Idle too high. How do I adjust

[Jay Vessels]

Hi there!

On my truck ('87 2.8V6 TBI 1227429 ECM) you can set base idle speed with
the following method:
Key off, short ALDL pins A&B.  Key on for 30 seconds or so, then unplug IAC
with the key on.  IAC pintle should be fully extended here.  Start engine
and adjust base idle speed (minimum air).  Turn engine off.  Key on, adjust
TPS voltage to spec. (0.50 volts +/- .05V seems to work for most cars).  If
the voltage was way off (i.e. you moved the idle screw a long ways) then
you might want to repeat the process until you get a base idle speed you
like and a TPS idle voltage in spec.  This also seems to work for Dad's '86
Caprice ('85 F-body TPI system, '85 ECM number slips my mind).

This did not work for my '84 Cavalier Type 10 convertible (2.0I4 TBI,
unknown ECM).  It had a plug that you were supposed to install in the air
horn of the TBI to block the IAC before you set minimum air.  

When I got the car, it idled at about 2000 RPM and slammed into gear.  I
replaced all the gaskets, vacuum lines, and the IAC to no avail.  I could
not adjust the speed lower with the idle speed screw.  In a fit of anger, I
removed the IAC and pulled on the pintle.  It almost came all the way out.
I threaded it back in so that the TBI's pintle seat itself would push the
pintle in as I reinstalled it.  Started the car, idled fine.  I guess the
IAC was somewhat retracted and the ECM never commanded enough steps to
fully seat it.  Either that, or it was stuck (albeit new) and my brute
force method fixed it.  Try this at your own risk.

On my truck, I don't have the VSS connected at all and the ECM is wired
such that it is always in Neutral.  It idles fine and fuel economy is
decent.  What benefits would I gain from attaching the VSS signal (didn't
have a VSS when I converted to TBI, now I do since installing the cruise
control)?  Currently set up like it is to disable EGR complaints.  Would
pulling EGR code out of the chip and connecting the VSS signal be a better
solution, or just more work?

Jay Vessels
1982 Chevrolet S-10 Sport, 2.8V6 TBI
1994 Chevrolet Cavalier Z24 Conv., 3.1V6 MPI
http://www.vessels-clan.com/jay (now W3C compliant)

At 05:22 PM 9/11/01 -0500, you wrote:
>
>> As far as connecting a VSS, Couldn't I just connect it or would I have to
>> mechanically power it as well? 
>> 
>> I dont see the connection logically between the VSS and idle, as it idles
>> perfectly, other than too high for the boat. In the vehicle, 800 would be
>> very suitable. 
>
>You could likely connect it once and give it a signal > 36 mph (isn't that
>right, fellas?) to allow the IAC to reset itself, then disconnect it.
>Problem is that the IAC baselines itself to certain numbers of "steps"
>from being closed...then after some period of time the ECM attempts to
>adjust to a particular RPM, given other parameters. Other parameters
>include stuff like decel, dashpot, temp, A/C, PNRDL, etc. My car would run
>at 2500 RPM for a few minutes before it idled down, and it never did make
>it to true idle speed. IAC didn't know where "closed" was after I took
>the TBI apart for cleaning. Sounds like you don't have that problem...yet.
>
>If you don't have the ability to burn an EPROM, that idea of blocking IAC
>and adjusting idle speed with a screw looks better and better. I won't
>pretend to be anyone that is worthy of burning chips for people (can you
>say engine go poof?), but if your calibration (the 4 letter code) is one
>of those listed in the bin_library email me directly and I'll give it one
>shot in the name of neighborly help. Today seems like a good day to offer
>some of that.
>
>-greg
>
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