[Gmecm] ? about pre-efi gm ecm

[Jay Vessels]

Hi there!

> My objective is simply to get this vehicle to EFI with absolutely as
> little modifications as possible.

I understand that.  I did a similar swap on an '85 Caprice, from CCC 
Q-jet on a 305 to TBI, grabbed from an '87 Caprice 305.  It wasn't that 
bad.  The pre-existing dash wiring is nice since with some small changes 
you can re-use it.  The rest of the CCC harness is scrap.  The existing 
O2 sensor can be re-used, as can the knock sensor/module and CCC 
distributor.

> I was under the impression GM's initial TBI was essentially the next
> step in evolution from carb to EFI, transitioning through the CCC
> (short lived) phase

CCC wasn't short-lived enough.  It should have never seen the light of 
day.  GM knew how to do EFI before unleashing CCC on the world, and they 
were doing TBI as early as 1982.

TBI is just as valid an EFI system as port injection (MPI/TPI/SFI) and I 
like it a lot.  It has some advantages.  Simplicity and compactness come 
to mind.  Port injection certainly brings a lot to the table, but a 
properly-tuned TBI system isn't bad.  Check out the Ultimate TBI stuff 
going on over at Thridgen.org.

> Will the throttle body for a TBI directly replace the CCC carb, or is
> it absolutely necessary to lift the manifold and replace it?

Stock GM TBI units have three bolts in a roughly triangular pattern, so 
it will not bolt to a stock intake.  You can get/make an adapter plate 
to put the TBI on a carb. intake.

Your other choice is to get a TBI intake.  GM used TBI on cars and light 
trucks from around 1985-mid '90s.  Later model (1987+) intakes have the 
center four bolts angled differently than the older intakes.  There are 
ways around this, none of which are fun.  I redrilled a newer intake but 
was never really happy with it.  If I were going to do it again, I'd 
either get the adapter plate or I'd get something like the Holley 300-49 
TBI intake for older heads.  Knowing me, I'd probably spring for the 
Holley ;)

> If I can use the existing manifold, shouldn't all the linkage and
> kickdown cables essintially just hook up

It depends on your donor, but the answer is "probably".  When I get 
parts from a donor, throttle/TV/cruise cables are on the list.

> We've a local show and swap event next weekend, so I'd like to have a
> 'shopping list' in my pocket of things to try to find. With that in
> mind, will the 846 work w/ most any GM throttle body from an
> equivalent engine, ie from a truck w/ 305 or can I expand my search
> to include throttle body units from the 350 as well.

Almost all 2-barrel GM TBIs are the same mechanically and electrically, 
with the exception being throttle bore size and injector flow rate. 
4.3/305/350 all share the same bore, using different injectors.  Some 
4.3 systems have injectors with two different flow rates, so be careful. 
  The 454 TBI has a larger throttle bore than the 4.3/305/350, and the 
2.8 TBI has a smaller bore.

If you use a TBI from a different engine size than yours, you'll either 
need to change the injectors or change the cal to know what injectors 
you've got.  Either choice is pretty simple.

The TPS sensor changed physical plug style sometime in the early '90s, 
but it's electrically the same.

> carburetor.  It is much easier to remove the entire factory harness 
> for the feedback emissions system and use the Painless harness. That 
> is what I did.  The factory ECM harness is accessible by unbolting 
> and moving the right front inner fender.

I have used stock ECM harnesses and the Painless harness, and both are 
good choices.  The stock harness has some rather oddball wiring routing 
choices, but it's not that bad to use.  The Painless harness is nice but 
doesn't usually contain provisions for AIR, etc. and the instructions 
with my harness had some bad advice (i.e. hooking up the P/N wire to 
avoid having to use a VSS sensor is bad advice).  The harness itself is 
pretty decent, though.

Jay Vessels
1982 Chevrolet S-10 Sport, 2.8V6 TBI
1984 Chevrolet S-10 Blazer Sport, 2.8V6 (TBI pending)