[Gmecm] 7747 stumbling acceleration when cold

[Charles McDowell]

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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