[Gmecm] TBI and EFE

[Carl Rumberger]

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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