[Gmecm] TBI and EFE

Tue Dec 5 14:46:12 UTC 2006

  It could be the IAC Steps Vs Coolant Temp table in the ECM calibration.
  When tuning my Nova, I could never get it to idle correctly until I lowered the values in the table locations near normal operating range. Symptoms were similar to yours. I have all the table entries in the normal operating temp range set to "63."
  The engine was only slightly modified but it was enough to affect the idle.
David

----- Original Message ----- 
From: <jryan at caminofx.org>
To: <gmecm at diy-efi.org>
Sent: Tuesday, December 05, 2006 8:19 AM
Subject: Re: [Gmecm] TBI and EFE

> Today was another cold morning (30 F) and it ran well.  (I know cold  
> in Houston is not the same as in other parts of the country.   
> Fortunately, when it freezes here, there is almost never  
> precipitation.  Right now it's 38 with a dewpoint of 29, relative  
> humidity 70%, which is more humid than it has been.)
> 
> I haven't been letting it idle and warm up at all in the mornings.  I  
> just start it and go.  It comes up to operating temperature much  
> faster in normal driving than idling, anyway.  Even so, with the iron  
> block and heads, I'm on the freeway before it comes up to thermostat  
> temperature.
> 
> This morning I noticed that when I parked at work and moved the  
> gearshift from OD to P, the idle speed stayed high, about 1000 RPM.  I  
> moved the lever back to OD, then back to P, and it settled down to the  
> speed set in the PROM.  Could this be a misadjusted Park/Neutral  
> switch?  I have seen this on occasion on this car, and it seems like  
> every time, putting it back in gear and then back in park makes it  
> settle down.
> 
> I'm actually very surprised at how well the 1989 9C1 PROM works,  
> considering I'm using full-size pickup injectors, which to my  
> understanding are 61 lb./hr. while the 9C1's might have been 68  
> lb./hr.  When there are no fuel leaks at the throttle body (and I hope  
> that is cured for good), the drivability is overall excellent.
> 
> Quoting Rick McLeod <dunvegan at sbcglobal.net>:
> 
>> J, good news. It was kinda as suspected, something amis from the   
>> work being done. Since you have an IAT sensor, it appears to be   
>> compensating adaqately, goodness.
>>
>> I'd suspect a minor tweak on mechanical min idle will cure the hunt,  
>>  you may have a sensitive setup that it's between counts for correct  
>>  idle on the IAC and it's trying to settle, which mechanical min  
>> idle  setting might correct, try just a little bit backed off of min  
>> idle  to see if that cures it, or maybe add just the slightest bit.
>>
>> Good luck, stay warm
>>
>>
>> ----- Original Message ----
>> From: "jryan at caminofx.org" <jryan at caminofx.org>
>> To: gmecm at diy-efi.org
>> Sent: Monday, December 4, 2006 2:49:06 PM
>> Subject: Re: [Gmecm] TBI and EFE
>>
>>
>> Well, once again, apparently I fail.
>>
>> It turns out that the injector cover gasket was leaking.  I took a
>> look at it with the air cleaner lid off, and I noticed that the
>> left-side throttle plate was wet, but the right-side plate was dry,
>> and the part of the gasket on the left side was glistening.  I have a
>> dual-plane intake manifold (Edelbrock 3704), so one-half of the
>> cylinders were getting a very rich mixture, while one-half were
>> getting a lean mixture, apparently.
>>
>> So I went to the parts store and got a TBI gasket kit, and I replaced
>> everything I could without taking the throttle body off the manifold
>> (no reason to suspect the base gasket).  I changed the injector
>> O-rings, injector cover gasket, IAC valve gasket, etc.
>>
>> I don't know if this is a best practice but I used petroleum jelly on
>> the injector O-rings to try to protect them during installation, as I
>> have mashed a lower O-ring in the past by accident, causing a
>> super-rich running condition.
>>
>> At any rate, I started it up and it ran fine, much smoother.  After it
>> warmed up, I had to do the whole minimum-air reset procedure, because
>> it idled much too fast (due to the mixture being much leaner).
>>
>> For what it's worth, I actually do have an IAT sensor.  I have a
>> strange hodgepodge of components.  The serpentine belt system, 1228746
>> ECM and air cleaner are all from a Camaro.  The intake fits under the
>> hood of the El Camino nicely and looks like it belongs there.
>>
>> The other thing the 1228746 has that the '747 seems not to is a
>> control for the vapor canister solenoid.  Cars seemed to have a
>> solenoid, while pickups did not seem to.  This may also play into your
>> thinking - truck owners may not be as sensitive to small variations as
>> car owners.
>>
>> It was 30 degrees F when I started out this morning, and it ran fine -
>> no stumble, no rough idling, no black smoke (lotsa white stuff out the
>> pipes though!), so apparently it is not critical to preheat the intake
>> air.  With the Camaro air cleaner, duct and scoop, it's getting cold
>> air from right behind the driver-side headlight, not heated by the
>> engine compartment or radiator.
>>
>> There is one small issue left, and this may be because I'm using the
>> ANLU (9C1 350) PROM with a camshaft a little bigger than it was made
>> for.  It idles very nicely at all times, very smooth, very close to
>> the target speed set in the PROM, but it does something a little funny
>> after I have been driving a while and I shift from OD/D to P.  The
>> idle jumps up slightly due to the decreased load, then the tachometer
>> needle does kind of a pendulum thing, going up and down about 100 RPM
>> total.  After about five or six cycles, it stabilizes and stays at the
>> set idle speed.  It only does this when shifting from OD/D to P, not
>> from P to OD/D.
>>
>> It acts like it's chasing the target idle speed, but it's such a small
>> variation that many people might not notice it happening.  I wonder if
>> I should get a scantool and watch the IAC counts, and try fine-tuning
>> the throttle plate opening based on the IAC counts.
>>
>> Thanks to everyone for all the advice.  I am enjoying learning how
>> this system is really supposed to work, and starting to really enjoy
>> driving the car.
>>
>> Quoting Carl Rumberger <perfdyn at cwnet.com>:
>>
>>> I have read a number of replies to your cold drivability issues that
>>>  address throttle body icing. Though icing may indeed be a real
>>> concern, especially from a safety standpoint and may contribute to
>>> your cold run issues, it is at least equally possible the problem is
>>>  related to the "747" TBI system not having any Intake Air Temp.
>>> (can  you say air density) sensing.
>>>
>>> GM choose not to use IAT sensors in Pick-Up applications where the
>>> typical owner wasn't likely to be very picky about slight running
>>> issues . . . combined with the fact that in these engines the intake
>>>  and coolant temperatures closely follow each other under most
>>> operating conditions (a coolant temp. sensor and single temp. table
>>> could substitute for both sensors and tables).
>>>
>>> The truth is these engines can be sensitive to cold intake air /
>>> lean A/F ratios. Of course the A/F: vs. Coolant Temp. table can be
>>> adjusted richer under cold operation but then you would likely run
>>> into slightly rich cold running when the weather turns warm. And yes
>>>  a TAC system will tend to minimize intake air temp.(air density)
>>> variations. I have had the misfortune of tuning a TBI "747"
>>> conversion that the customer choose an Edelbrock Performer RPM Air
>>> Gap intake manifold combined with a cold air intake system on a
>>> Chev. 350 with a mild cam. Trying to get the fueling correct was
>>> frustrating not only because of the cold air system but with this
>>> intake manifold the changes in IAT no longer even remotely followed
>>> the changes to coolant temp.
>>>
>>> It is also interesting to note that in those applications where GM
>>> thought the owners were more likely to be picky (Caprice / F body)
>>> they used a IAT sensor and the 1228746 ECU.
>>>
>>>
>>> ----- Original Message ----- From: Jared Ryan <jryan at caminofx.org>
>>> To: gmecm at diy-efi.org
>>> Sent: Friday, December 1, 2006 12:28:24 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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>>
>>
>>
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