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General E46 Forum
This is the place to get answers, opinions and everything you need related to your E46 (sedan, coupe, convertible and wagon) BMW!

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Old 10-03-2012, 08:05 PM   #1
dknightd
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why is the BMW coolant system the way it is?

OK. new to BMW. Fine car. But it seems like cooling system issues are common.

I understand that plastic fails with age, and that is a likely cause of system failures.

But I can't help wondering if the design is also part of the problem.

Many (most???) cars use an overflow tank and a pressure relieve valve in the radiator cap. If the pressure gets too high, the excess fluid is directed to the overflow tank. When things cool down fluid is sucked back in from the tank.

It seems BMW uses a closed system with an "expansion" tank. I guess the idea is that the radiator will always stay filled with liquid - as it expands it compresses air in the expansion tank to make room. If somebody overfills the system there is not enough room, and something explodes. Exploding seems common here. I wonder if perhaps this is due to not leaving enough air in the expansion tank. Or maybe it is user error - if you fill above max you will likely have an explosion since the expanding fluid has no place to go.

So, anybody know why they designed it this way? Is there any over pressure release valve at all in the BMW system? If not, seems like a recipe for disaster.
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Old 10-03-2012, 08:20 PM   #2
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Old 10-03-2012, 08:25 PM   #3
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I'm convinced part of the problem is that people overfill the ET.
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Old 10-03-2012, 08:39 PM   #4
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Look at the bottom of the ET cap. I think it says 200psi or something like that. I'm pretty sure the cap has a pressure release built into itself. Not 100% on that though
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Old 10-03-2012, 09:08 PM   #5
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e46's expansion tank is also its overflow tank. There is an overflow valve built into the expansion tank. As long as you don't fill it up above the overflow tube, you should be fine. This level is higher than the maximum fill level specified on your ET. Just don't fill it up above that level.

The overall design of e46's cooling system is not the reason why those parts fail quicker than, say, Japanese cars. BMW engines run hotter than most other cars' engines (good for efficiency), which means your cooling parts have to work harder than most other cars'. Solution? Just be on top of your maintenance, and replace them before they break.
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Old 10-03-2012, 10:12 PM   #6
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I have wondered about this. The e46 m3 system has a more "traditional" setup were if there's any excess coolant, it gets dumpe out a relief. You don't hear of similar problems on the e46 m3
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Old 10-03-2012, 10:18 PM   #7
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I assume the M3s dont have the big failures partly due to a better designed cooling system but mostly due to the owners taking better care of their cars due to the larger values of their cars and most are more likely to be enthusiasts.

This comes from a non m3 owner.
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Old 10-03-2012, 10:21 PM   #8
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I think that cars used to have an overflow tank, but I suspect that most are now have pressurized expansion tanks. It reduces parts count, and so reduces cost. The cap on the expansion tank vents any excess pressure, and if overfilled will vent the excess coolant as well.

Both of my other cars (Lincolns) also have pressurized expansion tanks and no overflow. Unfortunately, they also have a similar failure rate for the tanks and other plastic cooling system parts. On the good side, they have failsafe cooling, so no engine damage if the cooling system fails.

I've had other cars with pressurized reservoirs that had seemly bullet proof cooling systems. Maybe they used better plastic?
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Old 10-03-2012, 10:25 PM   #9
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I think that cars used to have an overflow tank, but I suspect that most are now have pressurized expansion tanks. It reduces parts count, and so reduces cost. The cap on the expansion tank vents any excess pressure, and if overfilled will vent the excess coolant as well.

Both of my other cars (Lincolns) also have pressurized expansion tanks and no overflow. Unfortunately, they also have a similar failure rate for the tanks and other plastic cooling system parts. On the good side, they have failsafe cooling, so no engine damage if the cooling system fails.

I've had other cars with pressurized reservoirs that had seemly bullet proof cooling systems. Maybe they used better plastic?
My Cadillac had this and I never had coolant issues which is shocking since it was a Northstar!
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Old 10-04-2012, 06:10 AM   #10
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Originally Posted by TerraPhantm View Post
I have wondered about this. The e46 m3 system has a more "traditional" setup were if there's any excess coolant, it gets dumpe out a relief. You don't hear of similar problems on the e46 m3
hmmm wonder if can be retrofitted into an non-M e46
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Old 10-04-2012, 07:37 AM   #11
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That's a bit pretentious. If you can't explain something simply, you haven't mastered that topic yet.
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Old 10-04-2012, 08:21 AM   #12
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That's a bit pretentious. If you can't explain something simply, you haven't mastered that topic yet.
I usually ignore mercforhire posts, but perhaps I should have answered his questions in case he has useful info to share. I'll go do that
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Old 10-04-2012, 08:24 AM   #13
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Are you knowledge enough to understand the answer to your question?
I would not have asked the question if I did not think I could understand the answer.

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Originally Posted by MercForHire View Post
What's your major?
b.s. engineering physics, phd atmospheric science. I've been out of school for 25 years, so, I guess my major is living and learning. Can you help me with that?

Oh, I guess I should be polite and ask what your major is.
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Old 10-04-2012, 08:29 AM   #14
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My guess is the usual German thing where they over engineer something that in testing is better but in practice wears out after 5+ years.

In theory a closed system is better, until that system isn't closed anymore then its way worse.
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Old 10-04-2012, 02:00 PM   #15
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I usually ignore mercforhire posts, but perhaps I should have answered his questions in case he has useful info to share. I'll go do that
he's a Troll. are you kidding, you think he has something useful to share? hasn't yet. people call him V.I.

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I would not have asked the question if I did not think I could understand the answer.
b.s. engineering physics, phd atmospheric science. I've been out of school for 25 years, so, I guess my major is living and learning. Can you help me with that?
Oh, I guess I should be polite and ask what your major is.
Haha, classic. the troll walked into that one. he's a 19 yrs old pizza delivery boy.
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Old 10-04-2012, 02:05 PM   #16
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I have wondered about this. The e46 m3 system has a more "traditional" setup were if there's any excess coolant, it gets dumpe out a relief. You don't hear of similar problems on the e46 m3
I was going to post this same thing. M3s have a different system. Not the attached expansion tank. We also all have both Mechanical and Electric fans. So it's not all the E46 cooling systems.

I've also wondered how often the caps fail to release pressure.
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Old 10-04-2012, 02:07 PM   #17
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Old 10-04-2012, 02:17 PM   #18
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My theory has been that the reason why they use plastic is because 1) its cheaper to make I bet and 2) they need parts on these cars to only survive the warrenty period. what happens when the warrenty is over for the second or third owner? they pay right out of pocket. i believe thats why some things on these cars have been designed the way they are
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Old 10-04-2012, 02:18 PM   #19
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I have no gripes with our cooling system, who am i to question it's design anyways.

However I have been driving BMWs for 8 years, and the initial shock of maintaining them has long long passed.

Bottom line, the design of this system is rock solid when well maintained, all that is required of you is to maintain it, and do it correctly.
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Old 10-04-2012, 02:29 PM   #20
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My theory has been that the reason why they use plastic is because 1) its cheaper to make I bet and 2) they need parts on these cars to only survive the warrenty period. what happens when the warrenty is over for the second or third owner? they pay right out of pocket. i believe thats why some things on these cars have been designed the way they are
Plastic is cheaper in quantity, but there's another reason too. You can make plastic parts in shapes and complexities that just aren't practical for metal.
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