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E39 (1997 - 2003)
The BMW 5-Series (E39 chassis) was introduced in the United States as a 1997 model year car and lasted until the 2004 when the E60 chassis was released. The United States saw several variations including the 525i, 528i, 530i and 540i. -- View the E39 Wiki

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  #26  
Old 03-08-2013, 07:38 AM
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Originally Posted by ztom View Post
This seems good information. If combustion gas isn't getting in, then the cap doesn't make a difference, it's only a pressure release if a problem. If combustion gas is getting in i.e. cracked head, then the pressure will go the limit regardless of engine or coolant temps, and then a lower bar cap may help by not overpressurizing the system and getting a burst.
........and lower the chances of blowing the head gasket (which is a major +).
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  #27  
Old 03-08-2013, 08:02 AM
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Originally Posted by mifesto View Post
people here dont understand the cap doesnt cause pressure, the heat does. heat is produced and maintained by thermostat opening and closing to route water to the radiator. rad cap is simply a safety device to allow pressure to escape if necessary. venting in a car ejects precious coolant and introduces air into the system; this is NOT the solution to lower pressure. lowering temperature is the ONLY way to reduce pressure of a ethylene glycol/water mixture. this total failure of people's understanding may be especially bad for the 540is(as they run hotter) and hit that 1.2 bars sooner then the i6 guys.
Finally, a ray of light. BMW owners seem (I do too at times) to jump at any perceived superior device than what BMW gave us. The pressure that the cap will hold will make absolutely no difference in how your car performs, how you will survive say a massive radiator leak while stuck in traffic, or in normal driving. You could put a 3 bar cap on with no difference in how the car performs, the temperature the car attains nor will it possibly save you in a failure. Where it may make a difference is if you accidentally over-fill the system to the point where the coolant has no place to expand to. The cap should (if the coolant reaches an excessive pressure) release the "extra" coolant. The tests performed here on this post demonstrate that this will exceed 1.2 bar or 1.4 bar and the longer you can keep your coolant in the closed system the better. Remember the cap is rated in pressure and no relation to temperature. A cooling system full of just water will increase pressure at a lower temperature than one filled with coolant. Cap Has no effect on temperature, only on pressure.
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  #28  
Old 03-08-2013, 08:15 AM
Dragan Dragan is offline
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Originally Posted by 540iman View Post
Finally, a ray of light. BMW owners seem (I do too at times) to jump at any perceived superior device than what BMW gave us. The pressure that the cap will hold will make absolutely no difference in how your car performs, how you will survive say a massive radiator leak while stuck in traffic, or in normal driving. You could put a 3 bar cap on with no difference in how the car performs, the temperature the car attains nor will it possibly save you in a failure. Where it may make a difference is if you accidentally over-fill the system to the point where the coolant has no place to expand to. The cap should (if the coolant reaches an excessive pressure) release the "extra" coolant. The tests performed here on this post demonstrate that this will exceed 1.2 bar or 1.4 bar and the longer you can keep your coolant in the closed system the better. Remember the cap is rated in pressure and no relation to temperature. A cooling system full of just water will increase pressure at a lower temperature than one filled with coolant. Cap Has no effect on temperature, only on pressure.
I'd like to just point out that temperature and pressure are directly proportional to one another. One goes up, so does the other assuming the volume of the system remains unchanged. If it the temperature/pressure of the cooling system gets high enough (2 bar in this case), the spring-loaded cap will vent the excess pressure to atmosphere.
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  #29  
Old 03-08-2013, 10:53 AM
mifesto mifesto is offline
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Originally Posted by Dragan View Post
the spring-loaded cap will vent the excess pressure to atmosphere.
not just pressure but also coolant!! thats whats scary... some misguided people are using this as a means to controlling system pressure on a regular basis! rad caps are meant to be a last ditch device to release pressure....
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  #30  
Old 03-08-2013, 12:00 PM
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Originally Posted by mifesto View Post
not just pressure but also coolant!! thats whats scary... some misguided people are using this as a means to controlling system pressure on a regular basis! rad caps are meant to be a last ditch device to release pressure....
Am I correct in assuming that, in normal operation, there is no venting going on, right?
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  #31  
Old 03-08-2013, 12:06 PM
Dragan Dragan is offline
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Am I correct in assuming that, in normal operation, there is no venting going on, right?
Correct. The cap is simply a relief valve for safety purposes. I feel as if people are under the assumption that the cooling system operates at 2 bar. It's nominal operating pressure is likely much less. The system is always completely sealed unless it's exposed to atmospheric pressure either through the cap (which houses the relief valve) or through a leak somewhere in the system.
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  #32  
Old 03-09-2013, 09:10 AM
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540iman 540iman is offline
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Originally Posted by doru View Post
No.
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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  #33  
Old 03-09-2013, 11:46 AM
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chiefwej chiefwej is offline
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The cap may look nice, but to reduce the pressure in the system you need to change the coolant not the cap. I have Evans NPG in it with a zero pressure cap on, (I removed the O-ring on my cap).
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  #34  
Old 03-09-2013, 01:08 PM
Mikes530 Mikes530 is offline
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Quote:
Originally Posted by 540iman View Post
Finally, a ray of light. BMW owners seem (I do too at times) to jump at any perceived superior device than what BMW gave us. The pressure that the cap will hold will make absolutely no difference in how your car performs, how you will survive say a massive radiator leak while stuck in traffic, or in normal driving. You could put a 3 bar cap on with no difference in how the car performs, the temperature the car attains nor will it possibly save you in a failure. Where it may make a difference is if you accidentally over-fill the system to the point where the coolant has no place to expand to. The cap should (if the coolant reaches an excessive pressure) release the "extra" coolant. The tests performed here on this post demonstrate that this will exceed 1.2 bar or 1.4 bar and the longer you can keep your coolant in the closed system the better. Remember the cap is rated in pressure and no relation to temperature. A cooling system full of just water will increase pressure at a lower temperature than one filled with coolant. Cap Has no effect on temperature, only on pressure.
No disagreement (in theory) to any of this. There are questions though:
- why is the industry standard 1.1 - 1.2 bar when the materials and system designs are fundamentally the same as our cars?
- why did BMW specify a 2 bar cap for the e30 and subsequently reduce it to 1.4 bar? I assume that there was a reason and it wasn't done on a whim.

Two possibilities (based more on speculation than anything):
- people could grossly overfill their cooling systems, either because they didn't know any better or because their indicator rod was stuck (seems to happen quite often). This will result in a higher system pressure and a 2 bar cap will prevent venting. Venting is bad for environmental and/or owner perception reasons. If you don't think that owner perception is huge at BMW refer to the (useless) buffered temp gauge and "lifetime" tranny fluid.
- if someone was to check their coolant level in very cold ambient temperatures, the coolant level would appear low. Topping up would again result in an overfill situation compared with the level in a more normal ambient temp.

The problem with preventing venting when it might be appropriate is that something's gotta give.....
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  #35  
Old 03-09-2013, 06:00 PM
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Originally Posted by doru View Post
I think BMW did a mistake with the 2 bar expansion cap, and fixed it on the newer BMW models to 1 bar expansion caps to prabably avoid the catastrophic cooling system failures (ruptured radiator necks, split expansion tanks to name a couple). Pretty much for the same engine (N52 vs M54 - not much different).

Edit: The above statement is wrong. The source I quoted was off.
thats right , E60 has 2 bar expansion cap but their coolant system does not fail like E39
The only complains about WP , all the rest last > 100k miles easy
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  #36  
Old 03-09-2013, 06:50 PM
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Originally Posted by doru View Post
From Gary (German Auto Solutions). It's a thing of beauty. The top is engraved steel and the relief valve is "Reutter" - the OEM supplier for the OEM expansion cap. The beauty is that "if" the temp is getting too high, it will release the pressure much sooner than the OEM cap, thus having a chance for the plastic parts to last longer. I would venture to affirm that if the cooling parts will rupture due to pressure, at a lower pressure the damage could be smaller, i.e. the crack could be just that: a smaller crack, which could allow you save the engine. At higher pressure, a crack will develop very fast in a big rupture, where one could loose cooolant very quickly.

Thanks Gary. Beautiful product.



Just did a complete cooling system overhaul and I'm still trying to understand all of this talk to see if this will really be better for my cooling system.

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  #37  
Old 03-09-2013, 09:06 PM
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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.

Gary
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Last edited by gary@germanautosolutions; 03-09-2013 at 09:09 PM.
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  #38  
Old 03-10-2013, 07:13 AM
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540iman 540iman is offline
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Gary, we agree on everything, but your statement, as written in #5 is simply not true UNLESS you also include your assumptions! Really not trying to nitpick, I swear! You can't just make a statement like that..It is probably an oversight, but is really misleading as written. You must state what material you are heating to 126 Degrees C to equal 1.2 bar and how much free air space existed. If you want to say that the E39, when filled to the correct COLD level and containing a perfect 50-50 mix of anti-freeze will be at 1.2 bars of pressure if it reaches 126 degrees C then you have a possibly valid statement. I trust your math, but you can't make a statement without specifying the liquid involved and the air gap. Like you pointed-out earlier in a way-air can compress, a liquid can't. Water can only compress if it changes state; like from a liquid to a solid.

The single most compelling reason in my mind for the lower pressure cap is for those that over-fill their reservoir on a regular basis. Then you stress the entire system to 2 bar to let the excess coolant out so the system can return to under 2 bar. If you fill the system full, the pressure in the system will exceed 2 bar and release ONLY enough coolant to get the pressure back to under 2 bar...say 1.9 bar for arguments sake. The next time you start and run your car, the coolant level will still be too high and the pressure will likely not reach 2 bar a second time, but may reach 1.9 bar quickly and stay there, in theory, forever, or until you somehow lose enough coolant to create the proper air space. For this reason, the lower pressure cap makes a lot of sense, but not to control your temps or what your cooling system wants to do. It is only an advantage for those who A) like the look and B) over-fill their systems regularly.

A system will only be at 1.2 bar @ 126 degrees with a very certain liquid in the closed system. I seriously doubt that it is possible to know exactly that you are at 50-50. Maybe close, but you never have a completely bone dry system to start with. Let's agree to not mix pressures and temperatures- just to know the relationship and the properties of the liquid in the system to make "assumptions". I really appreciate your testing and your thoroughness. Your statement was obviously making some assumptions that some might not get while others would. I especially like your objective evidence of the pressures observed with the proper fill level and the right mix of coolant. The car never exceeded even 1 bar. That was enlightening to me and thank you for that. Beautiful piece BTW! definitely FTW. Link to order please guys.
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  #39  
Old 03-10-2013, 07:36 AM
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bluebee bluebee is offline
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Quote:
Originally Posted by gary@germanautosolutions View Post
We found the following
This is excellent information for the thread on how the cooling system works!
-> E39 (1997 - 2003) -> What is the coolant temperature of an E39 (I6 or V8) under normal operating condition

See also:
BMW_M62_Engine_Details.pdf (570.8 KB, 125 views)
BMW_M62_Thermostat_Details.pdf
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Last edited by bluebee; 03-10-2013 at 07:40 AM.
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  #40  
Old 03-10-2013, 07:45 AM
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540iman 540iman is offline
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  #41  
Old 03-10-2013, 09:13 AM
JimLev JimLev is offline
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Originally Posted by Dragan View Post
Has anyone actually had a pressure gauge hooked up to the cooling system in the following conditions:

Idling when cold

Idling at operating temperature

at 2500 rpm with fan blowing air through rad to simulate highway cruising

I'd be curious to see all the pressure readings under these conditions.
And another thing that hasn't been clarified, is the 2 bars of pressure gauge or atmospheric? I assume it's gauge since that's how we measure everything else (tires, fuel pressure, etc.).
Here ya go. This is on my 2000 540 (M62tu engine) with the 2 bar cap. I do have a 1.4 bar cap but didn't use it as the cap doesn't effect the pressure.

I did this last fall, so the outside air temps were not in the 90's, no AC either.
I'll do it again this summer and see if it changes much.
It was posted and graphed on the other message board.
I heated the engine up in my driveway with the electric fan off to allow the engine to heat up quicker.
When the KTMP reached 103C I took the car out for a spirited drive, the temp never rose above 105C.

KTMP Reading/PSI Reading
40C/1.5 PSI
45C/2 PSI
50C/2.25 PSI
55C/2.5 PSI
60C/3.0 PSI
65C/3.25 PSI
70C/3.5 PSI
75C/4.0 PSI
80C/5 PSI
85C/6 PSI
90C/6.5 PSI
95C/7.5 PSI
100C/9.0 PSI
105C/16.5 PSI

When I returned from my drive I released some pressure via the cap. It was then down around 5 PSI. I went out for another drive, when I returned it was still at 5 PSI.
I'm going to modify the t-stat heater circuit to drop the temp which will drop the pressure.

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  #42  
Old 03-10-2013, 09:16 AM
Dragan Dragan is offline
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Thanks for that! Now if someone would do the same for the I6.
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  #43  
Old 03-10-2013, 09:20 AM
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doru doru is online now
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Quote:
Originally Posted by 540iman View Post
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.

Actually I don't make ANY assumptions. It's my car, and I service it. I know what coolant I have, and what proportions of coolant/water I have. Maybe you don't know what you have in your car, but I do know what I have in mine.
And just for your info: it's a 50/50 BMW coolant/water mixture.
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  #44  
Old 03-10-2013, 01:34 PM
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gary@germanautosolutions gary@germanautosolutions is offline
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540iman,
Quote:
Gary, we agree on everything, but your statement, as written in #5 is simply not true UNLESS you also include your assumptions! Really not trying to nitpick, I swear! You can't just make a statement like that..It is probably an oversight, but is really misleading as written. You must state what material you are heating to 126 Degrees C to equal 1.2 bar and how much free air space existed. If you want to say that the E39, when filled to the correct COLD level and containing a perfect 50-50 mix of anti-freeze will be at 1.2 bars of pressure if it reaches 126 degrees C then you have a possibly valid statement. I trust your math, but you can't make a statement without specifying the liquid involved and the air gap. Like you pointed-out earlier in a way-air can compress, a liquid can't. Water can only compress if it changes state; like from a liquid to a solid.
Yes, I was making the assumption of a 50/50 mix of water and OEM BMW antifreeze, filled to the correct cold level, and as measured in an m52 or m54 engine. It was a bullet point list and I didn't see the need to get much into the engineering math behind the measurements. But, you are absolutely correct.

Quote:
The single most compelling reason in my mind for the lower pressure cap is for those that over-fill their reservoir on a regular basis. Then you stress the entire system to 2 bar to let the excess coolant out so the system can return to under 2 bar. If you fill the system full, the pressure in the system will exceed 2 bar and release ONLY enough coolant to get the pressure back to under 2 bar...say 1.9 bar for arguments sake. The next time you start and run your car, the coolant level will still be too high and the pressure will likely not reach 2 bar a second time, but may reach 1.9 bar quickly and stay there, in theory, forever, or until you somehow lose enough coolant to create the proper air space.
Thank you for pointing this out as I forgot to mention it. If you over fill the expansion tank, the system can run at excessive pressures for many drive cycles, not just one.

Quote:
It is only an advantage for those who A) like the look and B) over-fill their systems regularly.
I disagree with this statement. Due to any number of reasons your vehicle can overheat. Just a few examples would be broken fan belt, bad clutch fan, bad electric fan, clogged radiator fins due to leaves or dirt, etc. In any of these situations you have ample warning via the dash display and temp gauge that something has gone wrong. I would rather the cap limit the max pressure to 1.2 bar than to have something in my coolant system explode, leave me stranded and cost me much more money.

Quote:
98-104 degrees C iirc
Correct, the m54s run a little hotter at times due to the ECM controlled thermostat which allows hotter temps at part throttle cruise conditions.


Dragan,
Quote:
Thanks for that! Now if someone would do the same for the I6.
That is what all the data from my post was taken from.
.
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  #45  
Old 03-10-2013, 02:11 PM
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Quote:
Originally Posted by JimLev View Post
KTMP Reading/PSI Reading
Technical question... (physics I guess). I never understood gauge pressure variants.

I was trying to convert JimLev's numbers to 'bar' to see how they related to the 1.2 bar cap versus the 2 bar cap ... when I realized there are TWO separate psi-to-bar conversions (depending on test conditions and gauge used):
  • Differential pressure, where 1 bar = 14.5037738007 psi
  • Gauge pressure, where 1 bar =-0.1921755993000005 psi (absolute)
I'm assuming the former rather than the latter, in expanding the JimLev table below in order to see the temperature relationship to pressure in bar units:
  • 40C = 1.5 PSI = 0.10 bar
  • 45C = 2 PSI = 0.14 bar
  • 50C = 2.25 PSI = 0.16 bar
  • 55C = 2.5 PSI = 0.17 bar
  • 60C = 3.0 PSI = 0.21 bar
  • 65C = 3.25 PSI = 0.22 bar
  • 70C = 3.5 PSI = 0.24 bar
  • 75C = 4.0 PSI = 0.28 bar
  • 80C = 5 PSI = 0.35 bar
  • 85C = 6 PSI = 0.41 bar
  • 90C = 6.5 PSI = 0.45 bar
  • 95C = 7.5 PSI = 0.52 bar
  • 100C = 9.0 PSI = 0.62 bar
  • 105C = 16.5 PSI = 1.14 bar
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Last edited by bluebee; 03-26-2013 at 08:17 AM.
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  #46  
Old 03-11-2013, 09:44 AM
dvsgene dvsgene is offline
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I take it as the former too as that weblink you provided also states:

Quote:
The above form works if you are measuring differential pressure, such as the difference in psi between two points. It also gives the correct answer for absolute pressure, assuming you are measuring psia, which is the pressure relative to absolute zero vacuum
I presume the typical dial guage is calibrated to zero vacuum/pressure at a zero reading.
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Old 03-11-2013, 10:26 AM
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chiefwej chiefwej is offline
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Would not any temp/pressure assumptions would be different based on altitude?
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Old 03-11-2013, 11:24 AM
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540iman 540iman is offline
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ABSOLUTELY! But, not by enough to matter a great deal if you are say in Denver at 4000'
This thin air will be reflected in fuel mixture more than in coolant pressure. Car learns and can negate a lot of it, but mileage will suffer as will power.
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Old 03-11-2013, 03:38 PM
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gary@germanautosolutions gary@germanautosolutions is offline
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I'm sure most people have gathered this from the ongoing discussion, but it still probably bares mentioning for those how are interested in the finer details; the coolant cap is a differential pressure relief valve. It will vent at 1.2 or 2.0 bar above whatever the ambient absolute atmospheric pressure is at the time. So technically it will vent at a little different absolute pressure even based on weather conditions.
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Old 03-12-2013, 07:01 PM
L0F L0F is offline
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So there's a bit of discussion on what "normal" operating temp range is for our cars once warmed up. Most of the figures I've seen for my m52 have been ballparks ~88-95. We know when and at what temp they overheat at, but what I would like to know is the factory stated standard operating temperature. With out knowing this value it leaves a grey area between "running hot" and over heating. I don't like grey areas.
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