Cracked coolant expansion tank...

[P1]

Wife saw a "low coolant" warning when she was out with my son earlier this week, but the light went off. Two days ago, I'm on my way to take my son to the doctor and the low coolant light is on from the second I start the car. I remember Poppyboy saying I could just top it off, so didn't think about it. Not even a few miles from my house and the white smoke starts coming from under the bonnet. Park it on the side of the road, call my (excellent) SA and he tells me to just get it towed to the dealership.



Apparently the coolant expansion tank had cracked, and it blew the vent hose at he same time. Strange thing was that I never received a "car is overheating" warning on my dashboard. The SA thinks that it probably happened right after I shut the car off. CPO took care of everything but the hose and clamp.

Apparently, the coolant expansion tanks are a weakness as they are made of plastic, and can start cracking over time.

17127536235 - VENT PIPE
34321156893 - HOSE CLAMP
17137647290 - EXPANSION TANK
82141467704 - COOLANT

Picked her up earlier and she drives great as always again!

[denaliman]

This is a known issue. Ask any tow truck driver LOL!. Same thing happened to me while I was sitting on a bridge at a border crossing from Canada to USA last summer.

[mitch808]

Apparently, the coolant expansion tanks are a weakness as they are made of plastic, and can start cracking over time.

Most coolant expansion tanks are plastic, for most makes/models of cars. It works, it's low cost, and it's a flexible enough material to be molded to fit almost anywhere. It's not a design flaw. It's just one of those things that happen. I'm glad it was an easy fix and not catastrophic.

[Falkon]

I think the key issue here is that these Bimmers run on a closed circuit pressurized engine cooling system with no vent. Meaning that any pressure developed by the expansion of hot coolant will be handled by the air gap in the expansion tank. Also note that the Bimmer Expansion Tank doubles as the primary coolant reservoir.

Thus, the tank and other flexible hoses in the cooling line are consistently subjected to cyclic stresses, expanding/contracting, due to pressure from heating/cooling coolant fluid. The Expansion Tank, as it seems, being the least elastic member of the system would likely be the first to fail under extended fatigue condition.

Some other car makes/models equally have coolant expansion tanks, albeit as a secondary coolant reservoir, but with a vent tube. Therefore excess pressure build up could be relieved to the outside environment of the fluid reservoir. Many Lexus/Toyota operate on this model.

[Poppyboy]

You freaked me out at the beginning of your post. I thought I gave you bad advice leading to the destruction of your engine!

[P1]

Originally Posted by Poppyboy

You freaked me out at the beginning of your post. I thought I gave you bad advice leading to the destruction of your engine!

Luckily it never got to that!

[Pierce330]

Known/common issue even on the E46/E90...I replaced all expansion tank, water pump (OEM uses plastic impeller) and thermostat...

[smyles]

I've read somewhere about a suggestion to replace the expansion tank cap with the one from a diesel version, as it has lower pressure tolerance and helps preventing various coolant leaks, esp. in radiators. Since I have 3.5d I didn't pay much attention, but something for non-diesel owners to consider, I guess...