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Hi all, I’ve got a 2002 M3 but I’m posting in the general forum as the same thing could maybe happen with any model.

So the issue. The car has been in storage for a couple of months but I needed it about a fortnight ago. All was good when I put it away and it started and drove fine when I got it back out however I’ve noticed that after about 15 - 20 mins the coolant in the reservoir starts to boil! Temp gauge is normal, coolant level is normal and the car drives fine. The coolant mix is correct ie 50/50. I service my own cars (and have for many years) so I’m confident re the mix.

My first thought is that the expansion tank cap is failing. The car has only done 70000 miles but it’s still 18 years old. Anyone have any other thoughts?
 

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Has the cooling system been overhauled? While the mileage consensus is that the system needs to be overhauled at 100k miles, 18 year old plastic parts are likely in need of replacement. First I’d check to see if there are any combustion gases in the coolant. There are test kits you can buy fairly cheaply (<$20; £16). Next I’d do a cooling system pressure check. If these tests come back ok, I’d bleed the cooling system just to make sure that there’s no air in the system.
 

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how can u tell coolant is boiling?
Yes, how?
He might have a clear glass reservoir tank :)

Yes, I wonder this too; did he open the tank cap to check boiling? and of course it boiled hard with open cap.
My guess: The presence of steam at the reservoir cap and audible bubbling of the coolant in the reservoir?

But, I would also like to know how do you know it's boiling?
 
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