I have a 2003 Chrysler GT, PT Cruiser with less then 60000 miles. It is a four cylinder 2.4L, that is turbo charged, and is automatic. I had a hole in the over flow tank and had it replaced with a new one. When I got it back from my mechanic it seemed ok. About a week later I took the radiator cap off and found a light brown milky substance in the radiator. No water in the engine oil. The temperature gauge never showed that the car ever got hot. It did however spray water all over the engine from the broken over flow tank. When I took it to the mechanic approx. 4 miles away. Any idea were the oil is coming from? The water in the radiator is dark metallic brown in color.
Does it bubble at all when radiator cap is removed (head gasket)? When's the last time you replaced your coolant (needs changing every 2 years) did the previous over flow tank have gunk in it? (mine did in my Holden 2001 and it got into the radiator/cooling system gave it a flush added new coolant, pipes, thermostat, maybe waterpump? I did when it happened to mine but I did it myself).
If you have none of those symptoms possibly the rear main seal. Which involves removing the transmission to get at it.