[Gmecm] Two '747 problems

[NICHOLAS HROMYAK]

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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