[Gmecm] 7747 stumbling acceleration when cold

Mon Nov 29 17:58:25 UTC 2010

No, I have no park/neutral switch.  I datalogged this morning and the
problem was very pronounced since it was fairly cold (40 degrees).  I
noticed that the "old high gear flag" was on when it was in closed loop but
not before.  It wasn't on in open loop.  The problem was, if anything, worse
in closed loop.  I have a heated O2 and go closed loop pretty quick.  Anyone
know if the high gear flag thing indicates anything important about the fuel
delivery?

Did I mention I have "cop car" injectors?  It seemed like a fun idea at the
time I got them, but I wonder if it's part of my problem.  I adjusted the
BPW to account for them, but maybe there are other things that need
adjusting as well?  I'm tempted to just get stock injectors so I can maybe
use a normal bin without so many mods.  I originally had 305 injectors
without knowing it and of course nothing worked.  I went straight to the cop
car ones, so I've never actually used the regular 350 ones.

Thanks for the help.

By the way, this list always wants to reply to the individual poster instead
of the whole list.  What's the easiest way around that?  I'm in gmail and
all I can figure to do is cut and paste the list address over the poster's
after I hit reply.

On Mon, Nov 29, 2010 at 9:48 AM, Mike Wilson <mwilson at bwarch.com> wrote:

> Do you have a functional park / neutral switch? I know it acts differently
> when it thinks it is in park. With no VSS and no heat riser with an open
> element air cleaner and headers there could be enough off in the tune to
> make it stumble when cold if it thought the trans was in park still.
>
>
>
> Just a thought.
>
>
>
> Mike
>
>
>
>
>
> *From:* gmecm-bounces at diy-efi.org [mailto:gmecm-bounces at diy-efi.org] *On
> Behalf Of *Charles McDowell
> *Sent:* Sunday, November 28, 2010 3:32 PM
> *Cc:* gmecm
> *Subject:* Re: [Gmecm] 7747 stumbling acceleration when cold
>
>
>
> Thanks for the replies.  This is a transplant onto a carb engine and it has
> never really run great.  Good enough for daily driver, but with quirks in
> cool weather.  I've tried various tunes over the years so I'm sure it's not
> the same, but I tried to start it one morning when it was cold soaked to
> about 15 degrees and it simply wouldn't start.
>
> I have headers, so the heat riser thermac thing is going to be cheesy at
> best.  The headers do have a little tube for thermac, but there's not much
> to it.
>
> Anyway, I datalogged some this morning but it was too warm to have the
> problem really show up.  I have a heated O2 and I go closed loop pretty
> quick.  I think the problem persists after going closed loop.  I noticed
> that the rich flag is almost always on.  It only turns off during constant
> cruise, but the BLMs aren't terribly far off.  The table had ranges from 118
> to 138 for a short drive.
>
> I also noticed the "old high gear flag" was on all the time.  Does it
> matter?  I tried searching for info on it, but I can't tell if the ECM
> changes anything as a result of that flag.  I don't have anything hooked to
> the ECM from my trans.  No VSS either.
>
> I'll try to get some data logged on a colder day too.
>
> Thanks.
>
> On Sun, Nov 28, 2010 at 2:56 PM, Scott Peitzsch <jlg-sep at comcast.net>
> wrote:
>
> Even with your caveat below, I'd suspect this is directly related to the
> lack of a
>
> properly functioning Thermac system.  Without a well regulated source of
> warm
>
> air, basically every 7747 I've ever tuned with an open air cleaner exhibits
> the
>
> symptoms you describe below.
>
>
>
> I'd suggest either returning to a stock style air cleaner and intake with a
> fully
>
> functional Thermac system, or consider switching over to a 1228746 ECM.
> The
>
> 8746 is pin-for-pin compatible with the 7747, and adds IAT capability,
> which
>
> should go a long way towards solving your issue.  I've used this solution
> with
>
> excellent success on a few recent trucks I've tuned.
>
>
>
> -Scott
>
>
>
> ----- Original Message -----
>
> *From:* Charles McDowell <charles at taildragger.info>
>
> *To:* gmecm <gmecm at diy-efi.org>
>
> *Sent:* Sunday, November 28, 2010 12:09 AM
>
> *Subject:* [Gmecm] 7747 stumbling acceleration when cold
>
>
>
> I'm hoping someone can point me in the right direction of what to change in
> my BIN to solve a bad cold acceleration issue with a 7747 TBI.  The truck
> runs great once warmed up, but when the engine is still warming up (and
> worse on colder days) it stumbles through it's acceleration, sometimes even
> backfiring.  It sort of comes off the line decently, then bogs down and
> gradually works its way up to speed.  Any pointers on what I might change to
> fix this?
>
> Not sure if it matters, but this vehicle has an open air cleaner and no
> heat riser stuff.  I don't think that should be the issue since this happens
> even when it's 55 degrees out, but I thought I'd mention it in case.
> Thanks.
> ------------------------------
>
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