Originally posted by SPORTIMAGE
Get a feul pressure regulator and a air feul ratio guage. When it gets cold outside you will have to adjust your feul pressure. Its just like running at the track you have to tune your car for the conditions.
A/F ratio guages read the voltage off the stock oxygen sensor. Data from the sensor is only used by the ECU when it is in open-loop mode. It switches over to open-loop at something like 74 percent or greater TPS and some RPM. Out of that range the voltage reading from the sensor jumps all over the place, which is totally inaccurate. Unless you have a wide-band oxygen sensor, any a/f guage is more of an "oh crap I'm leaning out" warning light on WOT runs than a tuning tool. Secondly, a fuel pressure regulator can only bump the whole fuel curve up or down a bit. It's not a very precise method of adjustment. The best way to tune is to get at the fuel and ignition maps inside of the ECU.
The most probable reason the car eats gas when it's cold out is the weather. Cold air is more dense which means more oxygen. The engine can then use more fuel to burn that extra oxygen, hence the higher gas consumption.