[Gmecm] SER# 1227747 ACSU promID Idle Problems (Update)
Dave Thomas
Dave.Thomas
Wed Dec 13 04:56:07 UTC 2006
Just thought I would give you guys an update. This weekend I tried the
following in order:
1. Capped all vacuum ports with the exception of the MAP sensor. (i.e. PCV,
Vapor Canister, Air pump, EGR, and brake booster. Unplugged the heater
damper vacuum reservoir plugged my vacuum gauge up. (no change)
2. Removed EGR checked for carbon, burrs, made a block off plate and
sandwiched in between the EGR and manifold. (Still no change.)
3. Swapped MAP sensor and checked for plugged hose with my 93 Suburban that
I drive everyday. (no change)
4. Plugged the PCV valve hose but left the PCV valve in valve cover,
Plugged air vent into air breather with a vacuum connection. I had
pressure of 2 psi no vacuum ( I assumed no lifter valley intake leak)
5. Changed ignition coil with 93 Suburban that I drive everyday. (no
change)
6. Changed Ignition module with 93 Suburban that I drive everyday. (no
change)
At this point, I was pretty discouraged so I hooked up my Diacom and
observed what was happening. The IAC counts at startup went to 65 then
reduced to 35 (Initial motor position no A/C) counts in Park, I then placed
in gear and the counts started decaying and the idle started to reducing
trying to reach the magic 500 rpm. Usually it would idle at 525 with 0
counts, then I would place the vehicle in park and neutral and I noticed
that the counts would stay at zero. The engine would idle up ~100 rpm and
settle back down. Then when I shifted into drive again, the engine would
die but the IAC counts did not increase at all. So I figured maybe the
computer was still trying to decay the counts to reach 500 rpm but the IAC
had no place to cut off any air flow because it was at 0 already. So maybe
if you haven't reached the correct idle, I was thinking the car may not
adjust the counts for the extra load. At this point, I decided to hook all
of the components back up to the TBI (i.e. vacuum lines, air breather, air
pump, vacuum brake booster) to rule out that maybe the computer was
expecting air from either the PCV valve and air pump. From here, I kept
decreasing the idle minimum air screw and as I go to around 5 counts at
idle, It was still stalling and cutting off. Next, I closed the idle screw
further and noticed this time at around 7 counts that the engine would run
a little longer after the shift into gear but it still stalled. Basically,
I kept reducing the screw until I got to around 20 counts and now it idles
as I shift between park and neutral at 500 rpms. I have not followed the
service procedure at this point but apparently if your minimum air setting
is to far open the engine runs on anyway with a high idle, but as you get
to the edge of the correct setting but still to far open the computer
cannot control the idle when you load and unload the engine, then comes the
"Zone" and after that my guess is that you will have the problem in the
other direction (i.e. counts to high). The IAC white paper was invaluable
for understanding what the computer was doing during idle.
I do have one more question though, it seems as though 500 rpm is extremely
low for this vehicle. Is this an emission setting for 1987? My 1993
Suburban idles at around 550 rpm. Also, I heard people say that you should
not see drips of fuel coming from the injectors, but I think I see what
looks as drips in the spray pattern at idle. Is this normal or not? I
have a movie I could send to someone if anyone is interested.
Thanks again to everyone for their suggestions,
David Thomas
Food & Packaging SBD Engineer
Festo Corporation
Phone: (704) 527-1427
Fax: (704) 527-7935
email: dave.thomas at us.festo.com
Jay Vessels <jay at vessels-clan.com> 12/08/2006 07:02 PM
Sent by:
gmecm-bounces at diy-efi.org 12/08/2006 07:01 PM
Please respond to
gmecm at diy-efi.org
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Hi there!
> This is after I ground the diagnostic pins on the ALDL connector, wait 10
> seconds for the IAC motor to zero out, remove ground from ALDL and start
> engine.
This may not be the same on all TBI systems, but I don't think that's
the right procedure for your system. Except for my '84 Cavalier (which
used a rubber plug installed in the TBI) all of the ones I've tuned use
a somewhat different procedure than you describe.
The difference from what you described is that you *don't* remove the
ground from the ALDL port before starting. Leave it there. Otherwise,
the ECM will try to control the idle, and you're fighting it with the
throttle stop screw.
Essentially:
1) Key off, jumper pins A&B on ALDL.
2) Key on, adjust TPS to proper voltage (0.5V on the ones I've touched,
yours may be different?)
2a) You may want to unplug the IAC, ensuring the ECM doesn't move the
IAC pintle, but leaving pins Z&B jumpered should do this.
3) Start engine and let it stabilize.
4) Adjust idle speed to spec.
5) Readjust TPS to spec.
6) Repeat 4 and 5 until both are satisfied.
7) Shut engine off, remove jumper, and reconnect IAC if unplugged.
Jay Vessels
1982 Chevrolet S-10 Sport, 2.8V6 TBI
1984 Chevrolet S-10 Blazer Sport, 2.8V6 (TBI pending)
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