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Old 03-09-2013, 09:06 PM
gary@germanautosolutions's Avatar
gary@germanautosolutions gary@germanautosolutions is offline
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Location: Grand Rapids, Michigan
Join Date: Jul 2010
Posts: 425
Mein Auto: E39 528i, E60 M5
I don't get on here as often as I would like, so I just stumbled across this thread.

There is a lot of good information here as well as a lot of misinformation.

When developing our coolant cap we risked possibly damaging two engines, an M52 e39 and an m54 e46. We tapped a pressure sensor into the air space at the top of the expansion tank and data logged system pressures from cold start up to full overheat (temp gauge pegged). To cause an overheating situation we removed the clutch fan and slid a full size sheet of cardboard between the radiator and AC evaporator to block the flow from the electric fan. We ran tests at proper fill levels, low fill levels and high fill levels.

We found the following:
  1. The system runs below 1.0 bar at all normal operating temps.
  2. A lower fill level results in lower pressure and a higher fill level results in higher pressure.
  3. Overfilling the expansion tank beyond the proper cold fill level can result in system pressures over 2.0 bar, at which point the OEM cap will start to vent excess pressure.
  4. Since the cooling system pressure, with a proper amount of coolant in the system (not overfilled), does not exceed 1.0 bar at any normal operating temperature, or even moderate overheating, the use of a 1.2 bar cap will not effect cooling system efficiency in any way.
  5. 1.2 bar equates to a system temperature of approximately 126 deg C (260 deg F), which also equates to a fully pegged temp gauge.

For those that argue that the engine was engineered by BMW to use a 2.0 bar coolant cap and you shouldn't mess with it, consider this. BMW has used a 2.0 bar coolant cap on many models from 1982 to late 2000 something. These models have included iron block iron head engines, iron block aluminum head engines, and aluminum block aluminum head engines, in 4 cylinder, 6 cylinder, 8 cylinder and 10 cylinder configurations. Either they start from the coolant cap and design their engines from there, or the coolant cap is not an integral part of the engine design.

BMW has also moved away from the 2.0 bar system with most if not all current models using a 1.2 bar cap. I think the original thought process was to use a cap that was at the limit of the cooling system rated pressure to give the greatest amount of extreme overheat boil over protection. What they failed to take into account is that a 5 or 10 year old system has lost some of it's original strength, and that reaching a 2.0 bar pressure in these vehicles can cause things to go boom.

By the time my engine has overheated to the point that the bong and dash message have warned me, and my pegged temp gauge has warned me, I would rather pull over, shut off the engine, and let the 1.2 bar cap vent any excess pressure, rather than let the pressure rise until something expensive goes pop and I also need to call a tow truck. I also like the peace of mind that comes from knowing that if while in a hurry I overfill my expansion tank a little, that I don't run the risk of a catastrophic cooling system failure.

I would like to thank everyone again for all the support you have shown, and all the kind words you have expressed to German Auto Solutions. It's been a fun and busy first year with many good things in the works for the coming year.


"I'm a little rascal on my Little Rascal"
- unknown

Last edited by gary@germanautosolutions; 03-09-2013 at 09:09 PM.
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