1228746 TBI 305V8 stumble

[Jay Vessels]

Hi there!

>Hard to diagnose a problem such as "off idle hesitation."  'Specially without
>more info.
>Shannen

Fair enough.  I understand the difficulty of fix-by-wire.  Big problem I've
got is that the car isn't physically close.  It's at my father's house (90
miles away) so I don't get to work on it unless/until I make a trip.
House/wife/job gets in the way of doing that too often :|  If it were
stable enough I'd drive it back to my house but if that were true, I'd give
it back to my cousin :)

Anyway, to answer some questions that hopefully will turn on a light for
someone:
ECM, TBI (including injectors) and PROM are all from the same donor car and
match.  ECM is 1228746, BCC is APFN, TBI is for a 305V8 (same as the
subject engine) as are the injectors, harness, all sensors (except knock,
kept the '85 knock sensor).  I essentially raided a '89 Caprice 305V8 TBI
car in a junkyard for all the parts I could stuff into the trunk of Dad's
'86 Caprice.  Target car ('85 Caprice, 305V8) has the same engine except
originally equipped with a CCC Q-jet.  

>> The '89 used the small-cap HEI which we didn't want to use.  
>Perfectly acceptable setup.  Why discard it?

Small-cap HEI from the donor had a cracked module connector and did not
look to be in that great of shape.  In an effort to keep the new
cap/rotor/coil/plug wires that were on the target car, we just re-used the
original large-cap distributor.  I removed the small-cap 4-wire module plug
from the TBI harness and spliced in the 4-wire module plug from the CCC
harness original to the target car.  No codes set.  Timing is adjustable
with the bypass wire disconnected.  Timing goes to about 12 degrees (from a
base of 0) with bypass plugged in so I believe the advance circuitry is
right.  

I saw a disassembly of a version of code for the 1228746 on GM-ECM FTP site
(for a 350V8 Caprice, not APFN) and downloaded it at Dad's house, but now I
can't find it on the site.  Anyone know where it is?  I checked bin,
bin-lib, and incoming but must have overlooked it.  I'll keep searching but
if anyone knows where it is, I'd appreciate being pointed in the right
direction.

>More details, please.  Lean/rich off idle?  What happens w/ O2 sensor
>disconnected?  Warm engine or cold?  Which bcc?  Throttle body match the

Haven't tried disconnecting the O2 sensor yet.  A&B tied together while
running at or near idle, CEL seems to spend more time "off" than "on" (I
know that's subjective).  If memory serves, this means the ECM thinks it's
lean.  Gotta get a 160 baud scan tool!

>basic spark issues?  TCC problems?  Flat tires? <g>  Faulty wood grain on the

Spark seems right in that an inductive timing light shows fire on all 8
cylinders and no obvious miss at idle.  Will have Dad pull plugs and
report.  TCC operation seems right during test drive.  Wood grain not
available on this trim level Caprice :)

This system originally had a no-start condition that we "fixed" by pulling
the fuel pressure regulator apart and reassembling.  Before, no fuel flowed
out of the return port of the TBI, and after reassembly, it flowed fine and
the car starts and runs.  Dad's wondering if it's not a fuel return line
restriction or a restriction in the TBI iself.  Injector spray pattern at
idle as observed with an inductive timing light triggered off of cylinder
#1 shows a nice conical spray from both injectors.  Fuel pressure gauge T'd
into the line after the filter shows 12psi, and a flow test gives about 8
ounces in 5 seconds.

Been fooling with GM EFI since '92 and the more I do the more I find I
don't know :(  I know what I've got is not as helpful as scan tool data,
etc. but any suggestions as to where to look are appreciated.  Heck, anyone
got a DIY 160 baud scan tool they like?  I can sort out the protocol if
it's an unknown once getting the hardware built.

Thanks,
Jay Vessels
1982 Chevrolet S-10 Sport, 2.8V6 TBI
1994 Chevrolet Cavalier Z24 Conv., 3.1V6 MPI


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