On a cold day I would go out start the car hold your foot on the brake and put it in drive, next rev the car up to about 2k RPM's for about 30 seconds (wont hurt the torque converter), then shut the car off. Open up the radiator cap if you got pressure blowing out most likely the head is cracked should be any pressure in the system on a cold start. Also check for oil on the rad cap. Beside the crappy plastic plenum on these 3800's that engine is pretty indestructible I'd be surprised if the heads were cracked or leaking.