The first failure wasn't due to mileage, it was due to time. May be a hotter ambient that you were running in, perhaps the vehicle didn't always have the proper coolant (just water), don't know. But yes, these tanks can fail. The second failure wasn't the same, it appears to have been a defective assembly, or rough handling. That is what they wanted to see. The OE part had a two year warranty (usually only if the dealer installs it, not for just a parts sale), the aftermarket part wouldn't typically have that warranty. If you want to claim parts warranty, though, you do have to go through the service department as they administer it.

Thanks JCL. I think I would have felt better if I was told it was out of the warranty period. I was actually surprized when they said 2 years. Never seen where the service dept got involved in a parts dept sales ticket.

Is there something to look for to see if the tank or hoses are getting near end of life?

All of my hoses look and feel fine. No cracking, not hard same with the tank. It looks fine. Replacing everything is well over 300 bucks and if needed I'll do it but I'd like to know it is necessary.

Thanks for all the great info here and all over the site.

A hose that is soft is on its way out.

Trouble is, that is a one way test. A soft hose should be replaced, but one that appears fine may go just as quickly.

If you have a slow coolant leak, it is worth tracking it down as it is likely to get worse, not better. Do it at a time and place you choose.

Those that are replacing all hoses and the expansion tank as a preventative maintenance step aren't really doing it because of the hose, per se. It is the same work to do it before or after failure. They are doing it because after a hose goes, if you don't pay attention and know about the potentially expensive consequences of overheating a BMW engine, you can be in for $$$. I haven't replaced my hoses or expansion tank based on time. But I also know that if I run out of coolant, I won't be driving it to the next town to get it fixed the next day. I will be pulling over because I don't want to worry about a warped cylinder head.

When I said not hard I meant as in when they get old and harder than when new like when they get hard and start cracking.

As I have no idea how old they are I am leaning towards peace of mind replacements but I would like to pick my battles and replace the things that make sense.

On a risk vs reward basis this seems like something to do because potentially destroying the motor is $$$$.

Something else to watch for is when those nice black plastic (nylon?) hose ends/quick connects start looking less black and look more brown. My experience has been that the plastic hose ends age faster than the rubber hose itself and become brittle and crumbly, self-destructing on a whim.

But maybe aftermarket hoses have better plastic, and the rubber dies before the ends do? I only have experience with OE. 10 years/100k is what I expect from the hoses, anything longer than that is a bonus, but pushing your luck.

I replaced the radiator this past fall at 130k, planned on replacing all the hoses this spring, but while bleeding out the system, the upper hose disintegrated at the radiator fitting. I replaced the other hoses along with that one.

Couple weeks ago, noticed the expansion tank is starting to form some vapor deposits along the seams...

Coolant Leak, Looks like expansion tank!
 

Hello folks! About a year ago I replaced my expansion tank. It wasn't OE, it was one of the other ones ECS sells. I don't think I got the cheapest one, I am trying to check my order history now. Nonetheless, I have a coolant leak now and based off this pic it looks like the expansion tank. (Should have got Genuine BMW)

http://s15.postimg.org/aau1flisn/coolant.jpg

I added coolant this morning with the heat on 90 deg F, fan low, and bleed screw open. What puzzled me was that nothing came out of the bleed screw. Last time I recall seeing a stream circulate in the expansion tank with the heat on and I would fill until some came out of bleed screw. I added about a quarter gallon and the float was at the max level. I probably added more than I should but nothing came out of the bleed screw! Why is this?

Actually it might have been OE! I ordered it from getbmwparts.com and there is only one option for the reservoir. It was purchased on Aug 2012 for $78.
Does not mention any brand.

getbmwparts is a dealer. BMW of Silver Spring | New BMW dealership in Silver Spring, MD 20904

nothing coming out the bleed screw means there is still air coming out the bleed screw. either park on an incline with the nose uphill while filling, or drive it and check it again after it has cooled down.

I'd contact them about it. Isn't there a 2 year warranty on all factory parts? Also, they have a few parts that if you get them installed at the dealer they have a lifetime warranty...BMW Service : Warranties

I installed it myself. So I don't know about the lifetime warranty. If I do find physical damage, maybe they can replace it.

I checked the coolant again and the float was a little bit lower. But I did see the stream circulating the coolant in the reservoir. I loosened the bleed screw and saw coolant come out. So either a: There was a good bit leaking since 10 AM this morning or b: There was an air pocket that got out, which required more coolant.

I'll check again tomorrow morning. Correct me if I'm wrong, but aren't you supposed to check the coolant when engine is cold i.e. first thing in the morning when engine hasn't been on since the previous day? The second time I checked it was after it cooled down for only 2 hours..