[Gmecm] Re: Off idle stumble

[Cowen]

I've had the same problem, and it had nothing to do
with the program in the ECM.  I haven't started
programming yet!  (Soon Grasshopper, soon...)

I put the MPFI from a 2.8l onto a 3.4l, which is about
a 20% increase in displacement.  That should probably
require about 20% more air at idle.

Unfortunately, I had not adjusted the idle stop
adjuster screw, so the throttle plate was too closed
for the 3.4l at idle.  "No problem," says the ECM,
"just command a few more IAC counts."  OK, but then
when I used the throttle to come off idle, the ECM
closed the IAC.  I was increasing the airflow by
opening the throttle, and the ECM was reducing
airflow, trying to maintain idle by closing the IAC! 
This conflict in airflow off idle results in a bad
stumble/bog off idle. It feels like the engine has no
torque - very weird, lots of slipping the clutch!

I simply reset the throttle stop screw using the
procedure from the manual, and no more problems.

I've had this setup in my Cherokee for just over a
year now, through two smog checks, and I can't believe
how well it works with the ECM from a 3.1 and no VSS
input (optical installed, ECM looking for magnetic). 
GM EFI is truly amazing!   I am still stunned at how
esaay it is to mix and match components from different
years, cars, engines, etc.  My only problem areas are
when out of open loop (e.g. it runs lean and slows
down at full throttle), but I'm looking forward to
learning to reprogram it too.

The next project though: a 1970 Buick Wildcat
convertible with a 455!  I was thinking TBI for
simplicity, but the 454 TBIs are scarce.  Now you've
got me thinking!

Good luck.  

Duncan

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