[Gmecm] PE lean issues on a ?747 TBI

[Jay Vessels]

Hi there!

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

Air density changes are made using MAP data (in the VE tables), but 
there is no specific table associated with air density correction.  It 
would be nice to have inlet air temperature in the equation, but the 
'7747 doesn't have that.  The 1228746 ECM (F-body and B-body cars) does 
have IAT, though I don't think the tables are all that prominent in the 
equation.

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

I haven't looked in a while but I think it's one of the bits in the mode 
flags.  I didn't see the mode flags logged in your data.

> 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...

If the fuel pump voltage is too low then the regulator may not be able 
to compensate.  The ECM has a table in it to compensate for lower 
battery voltage.  Lower battery voltage also makes the injectors open 
slower, so at lower power you're getting less fuel for the same pulse 
width.  I'd definitely give the charging system a good once-over.

Also, on your slower run your O2 readings are a lot leaner.  I'd check 
out the fuel system for pressure and flow.  You need 10-15 psi and you 
need to be flowing at least a pint in 15 seconds at the TBI fuel inlet 
(post-filter).

Once you verify the fuel pump and charging systems are good, make 
another pass and see if you're still running lean.  During normal 
driving, what do the BLMs look like?  Do you have plenty of O2 
crosscounts?  How healthy is that O2 sensor (i.e. can we trust the 
readings)?

> 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!

I'd disable the knock code in the chip.

> In other words, I guess I?m a bit on the dangerous side with this engine 
> in PE!

Only 'cause you're apparently lean.  The knock sensor is a failsafe to 
keep someone from lunching the engine in bad knock situations.  It's not 
a great tuning device and the ECM doesn't use it for fine tuning, just 
for safety.  If you're regularly tripping the knock sensor then either 
you have something mechanically wrong (or are using 
knock-sensor-unfriendly parts) or your tune is too aggressive.

Jay Vessels
1982 Chevrolet S-10 Sport, 2.8V6 TBI
1984 Chevrolet S-10 Blazer Sport, 2.8V6 carb. (for now)