P4 ECM #1227748 questions

[Nick McAlpin]

I've been chasing this "problem" since I bought the car two years 
ago.  When I asked the fiero-specific lists about a year ago, I also 
got the vacuum leak response.  Since i've replaced most of the 
vacuum lines and most of the vacuum devices (and listened 
carefully several times for vacuum leaks),  I really do think it's in 
the ECM.  Here's a good reason why I just found this morning:

I was reading in the GM service manual about the minimum idle 
speed.  I figured that if this was set wrong, the ECM might get out 
of it's calibration range and act funny.  I also figured that if I had a 
vacuum leak, it would idle much higher than expected (even with 
the IAC fully shut).  So I checked it.  After following the procedure 
to step the IAC fully in, I found that my minimum idle speed was 
too LOW.  (supposed to be 600rpm, mine would try to hold 
~300rpm, fail, and stall).  I'd suspect that if I had a vacuum leak, 
this woudln't be the case.  So I turned in the screw a bit until the 
car would hold 600RPM.  When I hooked the IAC back up, the 
ECM brought the idle up to 1400rpm.  now this is a cold-idle rpm I 
like!  and I figure that I fixed my problem.  I shut the motor down 
and re-start again to be sure, but it fires back up at ~2300rpm and 
then takes 30 seconds to step back down to 1400 =(

So all that really happened is the 300rpm I added to the minimum 
idle speed, is now added to the high-rpm idle thing that the ECM 
seems to be doing.

Why is the ECM doing this to me??  for some reason, with no 
other change apparent to me, the ECM stepped the IAC all the way 
out to 2300rpm.

nick.

> I think you have a vacuum leak.  The Fiero 2.8 V6 behaves
> similarly when it has a leak (ie, a really high cold idle that
> slowly comes down to normal, and has to be relearned every
> time.)

> > when i finally come off the highway an hour
> > later and come to a complete stop, the ECM still is "stuck" at high
> > idle ~2000 rpm.  If I sit at a complete stop, the ECM will step down
> > the idle in fairly noticable ~100rpm steps until it reaches 1000rpm.

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