message-----
> From: NICHOLAS HROMYAK [mailto:nhromyak at comcast.net]
> Sent: Thursday, March 10, 2005 9:36 PM
> To: David Haggard; gmecm at diy-efi.org
> Subject: Re: [Gmecm] Two '747 problems
> 
> I have the same problem with my TBI based 402 BBC.  I keep thinking I
have
> an exhaust leak
> But that should make it go more rich.  When it is in open loop and a
> little
> rich, I get a smooth idle.
> perhaps we have a lazy O2 sensor, or perhaps a vacuum leak.
> 
> After chasing it down for the longest time, I changed my O2 voltage
> readings
> at idle.  I am not sure if this can be done in the 808, but I think it
can
> be done in the 747, it certainly can be done with the 7730.
> 
> I too have Holley TBI (4 barrel).   To the best of my knowledge these
> aren't
> leaking, I have had them for six years now.
> 
> Good luck.
> Nick in Sacramento.
> 
> ----- Original Message -----
> From: "David Haggard" <david at newcovenant.com>
> To: <gmecm at diy-efi.org>
> Sent: Thursday, March 10, 2005 5:04 PM
> Subject: [Gmecm] Two '747 problems
> 
> 
> I have two tuning problems I can't seem to figure out. The ECM is a
> '747, '88 305, moderately cammed, 454 TBI and Holley Pro-jection
intake.
> 
> 1. After cold startup, the idle will never settle down to a smooth
idle,
> even after driving and fully warmed up. The idle oscillates between
the
> target setting and about 1800 rpm. The throttle follower seems
> overactive. But once the engine is warmed up above the closed loop
> threshold, if I shut the engine off, wait ten seconds, then restart
it,
> I get a smooth idle at the target setting, and the throttle follower
> acts perfectly.
> 
> 2. I have a bad flat spot under gentle acceleration, between 1600 and
> 1800 rpm. I experimented with lowering the PE TPS threshold to 20%,
and
> the flat spot is gone if I open the throttle past the 20% point and PE
> is enabled. The flat spot is there under any acceleration below the PE
> TPS level. The main fuel table has been tuned to give BLM's of about
126
> in that RPM and throttle range, using WinALDL. The spark table has a
> smooth, straight-line advance curve through that RPM range.
> 
> Any ideas would be greatly appreciated.
> 
> David Haggard
>    '88 Formula Firebird "SR-71 Black 'Bird"
> 
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