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Some BMW models are notorious with getting cracked coolant reservoirs. Yesterday I did a UV dye test to locate my leak and it appears that in my case coolant reservoir has one of these small hairline cracks as well.

Now, before I try to fix anything, I am trying to understand what could have contributed to the crack in the first place.

So, I have brainstormed some possible root causes that may have contributed to the reservoir's crack. I would like to hear your subjective (or objective) opinion w.r.t. their likelihood contributing to the crack and signs that I would have to look for to prove or disprove each of these potential root causes:

  1. coolant cap unable to release excess pressure allowing pressure build up in cooling system?
  2. leaking head gasket where combustion pressure gets into the cooling system?
  3. non-BMW coolant used in the past that may have different expansion ratio than "BMW gray" coolant or that possibly may have caused corrosion in the reservoir's walls (since I don't know what additives are in the coolant this is pure speculation from my side)?
  4. overfilled coolant system?
  5. anything else that I may have missed?
  6. maybe I am overthinking all of this and the cheap plastic BMW coolant reservoirs are meant to last only 5 years. Main

Closeup This is 2008 BMW x3. While I don't have full vehicle service history there is possibility that in 2012 it already had coolant reservoir changed due to a similar coolant leak. Also 6 months before the low coolant light came on first time it had "oil filter housing gasket" or "engine gasket" changed - can't quite tell from the handwriting on the receipt.

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  • Having an image of exactly where the crack is at would help tremendously here. There's a single (and very likely) cause for your issue, which would be vibration induced crack from a stress riser. Without the image, we can't tell you if this is a possibility. This could really just be a design flaw.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    May 21, 2017 at 19:52
  • @Pᴀᴜʟsᴛᴇʀ2 I added two pictures. The crack appeared to more or less where is the small red dot in the red circle.
    – user2138912
    May 22, 2017 at 6:07
  • Seems very unlikely that excessive pressure could build up in the system able to break the part at that place. It's probably just a cheaply made part suffering fatigue from vibrations. Had BMW spent a mm or two of extra matierial it'd be probably fine for the life time of the vehicle. You could try to put a piece of rubber or something under the hose to absorb some of the vibrations to prolong the next part's life. - Or, BMW may have realized the problem and improved on the part, so that your new part won't suffer that problem again. Happens from time to time.
    – JimmyB
    May 22, 2017 at 11:24
  • @JimmyB Actually on this BMW X3 there is already a piece of rubber around hose that enters coolant reservoir. It is in resting position and I believe was intended to mitigate vibrations. While this cracking may still be caused by regular vibrations I am wondering if blown head gasket may have actually caused periodic pressure fluctuations in the cooling system and that somehow contributed to the coolant reservoir wall fatigue. I will get one of these blown head gasket kits and check for exhaust gases.
    – user2138912
    May 26, 2017 at 6:18

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Yours is not the only mark \ model that has this type of issue : jag x type are known for leaking coolant tanks, again made of plastic.

The plastic tank is a "cheap" solution but has a limited life. A copper or brass metal tank will have a longer life, but weight and cost come into play.

As long as your pressure cap is working correctly then excess pressure should not be an issue.

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  • I replaced the coolant reservoir and in a year hose started to leak. Now wondering if pressure cap is stuck in "failed-closed" state. Will have it tested.
    – user2138912
    May 24, 2018 at 22:57

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