P4 ECM #1227748 questions

[rr]

Nick,

Try some ether or carb cleaner around the tb base & manifold. If no
changes, then no leak. Have-ta' check this stuff first.

When all is ok there, it's time to delve into the bin. There is a coolant
vs IAC position for startup. Dropping these values typically takes care
of the problem. Can go too low and get stall saver though.

A stock 3.1 '92 f-body 3.1l would go to 1800-1900 rpm on a cold
start. Shoulda' heard it once I put a perf muffler on it!

BobR.


Nick McAlpin wrote:

> 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.
>
> ----------------------------------------------------------------------------

----------------------------------------------------------------------------
To unsubscribe from gmecm, send "unsubscribe gmecm" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org