[Gmecm] PE lean issues on a ?747 TBI

[David Haggard]

The fix is to start lowering the numbers in the "Power Enrichment AFR vs. RPM" table, thus making it richer. There's a big difference between the requirements for a heavy '88 van and a Monza. I believe the weight difference in the vehicle is what is making it go lean.
 
I make adjustments in this table by values of 0.5, then do stopwatch runs bracketing the rpm ranges in the table. Best acceleration time in a particular range means the mixture is best for that range.
 
If this doesn't give you the richness you need, then increase the fuel pressure beyond 14 psi (which is the normal pressure you really want).
 
David Haggard

________________________________

From: gmecm-bounces at diy-efi.org on behalf of Per Lonnborg
Sent: Wed 7/6/2005 10:40 AM
To: gmecm at diy-efi.org
Subject: [Gmecm] PE lean issues on a ?747 TBI



Hi, 

I run a Monza with a stock 350TBI with the 747 ECM from a ?88 Van, the only 
modifications so far is headers into a 3" single free-flowing exhaust. 
I haven?t made any bigger mods in the chip more than lower the Main fuel 
Tables a bit because nearly all BLM values where on the rich side. 

I have made 2 trips to the strip with this setup, the first time approx one 
month ago I did a 14.64/95 run. 
Last Sunday best ET was slower, 14.98/94 despite the fact I had switched 
to a lower gear rear end and with POSI! 

Last Sunday the weather temperature was much higher(=lower density). 

I logged the runs with WinALDL,(I?ve attached parts of the data) and after analyzing 
the values one can clearly see that the car goes lean on WOT, especially 
last Sunday. 


Now to the questions: 
--------------------- 

1. How does a 747 ECM compensate for air density changes in PE mode? Or maybe 
it can?t? 

2. Is there a way to see when this ECM is in PE mode? 

3. If you look in the attached file, you can see that the battery voltage was 
about 0.5V lower the slower run, can that be an issue? But I guess the fuel 
pressure regualator should take care of that part... 


Oh, I should also mention that the knock sensor is disconnected, the cable to 
it broke, and now I can?t reach the knock sensor due to that the headers 
are to close! 
In other words, I guess I?m a bit on the dangerous side with this engine 
in PE! 

Any suggestions would be appreciated. 

thanks, 

/Per 
Stockholm, Sweden 

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