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:
- 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.