I just had to replace the battery in my 2003 eclipse gt. As soon as i put the new battery in, my car wouldnt hold an idle. If i put the car in park it dies. Can anyone help me figure out how to get it to idle again.

 

I had this problem recently in my '01 GT. Here is what I did:
1. Unhook the positive from the battery for 10 minutes.
2. Replace cable and start engine without ANY accessories on (A/C, fan, radio, etc).
3. Allow the engine to idle for 15-20 minutes (if it dies, restart).
4. By now it should hold an idle w/o accessories, so turn each on one by one. Make sure it holds idle.

Basically, the 3G (perhaps others?) are known for telling the chip to completely reset when there is a complete power loss. Therefore has to learn at what RPM to idle properly. Also, remember that your car will also need to relearn how you drive, so be gentle for the first 20-30 miles.