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3d dwg files download. Some BMW models are notorious with getting. 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: • coolant cap unable to release excess pressure allowing pressure build up in cooling system?
• leaking head gasket where combustion pressure gets into the cooling system? • 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)? • overfilled coolant system? • anything else that I may have missed? • maybe I am overthinking all of this and the cheap plastic BMW coolant reservoirs are meant to last only 5 years. 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. 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. – May 22 '17 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. – May 26 '17 at 6:18.