[Gmecm] Re: [Gmecm] 305 TBI Hesitation

[Steve Ravet]

> -----Original Message-----
> From: gmecm-bounces at diy-efi.org 
> [mailto:gmecm-bounces at diy-efi.org] On Behalf Of David Allen
> Sent: Monday, July 10, 2006 9:55 AM
> To: gmecm at diy-efi.org
> Subject: [Gmecm] Re: [Gmecm] 305 TBI Hesitation
> 
>   Does your calibration have a table that modifies idle speed 
> vs. battery voltage?  Could there be parameters in the 
> calbration which are not called out in the definition file?  
> I caught heck with a project this year having a similar 
> problem.  It would idle very high and according to scan data 
> the ECM was calling for this idle- just like in your case.
>   My calibration had a table for desired idle speed vs. 
> battery voltage. 
> There is something wrong with the calibation or the 
> definition file because the ECM only responds to the LOWEST 
> battery voltage range in this table. 
> Even though the ECM reports a battery voltage of 14 with the 
> engine running, it will idle at the speed called for in the 8 
> volt table location. I figured this one out by trial and 
> error and error and error and error.....
>   No one (including the creator of the definition file) has 
> been able to offer any additional insight.  I simply put 750 
> in all cells of this table and the engine idles at 750 rpm.
>   Probably not the most helpful advice but...
> Later,
> David

My reported battery voltage was also correct.  This idle problem arose
after years of OK operation, even though the P/N switch wasn't
connected.  It didn't seem likely that the computer or calibration would
be wrong in this case so I didn't pursue that.  Some reading on the
thirdgen board turned up a possibility.  When AIR is enabled some
calibrations change the O2 switchpoint slightly to compensate for the
air being added to the exhaust stream.  I removed the AIR pump on mine,
and the timing of that may correspond with the start of my idle
problems.  BUT, in my case the O2 sensor is in the driver's side
downpipe, and the only AIR fittings are on the passenger side.  So maybe
not.

The other thing I meant to mention last night is that there is a very
comprehensive document about the TBI idle strategy on the gmecm page.
Lots of tables and data values and a very good discussion of what they
do.  So if you wanted to dive into the idle routines and tables start by
reading that.

Something else I did while troubleshooting this was to jumper the A/B
connectors with the key on (fully seats the IAC) then disconnect the
IAC.  Then start the engine.  It will probably barely idle.  Plug the
IAC back in, put the trans in gear, wait for the ECM to bring the engine
up to a reasonable speed, then unplug the IAC again, this time leave it
disconnected.  David's solution is better since you still get stall
saver and other features with the ECM controlling idle, but this worked
in a pinch.

--steve




> 
> 
> 
> ----- Original Message -----
> From: "Steve Ravet" <Steve.Ravet at arm.com>
> To: <gmecm at diy-efi.org>
> Sent: Sunday, July 09, 2006 9:08 PM
> Subject: RE: [Gmecm] 305 TBI Hesitation
> 
> 
> >>
> >> THis one has plagued me for a year now and like a man it is
> >> finally time to ask for help.
> >> The Project:
> >> 305 CID Small block TBI  7747.
> >> I'M  trying to figure out two things still. I have a high
> >> idle problem that cannot be lowered by altering convention
> >> cell settings for the idle. Something in another area is
> >> holding the idle up to 1100 RPM no matter what is done, it is
> >> not mechanical. The computer wants to increase the idle in
> >> gear at a stop sign.
> >> This might be related as well .If the throttle is opened up
> >> slowly under load, the engine hesitates or stumbles as though
> >> it is going lean. If you let off the throttle ( around 1100
> >> RPM ), it hesitates and
> >> the engine surges back as though it went suddenly went 
> rich.   Now if
> >> you step on it Hard, the engine jumps with all kinds of power
> >> and runs as it should. My question is has any run into this
> >> before and which cells are responsible or maybe over riding
> >> the others? I need some more heads on this one....Fred
> >
> > I had a similar problem some time ago, you can search in 
> the archives
> > for "creepy idle".  With a scan tool I could see the 
> "desired idle rpm"
> > creeping up while stopped and in gear.  From a fully warmed 
> up desired
> > RPM of 550 it would slowly creep up to 800 or so, opening 
> the IAC to 180
> > or more in the process.  I got used to shifting to neutral 
> when coming
> > to a stop.  While looking into it I discovered that a 
> wiring error meant
> > that my P/N switch had never been connected correctly.  It 
> was always
> > indicating in gear.  (This is a swap project, 350 TBI into an S-10
> > Blazer, I've been driving it for 9 years now).
> >
> > Fixing that, however, did not solve the problem.  
> Unfortunately I don't
> > know what did solve it, as it just gradually quit doing it. 
>  I checked
> > CTS, VSS, etc. on the scan tool and all were correct.  
> There were some
> > other suggestions in response to my original post that I never got
> > around to trying, you might try to dig them up.
> >
> > --steve

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