[Gmecm] TBI and EFE

Sun Dec 3 16:14:58 UTC 2006

OK, here is what happened, from the beginning.  The exact sequence is  
important.

I discovered the busted fuel injector O-ring that was the cause of the  
too-rich problems I had been chasing since April.  (The lower O-ring on  
the left-side fuel injector was mashed, letting fuel under pressure  
down into the barrel).  I replaced that, and the engine ran perfectly.   
It idled very slow (at the target idle speed programmed into the ANLU  
chip, about 550 RPM) but very smooth.  Drivability was excellent.

Then, after a few days, I noticed it would sometimes idle faster than  
it had been.  It was still smooth, but about 1000 RPM.  It would flare  
up when I shifted out of drive into Park or Neutral and not come down  
as fast as it had.

All of the preceding is with the daytime highs in the mid-70s and lows  
in the 60s.  Warm weather.

Well, I wondered if the IAC valve might be sticking.  I took out the  
valve and cleaned it.  It wasn't very dirty, hardly any carbon, but I  
sprayed Gum-Out on it and also sprayed it in the passage on the  
throttle body.  I put the IAC valve back in.  I didn't drive it after  
doing this.  Later that night, the arctic blast came through (well, for  
Houston, TX; lows in the low 30s, highs in the 50s at the most).  Since  
the day I cleaned the IAC valve, it has not idled well.  It acts like  
it might be too rich, and it won't idle slower than about 700 RPM.

I've done the IAC reset, grounding the diagnostic pin to force the ECM  
to extend the IAC plunger fully, and the slowest I can get it to idle  
with the IAC valve unplugged is about 600, and it's rough.  It's rough  
with the IAC valve connected, both cold and hot, and it idles faster in  
Park or Neutral than in Drive.  In Drive it's right about at the target  
idle speed, but still rough, but it won't idle down when taken out of  
gear.

By "rough" I mean a bad smell from the tailpipes, car shaking, surging,  
having to really hold my foot on the brake to keep it still at a red  
light due to the surging.

So could I have damaged the IAC valve with the Gum-Out?

I broke the scientific method and changed two things at once (OK, so I  
can't control the weather), and now I don't know if I have an acting-up  
IAC valve, a fuel leak, or if it just doesn't like the chilled air.

The pintle of the IAC valve looks like it could possibly wear over time  
and not seal against the throttle body, with or without carbon buildup.

  ---> Jared Ryan <---
jryan at caminofx.org | http://www.caminofx.org

On Dec 2, 2006, at 9:31 AM, davesnothereman at netscape.net wrote:

>
>  The TBI trucks w/ 7747 expect intake air temp around 90 deg F or  
> greater.  The intake air heater is calibrated for this temp.  Most  
> folks with older tbi trucks don't seem to know / care.  In an area  
> where 10 year old trucks are rotted out due to salt, snow, and  
> humidity it's not uncommon to see the few left running with no heat  
> stove left on the manifold and no tube connected to the snorkel.
>
> The question you need to answer is "How good is good enough?"  I have  
> open element air cleaners on my truck which used to run in temps well  
> below zero without causing any issues which I can remember.
>
> The old carby 2.8 S10's had an electrically heated grid under the carb  
> to prevent icing.  A 15-20 mile drive at highway speed at 50 deg temps  
> could cause enough restriction to bring vehicle speed below 30 mph if  
> the heater was inoperative.
>
> Zaphod
>
> -----Original Message-----
> From: jryan at caminofx.org
> To: gmecm at diy-efi.org
> Sent: Fri, 1 Dec 2006 1:28 PM
> Subject: [Gmecm] TBI and EFE
>
>
> Do TBI engines typically need the EFE ThermAC setup to run smoothly in  
> cold weather?
>
> I have an El Camino that has a heat stove on the passenger side, but I  
> recently changed to a Camaro air cleaner that puts the snorkel on the  
> driver side (this fits MUCH better with my Camaro serpentine belt  
> setup). I will need to put a heat stove on the left-side exhaust  
> manifold in order to use the ThermAC.
>
> I assume it wouldn't be there (on '88-'92 TBI F-bodies) if it didn't  
> help. It made a BIG difference in cold-weather running when I had the  
> QuadraJet carburetor, and TBI is also a wet-manifold system.
>
> The reason I ask is that the temperature has dropped into the 30s F,  
> which is cold for Houston, TX, and the idle is a little rough.  
> However, the cold weather moved in on the same day I removed and  
> cleaned the IAC valve. It is entirely possible I just need to reset  
> the minimum-air setting and the idle will smooth out.
>
> Off-idle performance has been perfect, just a little bit of a rough  
> idle since it turned cold. I will do the IAC reset this afternoon, but  
> I still wonder if making the ThermAC operational will help a lot.
>
> I notice that the TBI intake manifolds no longer have the exhaust  
> crossover under the carburetor/throttle body mounting pad.
>
> I hear a lot about ThermAC, like "It only closes in *really* cold  
> weather. When I had it set up properly with the QuadraJet, it would  
> close at least a little bit on sunny warm spring days! It would open  
> quickly, but it didn't only work on very cold days, for sure. On very  
> cold days, like what we have here, it often wouldn't ever open  
> completely.
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