I recommend you switch to a 1.4 bar expansion tank cap as soon as possible. Instead of the car letting you know you have a problem by letting go at the cap, the 2.0 bar is stronger than your brand new hoses, and let’s go by blowing slightly used cooling parts.

This isn’t some high performance cap. I know someone with a v12 Lamborghini that has a 1 bar cap. This is bmw perpetually making sure there is a client list for exploded tanks, hoses radiators and valley pans.

Swapped mine out years ago and never looked back.

Amen in to that! I swapped mine out when I did my cooling system refresh at 90k, I’m sitting just under 115k now so nearly 25k miles and no adverse affects (except my expansion tank probably won’t blow up again...) [emoji106]

BMW part number is: 17 13 7 739 023


2005 X5 4.4i (04/05 build date)
Titanium Silver w/Black Interior

Edit... part # is 17 13 7 639 023 - apologies for my fat fingers


2005 X5 4.4i (04/05 build date)
Titanium Silver w/Black Interior

I’m at like 50k miles since switching cap. Use car year round. So yea.

Ty for part number. I was too lazy to look for it lol.

Remember, the system is sealed, so whether you have a 1.4 bar or 2.0 bar cap, an overheating engine is going to expand into the ET which has a limited ability to relieve pressure before the entire system develops enough pressure to create a "new" relief (e.g., cracking the ET, blowing a hose, whatever is the weak point).

I have installed 1.4 bar caps on my BMW's, but as I've come to understand the system better, I'm not convinced it really makes much (if any) difference. The best thing you can do to minimize the chances of a pressure failure is to keep your coolant level at the minimum full level. I run mine with the top of the float stick just even with the top of the ET.

The more full you run your ET, the less capacity it has to absorb coolant/pressure from the system. Remember that the cap does not vent externally. It only vents to the ET.

AM.

Overheating an engine due to cooling system problems happens very quickly and can easily cause catastrophic damage. Cooling systems are designed for a specific maximum pressurization and boiling point. Changing the pressure release max of the cap is an unnecessary risk. Changing the specified mix of 50/50 coolant is also a risk. I suggest checking the components about every 10,000 miles and a PM interval of 75,000 to 100,000 miles using OE or OEM parts.

BMW used to vent the tank externally... but then again BMW's used to be a more low-maintenance vehicle too. :D

I agree on the 100,000 mile service regimen for the cooling system. I tend to do it all at once every 100,000 miles. Is it wasteful? Maybe. But it gives me the warm and fuzzies so that's what I do.

Every other car I've ever owned has vented externally if you overfill, making it simple to fill coolant after a flush or maintenance. That decision still has me a little baffled.

I'm pretty sure the coolant res is in the active loop and already pressurized. Not sure how you would vent into a pressure zone. That said I never saw an overflow tube either


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Yes it is. I tried to find where I read about how the cap equalizes the pressure between the rest of the system and the ET. In my futile search, I read a bunch of other contradictory stuff. In the end, I'm still not sure if it accomplishes anything to run the lower pressure cap. Why did BMW spec such a high pressure cap? Surely it wasn't because it was just a nice round number!? :dunno:

AM.