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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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  #76  
Old 03-28-2013, 12:47 PM
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Studawg Studawg is offline
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Well, you sold me. Just ordered one, in black. I was just noticing how ugly the factory cap was last night. My first "mod" on this car! Other than the ultimate cupholder, of course. Guess Im no longer a purist.
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  #77  
Old 03-28-2013, 04:24 PM
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Quote:
Originally Posted by Jason5driver View Post
Yes...!

Ever think about making a better/ less complicated Coolant Expansion Tank...?
He could start here:
- Does anyone have a BMW description of HOW the E39 M54 expansion tank actually works?

Diagram courtesy of JimLev on 3/27/2013
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  #78  
Old 04-17-2013, 06:42 PM
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Just ordered mine too. I just did a complete cooling overhaul at the beginning of the year and I'd like to keep all those parts going as long as I can.
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  #79  
Old 05-22-2013, 08:49 PM
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For cross reference, there is a related interesting discussion about the E30 expansion tank cap over here today:
- > E39 (1997 - 2003) > Radiator cap. lower bar?
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  #80  
Old 08-03-2013, 05:51 PM
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I was doing some research for someone needing a coolant bleeding DIY, when I ran into this cryptic sentence:
Quote:
Originally Posted by Aussie528iT View Post
The expansion tank does not have an overflow tube. Coolant is vented through the cap (cap is hollow inside) and any over flowing coolant will leave white stains around the top of the expansion tank. This is normal but leads some people to think their expansion tank is faulty and leaking
So, if the expansion cap is the one-way coolant overflow valve (with a two-way air check valve built in), then what is the purpose of the so-called expansion tank overflow tube that goes from the expansion tank neck to the top of the radiator?
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Last edited by bluebee; 08-04-2013 at 04:00 PM.
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  #81  
Old 08-04-2013, 09:35 AM
John in VA John in VA is offline
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Quote:
Originally Posted by bluebee View Post
So, if the expansion cap is the one-way coolant overflow valve (with a two-way air check valve built in), then what is the purpose of the so-called expansion tank overflow tube that goes from the expansion tank neck to the top of the radiator?
I thought that as the coolant was heated, it "expanded" and moved from cooling system (radiator) to the expansion tank. After the car is stopped (cools), coolant is drawn back into the radiator.
E28 & E30 expansion tanks have hoses that vent excess coolant to the ground.
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  #82  
Old 08-04-2013, 11:40 AM
Solo12 Solo12 is offline
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I just saw this discussion today and thought of an earlier post JimLev made about a 1.2 bar cap possibly presenting "a problem with the tu motors running at 108C on very hot days". Here is the link.

http://forums.bimmerforums.com/forum...light=pressure

This stuff is a bit beyond me and I did not read through this whole thread, but I thought this info may be useful to some of you who understand better. If this is just cluttering up the thread with unrelated info, let me know and I will delete it.
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  #83  
Old 08-04-2013, 12:15 PM
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Quote:
Originally Posted by John in VA View Post
I thought that as the coolant was heated, it "expanded" and moved from cooling system (radiator) to the expansion tank.
If that were true, then there would be no need for the expansion tank cap to vent the coolant, which, clearly, this thread indicates is what happens.

So, that leaves open the question of what the so-called "overflow" hose does.

Maybe that overflow hose might just an air-only equalizer?
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Last edited by bluebee; 08-04-2013 at 03:59 PM.
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  #84  
Old 08-04-2013, 12:46 PM
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I can't 100% certain, but from my analysis of the system I've learned the following:
  1. The cap has a blow off valve (OEM calibration of 2.0 bar, GAS calibration of 1.2 bar) that vents excess pressure through passages in the perimeter of the cap.
  2. The cap has a 1-way check valve that allows air to pass in the opposite direction if the expansion tank pressure drops below atmospheric pressure during cool down.
  3. The little hose connecting the top of the radiator to the top of the expansion tank helps to bleed off trapped air in the top of the radiator. I also believe, based on the flow path of that hose through the pressure relief section of the coolant cap, that it also makes sure that an overfilled system will vent excess coolant, and after successive heat cycles, return the coolant levels to normal. In a normal system there is coolant flow through that hose and if the system exceeds the blow off pressure of the cap, the cap will vent both trapped air from the top of the expansion tank and coolant flowing through the little hose. That is why you see coolant residue on the top of the expansion tank if the cap vents. If the cap only vented trapped air from the top of an overfilled expansion tank, the system would over pressurize every time the car was run and the system would remain overfilled.
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  #85  
Old 08-04-2013, 12:56 PM
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Quote:
I just saw this discussion today and thought of an earlier post JimLev made about a 1.2 bar cap possibly presenting "a problem with the tu motors running at 108C on very hot days".
We removed the manual clutch fan and disconnected the aux fans and tested both an M52 and M54 engine in real time while monitoring system pressures and temperatures up to full overheat temperatures (Gage pegged on full hot and dash warning displaying) and the system never exceeded 1.2 bar of pressure until temperatures exceeding 125 deg C.

Gary
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  #86  
Old 08-04-2013, 01:03 PM
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Quote:
I thought that as the coolant was heated, it "expanded" and moved from cooling system (radiator) to the expansion tank. After the car is stopped (cools), coolant is drawn back into the radiator.
This is how almost all US manufactured cars work, and I think it is the simplest and therefor best way to do things. In this method the expansion tank is not pressuerized and has an extremely low failure rate. In most European cars the expansion tank runs at full system pressure and actually has constant coolant flow through it. That is why there is such a high failure rate in this type of system. The problem is compounded by BMW's choice of a 2.0 bar venting pressure for these cars.

Gary
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  #87  
Old 08-04-2013, 03:58 PM
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Quote:
Originally Posted by gary@germanautosolutions View Post
I've learned the following
Thanks Gary.

I understood everything you said about the pressure cap, but, I just can't make sense out of the hose flow explanation.

However, this thread is clearly all about the pressure cap; so, I just opened a separate thread solely on the topic of how the overflow (aka bypass) hose works, over here:
> E39 (1997 - 2003) > Can someone explain the flow through the expansion tank so-called "overfow" hose?

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  #88  
Old 08-04-2013, 05:27 PM
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Quote:
I understood everything you said about the pressure cap, but, I just can't make sense out of the hose flow explanation.
Since the hose is attached the highest point in the system, and has a bleed screw port, it's the last and best place to bleed any remaining trapped air in the system.

The passage that the hose connects to enters a separate circuit between the two o-rings on the coolant cap. This is the return path of that hose flow into the expansion tank. Since this hose is passing coolant during normal operation, and that coolant passes through the cap just below the pressure relief valve, if the system pressure exceeds the cap rating, when the cap vents that pressure, it will vent both air and coolant.

This system helps to vent any steam first if the system has actually overheated, because the stream will enter the system at the top of the radiator. But also helps to rid the system of excess coolant if the excess pressure is caused by overfilling the expansion tank.

I hope this helps.

Gary
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  #89  
Old 08-04-2013, 07:24 PM
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Quote:
Originally Posted by gary@germanautosolutions View Post
The passage that the hose connects to enters a separate circuit between the two o-rings on the coolant cap.
I dug up my expansion tank autopsy photos (1) (2) (3) ...


Is this the start of that separate circuit in the expansion tank cap?


Does that hole in the expansion tank cap mate with this hole in the expansion tank neck?


Isn't that expansion tank filler neck hole the same hole that sprays coolant when the engine is off and the aux water pump is running with all the flap valves opened (as part of the normal bleeding procedure)?
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Each repair should invariably add to our knowledge base by the process of inexorable incrementalism.
Your job, in return, is to read the suggested threads, where the best people will always add value to those threads, either by pictures or by descriptions, so the next person with the same problem stands on your shoulders.
See also: E39 Bestlinks & How to easily find what you need

Last edited by bluebee; 08-04-2013 at 08:43 PM.
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  #90  
Old 11-26-2013, 05:50 PM
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For the record, this thread (basically belonging here) was opened today:
> E39 (1997 - 2003) > Possible coolant reservoir cap failure
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  #91  
Old 11-26-2013, 06:37 PM
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Ive had my GAS cap on for almost 15k miles now, all the way through a hot summer in SC and no problems. (Car has 116k now, some of the cooling system parts were replaced years ago, not even sure exactly which or when, although I know the radiator was replaced, and I replaced the fan clutch myself.) I will say the low coolant level warning came on just a couple weeks ago, and when it fully cooled down, I checked it and it was just a little low. I also noticed a little bit of dried coolant around the cap. I went to the dealer and picked up a gallon of coolant, ($24 :0 ) and it took maybe 6 ounces to bring it up to full again. All good. Probably less than 6 more like 4 ounces.
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Last edited by Studawg; 11-26-2013 at 06:38 PM.
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  #92  
Old 04-03-2014, 06:02 AM
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WOW, this is an incredibly technical post, and after reading the entire post I still dont know if I should change the cap on my 2003 530i with 39K miles. I want to do anything i can to avoid the cooling problems that seem to happen to these cars. So Im hoping to get a clear yes or no answer. To protect my engine should I get the 1.2 bar cap?

Last edited by Wooz; 04-03-2014 at 06:04 AM. Reason: missig info
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  #93  
Old 04-03-2014, 06:24 AM
JimLev JimLev is offline
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The only thing the cap does is vent the cooling system when the pressure exceeds the rating of the cap.
Being in Phoenix where it is HOT in the summer you could possibly exceed the rating of a 1.2 bar cap when there is really nothing wrong with your engine or cooling system.
The 2 bar cap will vent at ~29.4 PSI
The 1.4 bar cap at ~20.5 PSI
The 1.2 bar cap at ~17.6 PSI

If you want to change caps I'd go with the 1.4 bar cap.
I have a pressure gauge on my radiator and have seen it at 16 PSI many times here is MA where we rarely get into the high 90's.
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  #94  
Old 04-03-2014, 11:03 AM
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Jim, it probably depends on the car as well. The v8 produces more heat than the i6. Therefore, if you have an v8 maybe the 1.2 bar cap is a tad on the low side.
Since I replaced the cap with the 1.2 bar cap, I never had to top up the coolant, and it's been a while. During the summer months, it can be hot as well here, not as hot as Arizona, but stop-and-go in hot temps will raise the temp of the engine & coolant more than in the cold months, and will cool off slower while driving. The coolant level was still undisturbed.
I wouldn't buy the 1.2 cap for my X5 with the 4.4 engine. That thing runs "normally" about 10 C hotter than my e39, sometimes a few degrees more.
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  #95  
Old 05-22-2014, 07:27 AM
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Quote:
Originally Posted by JimLev View Post
Being in Phoenix where it is HOT in the summer you could possibly exceed the rating of a 1.2 bar cap when there is really nothing wrong with your engine or cooling system.
In addition to the heat in Phoenix, I would think that overfilling the expansion tank would have an additional pressure effect.

I just started to wonder if that overfilling-plus-heat exceeding the 2 bar would "only" effect the expansion tank.

Could that excess pressure due to both overfilling_plus_high_heat also crack the plastic radiator?
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Your job, in return, is to read the suggested threads, where the best people will always add value to those threads, either by pictures or by descriptions, so the next person with the same problem stands on your shoulders.
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  #96  
Old 05-22-2014, 10:52 AM
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I've been a advocate for not doing the lower bar cap because of the increase chance for boil-over. PS can someone post the link to jimlevs test


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  #97  
Old 06-26-2014, 03:00 PM
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bluebee bluebee is offline
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Quote:
Originally Posted by hh3uunp View Post
can someone post the link to jimlevs test
Here is a link to the JimLev results:
http://www.bimmerfest.com/forums/sho...9&postcount=78

See also:
- What is the proper coolant level bobber stick height (1) & what to do when your expansion tank coolant level sensor float bobber stick is MIA missing in action (1) & where to find leaks when the CHECK COOLANT LEVEL is lit on the instrument cluster (1) & where does the coolant level sensor cable go (1) & what is the o-ring size for the coolant level sensor (1) & what is the temperature of the coolant when the thermostat opens under normal conditions (1) & how to modify the cooling system expansion tank 2 bar cap to 1.2 bar to vent at a lower pressure (1) & how to retrofit a coolant level sensor to an E39 that doesn't have it (1) & what is the most often recommended coolant (1) & tricks for efficient flushing (1) bleeding (1) and a coolant refilling DIY (1) & some behr/hella cooling system autopsy photos (1)
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Each repair should invariably add to our knowledge base by the process of inexorable incrementalism.
Your job, in return, is to read the suggested threads, where the best people will always add value to those threads, either by pictures or by descriptions, so the next person with the same problem stands on your shoulders.
See also: E39 Bestlinks & How to easily find what you need

Last edited by bluebee; 06-26-2014 at 03:17 PM.
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