[Gmecm] Cold start with 1228746

[Jared Ryan]

I have a Chevy El Camino with a relatively new (not rebuilt) 350 long block, Edelbrock Performer TBI intake manifold, stock exhaust manifolds, and Crane 2032 roller camshaft.

I have been using a 1228746 ECM with an ANLU .bin from an '89 or '90 9C1 cop car.  I do not have an air pump but I do have the IAT sender in the air cleaner, and it is hooked up and gives a sane reading on the scantool, as does the sender in the thermostat housing.

In summer, the engine catches quickly and runs fine.  I haven't put forth much effort to really tune it because it drives fine and gets between 19 and 22 mpg on the highway (with 3.23:1 rear axle and TH-200-4R overdrive).

Once the temperature goes below about 80-85 F, the following starts happening:

Dead cold, the engine catches relatively quickly, and has a strong fast idle.  After about 10-40 seconds (seems to happen faster if it is colder outside), it will sound like it's slowing down gradually, then abruptly quit.  I crank it again, it fires quickly, and either it will keep running or the same thing happens.  I go through more cycles of this if it is colder; it is definitely temperature-dependent.

What little logging I have done actually showed values close to 128 at idle conditions, so I am not sure what is happening.  Previously, I have tried changing the open-loop and start AFR values in the chip, but nothing seemed to make a difference.

I have never tried a 1227747 ECM in this car; I have one on the way and I am tempted to try a .bin for a truck 350, just to see if it acts any different.

I am not sure what to be looking for.  It's very odd that it catches and runs very strong, then abruptly quits.  Then when I crank it again, it catches and runs fine (though it may quit again).  Once it has gone through this, it runs fine the rest of the day, and warm starts are never a problem.  It is only dead-cold starts that give me fits.

Based on how it begins to slow down then dies, it seems to me that perhaps the IAC needle is being moved from its starting position.  I don't see a way in TunerCat to adjust the rate at which this happens, or perhaps I don't understand what I'm looking at.

I never have gotten to the bottom of this, because I primarily drive my Civic when it gets colder, and it makes me lose interest in the El Camino.  I'd like to fix this, though.