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Old 03-09-2013, 09:10 AM
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540iman 540iman is offline
resident, old fart
Location: N.W. Indiana
Join Date: Sep 2008
Posts: 1,873
Mein Auto: 2002 540i sport
Originally Posted by doru View Post
Because the 1.2 bar cap holds the coolant pressure up to 126C or 260F. If you ever reach that temperature, chances are your engine has or starts to overheat. And if you reach that temperature, something else is wrong, not the expansion tank cap.
For example, I know 100% that if I see the KTMP going past 98C, one culprit could be a bad fan clutch. And with a bad fan clutch I noticed KTMP as high as 102C. Which is still below 126C. I adressed the issue, the KTMP stays where it should be now. That's for i6. For v8 the normal operating temp is higher.
So, yes, I think it's pretty safe. Thanks for pointing it out though. One can never be too careful.
This is what scares me when someone goes from temperature to pressure. They are in a proportion to one another, but pressure and temperature only have a certain relationship depending on the fluid. Water at 126 degrees C will have a huge difference in pressure than oil at 126 degrees C or a 50-50 mix of coolant, or a 75-25 mix of coolant. Just keep a 1.2 bar cap for what it is. Has nothing to do with temp-only pressure. You make your own assumptions as to what fluid is in the closed system and what amount of air gap exists that the fluid may expand to fill first. Cap will just vent to atmosphere at a lower pressure and that is all. If it does vent and your coolant level decreases, you have less coolant for the radiator to use to cool. Pretend you had a .6 bar cap. You would never over-heat until most of your fluid was all along the road behind you. So, the over-heating would be CAUSED by a cap releasing fluid when the pressure was too low....having NOTHING to do with temperature other than it gettinhot enough to overcome a .6 bar relief valve. Take what you said about a 1.2 bar cap holding the coolant up to 126 degrees. TYour first sentence is already making assumptions you may not have such as absolute mixture. What if your radiator was full of oil? Far fetched sure, but 1.2 cap would hold it just fine.

Last edited by 540iman; 03-09-2013 at 09:15 AM.
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