Ever wonder why E46 expansion tanks fail? here is why...

[cwarner]

I wondered about this, particularly after my second OEM tank failed in 12 months. So after I replaced it yet again, I decided to do a failure analysis on the failed tank, and was shocked at how bad a few of the mistakes were that lead to failure.

So here is the tank. I had already cut the top off and cut out the side port, so this is just to show the whole tank:



Next, looking down into the tank from the top. Note that I have removed a bunch of internal parts already, but one can quickly see that the side that bulges out is not supported to the center for most of the length. Not good.



To make things more clear, I then cut off one corner of the tank to expose the central rib that goes down the middle of the side which bulges out.



Now that the inside of that portion of the tank is exposed, the problem is remarkably clear. The mistakes here are really bad. They clearly blew it on multiple accounts including the decreasing thickness of the stiffening rib and the insufficient cross section of material connecting the side of the tank to the middle. The loading is clearly more than the fiber reinforcement to the plastic can take and it fails in shear (indicated by the 45 degree orientation of the cracks. Then the whole tank self destructs due to excessive deflections. It is not to supprising that these fail all the time.



Clearly if the side of the tank was connected to the middle at some point above the side port, the stresses would be significantly lower. More to the point, if the stiffening rib did not thin out as it does the deflections in the side of the tank would also not contribute to the problem. I also noted that one corner of the tank did not crack (the corner in the lower right corner in picture 2 above). This corner had a much higher radius than all the others. again stress concentrations.

This is very sloppy engineering. It is very much on par with the abismal design work done on the rear subframe mount. In both cases the engineer who did the design work shows a complete lack of comprehension of stress concentrations, how stress is distrubuted in a structure, and the design limits of the materials they are using.

I sincerely hope the new tank (part number 17 11 7 573 781) has structural improvements inside versus this one (part # 17 13 7 787 039)!

[ducky328i]

wow. if i had an e46 id be thanking you a ton right now...but i dont...so ill just thank you on behalf of all the people that dont get to see this thread that wish they would have.

[Sean323iSE]

WOW! I havn't got my e46 yet, so havn't had this problem...yet. but thanks for taking the time to check this out!

I have a question/suggestion about this, hopefully you can tell me if it would help or just be a waste of time...

Would puting some sort of brace around the tank, to "hold in" the side that bulges help delay this from happening?

Maybe a large metal jubilee "hose" clip tightly around the tank or a few strong cable ties?


Sean

[mfumbesi]

Thank you for your effort.
I have an E46 and I've replaced one of those already. I actually started a thread about it complaining, at that point I called it planned obsolescence. I have a friend who owns an E30 we talk about our frustrations......he called it non-conformances (He is a mechanical Engineer and a good reference point for me).
If we can only get a manufacture (independent entrepreneur) willing to take this issues up, I would be willing to pay more if I knew the part will last me more than the BMW part.

[mfumbesi]

By the way, my expansion tank also ruptured at same place as yours.

[Sean323iSE]

" I would be willing to pay more if I knew the part will last me more than the BMW part. "


+1

[cwarner]

WOW! I havn't got my e46 yet, so havn't had this problem...yet. but thanks for taking the time to check this out!

I have a question/suggestion about this, hopefully you can tell me if it would help or just be a waste of time...

Would puting some sort of brace around the tank, to "hold in" the side that bulges help delay this from happening?

Maybe a large metal jubilee "hose" clip tightly around the tank or a few strong cable ties?


Sean

A couple of us at work discussed this option. We all came to the conclusion that it would make things worse. The reason is that the square cross section of the tank would mean the band would put more pressure on the corners, increasing stress there. It would not be very effective at applying pressure in the middle of a flat side. What might help is something more like a c-clamp that presses on the middle of the unsupported span.

As to a redesign or new part by independant, that might be tough to get someone to do it. But with enough demand someone might do it.

[davidebby]

Thanks a lot. I am getting ready to remove mine because I found evidence of a small leak and I am getting intermittent low water fault indications. You didn't mention much about the low water sensor. I heard that tends to stick and needs to be serviced but I don't know exactly what it looks like or how to proceed. Any ideas?

Davidebby

[G. P. Burdell]

Excellent investigative work.

Looks like I'll be going with the Zionsville radiator and expansion tank at 60,000 miles.

[Mad Dragon]

If the reservoir was the only thing you replaced, of course it's going to fail again. Did you replace the water pump, thermostat, hoses, or cap?

This is very sloppy engineering. It is very much on par with the abismal design work done on the rear subframe mount. In both cases the engineer who did the design work shows a complete lack of comprehension of stress concentrations, how stress is distrubuted in a structure, and the design limits of the materials they are using.

What abysmal design? I've never seen one that has failed. Techs I work with that have 7+ years' experience have never seen them fail.

[323ie46]

Park this ish!!

[cwarner]

If the reservoir was the only thing you replaced, of course it's going to fail again. Did you replace the water pump, thermostat, hoses, or cap?

What abysmal design? I've never seen one that has failed. Techs I work with that have 7+ years' experience have never seen them fail.

I thought I stated that one year ago I replaced the water pump, thermostat, hoses and this expansion tank, but I probably missed that. The water pump was why I started down this path. All parts were OEM supplied through my local dealer.

In your statement about your tech friends, are you refering to the expansion tank failure or the the rear subframe mounting point failure?

If it is the rear subframe mounting point failure, I simply do not buy what you are saying. I had mine fail and there are over 150 cars with failed rear subframe mounts listed over at E46 Fanatics the last time I checked a few months ago (over 5% of all 2000 328Ci's sold in the US have been reported as failed). I have and would be happy to email you a copy of the BMW AG tech bulliten where they inspected all cars in Germany and used a engineering body cavity foam to reinforce the subframe mount. I analyzed that failure in depth and posted in the past a detailed explanation of why that fatigue failure occurs. My tech (> 15 years experience at BMW) admitted seeing the rear subframe failure many times in the E46. Now that he owns his own shop, he has fixed many of these using the Turner motorsports reinforcement kit. Talk to Turner Motorsports if you need more info, they will tell you how many they have fixed. It will shock you. Heck, there are at least three class action lawsuits over these failures in various states of litigation.

If you are refering to the expansion tanks, they fail even more frequently. do a search and you will see many posts here and at E46 Fanatics.

[cwarner]

Excellent investigative work.

Looks like I'll be going with the Zionsville radiator and expansion tank at 60,000 miles.

Thanks! I had not heard about the Zionsville radiator. Nice, but wow that is expensive.

[bong bong]

Thanks a lot. I am getting ready to remove mine because I found evidence of a small leak and I am getting intermittent low water fault indications. You didn't mention much about the low water sensor. I heard that tends to stick and needs to be serviced but I don't know exactly what it looks like or how to proceed. Any ideas?

Davidebby

this is what the sensor looks like.

[Vince S.]

Good job, cwarner!

Yeah the E46 expansion tank is famous for failing. These days, I'd rather replace them after 10,000 miles. It's a cheap insurance vs that happening on a track.

[mfumbesi]

Great investigation.
I wonder if anyone at BMW ever reads this stuff, I know that they know about the tank, but do they read what we think about their design.

[cwarner]

Thanks a lot. I am getting ready to remove mine because I found evidence of a small leak and I am getting intermittent low water fault indications. You didn't mention much about the low water sensor. I heard that tends to stick and needs to be serviced but I don't know exactly what it looks like or how to proceed. Any ideas?

Davidebby

That part, look a few posts up on this thread, is mounted in the bottom of the tank. You disconect an electrical connector and then it it removed by twisting and pulling out. I had no issues with mine, so I reused it. If yours is faulty I would have thought it would be just replaced.

When you take out the tank, remove the plastic cooling duct to the alternator to give you room to use a small screwdriver to wedge the tank free. IF you just yank on it from above you run the risk of cracking the plastic carrier that the tank mounts to (did that last year).

[geegee]

I have 2002 325ci that has been service for 5 plus years and has 60,000 miles and

[geegee]

I have a 2002 325ci that has been in service for 5 plus years and 60,000 mile,no sign of any leaks yet.What sort of mileage are guys having before the problem starts.I did have a problem with one of my e36's.It cracked where the upper hose connects to the rad.

[Critter7r]

What abysmal design? I've never seen one that has failed. Techs I work with that have 7+ years' experience have never seen them fail.

Don't feel bad, I've worked with BMW for over 15 years and I've only seen 1 in person. Heard about many, only seen 1. And I worked at a dealer for 10 years that serviced 40-60 cars per day, 15-20 techs (depending on the year), 38 hoists, so not some little po-dunk dealer.

[cwarner]

Don't feel bad, I've worked with BMW for over 15 years and I've only seen 1 in person. Heard about many, only seen 1. And I worked at a dealer for 10 years that serviced 40-60 cars per day, 15-20 techs (depending on the year), 38 hoists, so not some little po-dunk dealer.

You sound like the service manager at my local dealership when I brought my car in with the rear subframe failure. Except, he lied about having any previous knowledge about the issue even though they had two failed cars come throught the dealership (the body shop that worked with him to fix those cars let me know that little nugget). I do not expect you to understand why the rear subframe mount is an abysmal (yup, spelled it right this time ) design. I seriously doubt that you understand metallurgy or the nuances of fatigue crack initiation and propogation at stress concentrations and heat affected zones. I do, as that is what I do. BMW should, considering the large number of metallurgists they employ.

Just because you have not seen it in huge numbers does not mean it is not an issue, and that a very significant number of cars have not failed. All it shows is that you have a lack of understanding of what specific cars are having the biggest issue. The statistics I have compiled through last spring showed that the most serious issue was with model year 2000 328Ci coupes. Over 5% of those cars sold in the US have been reported on the web with this failure. According to the NHTSA it is common for 1 in 5 to 10 failures to be reported, so the actual failure rate in that particular model is probably much higher, probably in the area of 10-15%. Note that NHTSA takes >5% as a very significant finding in terms of failure rates. The sedans and 323i coupes are also failing, but not at equivalent rates based on sales records. That is probably due to differences in engine torque in the 323Ci's and sheetmetal geometry in the sedans. The absolute number of model year 2000 328Ci's is small, so I would doubt that you would see more than a few per dealership. But that does not mean that the failure is not very real and occurs at an unnaceptable rate to the customers of that model of car.

Face it, every car manufacturer makes mistakes, some worse than others. I fully accept that. The difference is how they handle it. In the case of rear subframes, BMWNA has chosen to deny any knowledge of a problem and did nothing about it. In contrast, BMWAG did something about it. I hope that the new part number for this part indicates that BMW has done something about this failure prone part.

To all the other posts thanking me for the analysis, you are more than welcome! Lets hope that by presenting data like this, BMW may begin to change their approach and fix things. I love the way my BMW drives, handles and is fuel efficient. I just wish they would build them better.

[Critter7r]

WHOAAAA! HEY!!! Hold up a second there! I wasn't the one arguing with you about it being/not being an abysmal design, just pointing out that Mad Dragon wasn't alone in his knowing people with lots of experience and not having come across this problem. I'm with you on the whole subframe being a problem and crossing my fingers that it never happens to me.

[Critter7r]

P.S. You obiously have a better grasp on the whole metallurgy fatigue crack propigation and initiation (or at least the lingo!) than I do and I don't propose to be an expert in that area. But I do have a huge amount of (mostly useless) BMW info stuck in my cranium.

P.P.S. Beatuiful write-up on the tank failure BTW.

[99mcab]

sorry, but i have to ask this question? Can someone explain what the expansion tank does? also, sometimes when im at the gas station filling up, when i get it to full, the gas leaks out from the top, maybe this is the problem?

[G. P. Burdell]

sorry, but i have to ask this question? Can someone explain what the expansion tank does? also, sometimes when im at the gas station filling up, when i get it to full, the gas leaks out from the top, maybe this is the problem?

The tank described above is the coolant expansion tank. It doesn't have anything to do with the fuel tank.