Here's a quiz for you DIY'ers
 

Ok, you expert DIY'ers, here's a lil' quiz for ya. Look at the picture; now what do you think caused this? The expansion tank is brand new.

Choose the correct answer below. Only one answer is correct:

A) The engine overheated expanding the coolant beyond what the cooling system could handle;

B) The expansion tank has a manufacturers defect;

C) The coolant was overfilled causing over-pressurization of the cooling system;

D) The coolant got extremely hot thus melting the expansion tank joints.

Since tank is brand new - the most probable answer is
B) The expansion tank has a manufacturers defect;

E) 2 Bar tank cap

A & D
Most people don't look at the gauges or care about the lights. They think they can drive 1 or 2 more miles and get home.

Why was the Overflow tank replaced? My guess is whatever the cause for the original tank going bad has occurred again with the new tank.

Symptom was treated, not the cause.

Just my guess...

C

I'm thinking a variation of answer C; perhaps the coolant and water aren't a 50/50 ratio with more water than coolant causing a higher pressurized (steam) system

Bingo, we got a winner! The correct 50/50 coolant mixer was overfilled causing over-pressurization of the cooling system. Happened twice- first there was hairline crack on the side of the tank. Replaced and 2 weeks later the leak as shown in the picture. The dealer replaced them assuming it was manufacturing defect.

The coolant level must be correct. It is correct when the end of the red float is aligned with the upper edge of the filler opening, or max. 0.8in/2 cm higher, i. e. up to the mark on the float (refer to the schematic diagram, next to the reservoir cap).

Yes, Kristophe, a winner too! Very important to have the correct mixture!

If it were a over pressure event then wouldnt pressure be vented out the expansion tank cap without any damage to the other components?