start up idle problem
Assuming no vacuum leak, you probably just need some small adjustment to your open-loop coolant correction table for warm-up.
It could be just a little lean at the very lower temps.
You're right that gas mileage drops during very cold weather. Warm up takes much longer, and denser air requires more fuel.
A V8 passenger car is hardly underpowered, so the cooler air & added power capacity are not needed during normal driving, and so gas mileage drops a bit.

