if it ain't broke....
 

After reading BimmerBreaker's comments, and remembering the fact that even on most '40-early 50's American cars, pressure caps were non-existent at first, then became 7 psi on some cars, then atmospheric 14.7 pressure (usually labeled 14-15-16 psi) by the '60's, I tend to agree with BimmerBreaker. There's a reason that manufacturers started using increased-pressure for their radiator caps.

I drag-raced with a person that also did some SCCA events (in a different car), and he used high-pressure caps on both cars. He was convinced they helped cool the heads better, but I never bought into that, at the time. Since my racing was only in 10-11 second bursts, I was satisfied using

• 1) water+"water-wetter" additive, as my coolant,
• 2) the stock 15/16 psi Stant cap I could buy anywhere,
• 3) an oversized, soldered-not-epoxied aluminum radiator (no plastic tanks, with a separate/isolated radiator & fan for the auto trans),
• 4) two high-powered electric fans,
• 5) no thermostat, just a flow restrictor instead,
• 6) a 55 gpm Meziere electric water pump,and
• 7) temperature sensors in radiators, intake, and heads

I could control my engine's temperature by selecting different combinations of switched fans & pump. My X5 isn't raced, so I'll let the system remain stock...and stick with the stock 2.0 bar cap for now, anyway. The tank is relatively new (under 3k miles, I think) so I don't think it'll blow any day soon, and the 2.0 bar cap seems to be self-regulated OK, so if it ain't broke....

If your system reaches a pressure of 1.4 bar, you are already in an overheat situation. Allowing that pressure to continue to build to 2.0 bar is only delaying the inevitable. The difference is that the ET or possibly an aging hose will let go before the 2.0 bar cap begins releasing pressure.

Your argument would be valid in the rare circumstance where the cooling system pressure spiked to somewhere between 1.4 and 2.0 bar, but then returned to normal. I can't think of a situation where that would be likely. In virtually all situations, if you hit 1.4 bar, 2.0 bar is arriving shortly (unless something else gives way first).

AM.

The pressure in the cooling system is increasing because it is overheating, it is not overheating because the pressure is increasing. It is true that decreasing pressure does decrease temperature - but a lower pressure cap doesn't decrease the pressure. It just vents coolant out once the pressure is reached and it will maintain that pressure.

The critical thing here is that it vents coolant - this means that coolant is removed from the system, decreasing the thermal mass of coolant in the system and decreases the ability for the cooling system to remove heat from the engine.

That lower pressure, as mentioned earlier, also means lower boiling point. To get really nitty gritty into the weeds, lower pressure also means less cooling ability (less movement of coolant molecules against the hot surfaces it is trying to cool).

So with a lower pressure cap, during an overheat - you've created a system that cools less efficiently due to lower pressure, boils prematurely due to lower boiling point, and vents coolant prematurely to remove coolant from the system.


And no, things do not just blow up at 2.0 bar. Things blow up prematurely after repeated heat cycles when overpressurized. And THAT happens when people overfill their cooling systems.

BMW's with lower pressure caps have different cooling systems and run different pressures from the factory. The same temperature that puts a BMW with a 2.0 bar cap at 2.0 bar pressure, would put a 1.4 bar car at 1.4 bar.


To put my money where my mouth is, here is my own personal car at 2.0 bar. I let it sit like this for a couple hours - nothing blew up. I even took it up to 2.2 later just for fun, still, nothing blew up.
And this isn't a brand new cooling system. In fact, this particular cooling system is in an engine swapped car of mine where hoses have been hodge podged together, quick connect fittings were cut off various hoses and hose clamped onto other hoses of unknown age and origin. Hoses have been cut, modified, all sorts of stuff.

https://static.wixstatic.com/media/d...9b9787~mv2.jpg

A factory cooling system in even remotely decent shape will fare better than that car and yet that car still withstands 2.0 bar perfectly fine.
A factory cooling system, that hasn't been stressed due to being overfilled, can easily withstand 2.0 bar on numerous occasions.


Lower pressure caps are not better because they vent earlier during an overheat situation, they are WORSE because of that fact. They exacerbate and hasten overheat conditions due to them providing less cooling ability in the first place due to less pressure, venting coolant prematurely allowing for less thermal mass to absorb and dissipate heat and lowering the boiling point allowing boiling in the head.

And during normal operation, a lower pressure cap changes nothing because during normal operation a car is running a pressure 1.0 bar or lower.

There is no benefit to a lower pressure cap.

Thanks for the thoughtful discussion. We'll have to just agree to disagree. Have a great weekend!

AM.

It seems the problem is realising the cause and effect. On BMW engines (that I know of) the expansion tank cap pressure is chosen due to the normal running temp the engine is supposed to work. Like said the system is supposed to be sealed/pressure proof thru the operating temp and pressure range. The expansion cap is only the ultimate fuse when all things have already gone wrong and the cooling system is not working as it's supposed to do. It will not alter how the engine or cooling system works but only sets the fuse to blow prematurely and possibly causing severe adverse effects on the engine itself when salvaging other cooling system components.

Bentley says to test it, go to 1.5 bar. It should not drop more than 10% in 2 minutes.

The last burp was probably caused by over filling it. I topped it off about a month ago. Weather was still hot then. It only need just a little bit. I added too much blue coolant. I figured it would burb if needed.

The fill level should be set when cold. If the last time it was filled it burped, it is probably still overfilled - it won't burp down to the correct amount (operating around ~1 bar) it will only burp to just below the cap's rating (2.0 bar). As such, your cooling system is probably constantly operating ~1.9 bar during normal driving.

These are not old American cars where the system will just burp excess coolant. More coolant is not better.
People doing this and blowing components in their cooling systems prematurely, is I believe the reason why BMW cooling systems have a reputation for being unreliable.

Let's say the coolant temp range is 120 degrees C, the heat expansion of 50% ethylene glycol solution is about 7%. On BWM coolant systems the total coolant capacity is roughly 10 litres. So the expansion tank needs to have at least 0,7 litres of empty (compressing gaseous) space if the level is checked at 0 C.

Yep, lower pressure cap needs more space so there is going to be less reserve before the coolant level light comes on.

Pressure test should be a little more than the cap rating, so 1.5 bar would be good for a 1.4 or less cap. The 10% in 2 minutes is pretty much meaningless, the tester itself could leak that and there could still be a leak if that holds. Always eyeball and listen for leaks ;)

Just as a heads up, RockAuto has the CXT100 (Standard Automotive) expansion tank on for $27 Canadian! Includes the trans thermostat and coolant sensor! I put one on my X5 a while back and it was $93 USD and just now ordered one for the wifes X3. This is supposedly a much improved tank and I will say it does look like it is.