[Gmecm] PE lean issues on a ?747 TBI

[David Haggard]

Your other questions--
1. How does a 747 ECM compensate for air density changes in PE mode? Or maybe 
it can?t? 
 
It can't, but it does. 
It "compensates" by MAP, RPM, and TPS. Air density has a direct affect on MAP, so the ECM can "compensate" based on the MAP reading vs. RPM and TPS. In reality, it's not compensating. It's calculating how much fuel to inject based on these readings and the values in the "Power Enrichment AFR vs. RPM" table and the two fuel tables. No other compensation for air density is needed.
 
2. Is there a way to see when this ECM is in PE mode? 
 
No. Not that I know of. But your O2 readings should go above 0.9 at WOT. That's a very good sign.
 
3. Half a volt should have no affect, unless it's under about 11 volts.
 
Other thoughts that I should have said earlier--
What is your fuel pressure? Stock specifies 9-12 psi, but 12 is preferred, and on some builds, 14 is better.
On my build (370 hp 305 with a 454 TBI on a Weiand TBI manifold), to get the fuel delivery I needed I installed a 1990 TPI fuel pump in my tank, and an external Mallory vacuum-modulated fuel pressure regulator in the outlet line from the TBI. My build likes the idel fuel pressure at 15 psi. At WOT it gets 22 psi.
 
Related to that, if your pressure is up to 12, can your fuel pump deliver the volume? Is the fuel filter partially clogged? Is the pump tired?
 
The knock sensor is not an issue unless you've got knock. You can turn off the knock sensor by setting a flag.
 
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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