Expansion tank keeps developing a crack! VERY STRANGE

[akshatp]

Car details:
2002 BMW 525i 4 Door
101,000 miles

(For those unfamiliar with BMW's, the radiator expansion tank on the BMW is the main filling point for antifreeze, and is under pressure just like a radiator. I wanted to make that clear for those more familiar with american vehicles where the overfill tank is just a reservoir for extra AF. )

So about 4 months ago, the expansion tank that sits next to the radiator developed a small hairline crack on the top surface causing the vehicle to overheat due to low antifreeze. No big deal, at around 100K miles this is one of the things to be replaced.

Had the part replaced at the mechanic, all was well until a few weeks later when the new tank developed a similar crack in the same general area. Had the vehicle towed home. First thing I checked was to see if something was fouling the tank causing friction and causing it to crack. Nope. Took back to mechanic, he replaced it for free and put it down to a defective tank.

Another three weeks go by, and I am sitting at the airport idling waiting for my parents flight to arrive, and all of a sudden i hear a POP sound and alot of steam from the engine. Get out, pop the hood, all the antifreeze is on the floor and upon further inspection I see the tank has come apart at one of the seams!!! It literally POPPED open. Had the vehicle towed AGAIN and back to the mechanic, who agreed it was strange that another tank was defective. He replaced it again at no charge.

Now fast forward to this weekend. We are in NYC, and at a TL car starts to overheat again!!! I couldnt beleive it. No flashlight so I couldnt see what was going on, but there was some antifreeze spewing out of somewhere so I knew I had to get towed again. I was pissed.

Next day checked it out, it was the tank AGAIN. This time, it had a crack down the side of the tank where the antifreeze leaked out under pressure.

Spoke to the mechanic again and he assured me he has been using authentic BMW parts. I went to another shop and they said the cap, which is supposed to auto-vent might be bad. So I bought a new cap and tank from the BMW dealer myself and had this new mechanic install the tank.

Anyone heard of anything like this? THREE replacement tanks all going bad within a month of installation??? Could the system be overpressurizing causing the tank to "rip"? What else could it be?

Fan clutch works, thermostat seems to be fine. What am I missing?

Cliffs:
Expansion tank keeps developing cracks
Keep replacing it, keeps getting cracks
What could be causing it?

Any feedback appreciated!

 

[Ronstang]

Sounds like you have a slightly cracked head/block or a leaky head gasket. Believe it or not you can have this problem with no steam out the tailpipe and the engine seems to run fine....ask my how I know.

 

[halik]

Originally posted by: akshatp
Car details:
2002 BMW 525i 4 Door
101,000 miles

(For those unfamiliar with BMW's, the radiator expansion tank on the BMW is the main filling point for antifreeze, and is under pressure just like a radiator. I wanted to make that clear for those more familiar with american vehicles where the overfill tank is just a reservoir for extra AF. )

So about 4 months ago, the expansion tank that sits next to the radiator developed a small hairline crack on the top surface causing the vehicle to overheat due to low antifreeze. No big deal, at around 100K miles this is one of the things to be replaced.

Had the part replaced at the mechanic, all was well until a few weeks later when the new tank developed a similar crack in the same general area. Had the vehicle towed home. First thing I checked was to see if something was fouling the tank causing friction and causing it to crack. Nope. Took back to mechanic, he replaced it for free and put it down to a defective tank.

Another three weeks go by, and I am sitting at the airport idling waiting for my parents flight to arrive, and all of a sudden i hear a POP sound and alot of steam from the engine. Get out, pop the hood, all the antifreeze is on the floor and upon further inspection I see the tank has come apart at one of the seams!!! It literally POPPED open. Had the vehicle towed AGAIN and back to the mechanic, who agreed it was strange that another tank was defective. He replaced it again at no charge.

Now fast forward to this weekend. We are in NYC, and at a TL car starts to overheat again!!! I couldnt beleive it. No flashlight so I couldnt see what was going on, but there was some antifreeze spewing out of somewhere so I knew I had to get towed again. I was pissed.

Next day checked it out, it was the tank AGAIN. This time, it had a crack down the side of the tank where the antifreeze leaked out under pressure.

Spoke to the mechanic again and he assured me he has been using authentic BMW parts. I went to another shop and they said the cap, which is supposed to auto-vent might be bad. So I bought a new cap and tank from the BMW dealer myself and had this new mechanic install the tank.

Anyone heard of anything like this? THREE replacement tanks all going bad within a month of installation??? Could the system be overpressurizing causing the tank to "rip"? What else could it be?

Fan clutch works, thermostat seems to be fine. What am I missing?

Cliffs:
Expansion tank keeps developing cracks
Keep replacing it, keeps getting cracks
What could be causing it?

Any feedback appreciated!

20 bucks says you overfilled it. The min-max lines are for when the car is hot... put in less coolant?

I was gonna suggest thermostat or fan, but that's apparently not it.

 

[akshatp]

Originally posted by: halik

Originally posted by: akshatp
Car details:
2002 BMW 525i 4 Door
101,000 miles

(For those unfamiliar with BMW's, the radiator expansion tank on the BMW is the main filling point for antifreeze, and is under pressure just like a radiator. I wanted to make that clear for those more familiar with american vehicles where the overfill tank is just a reservoir for extra AF. )

So about 4 months ago, the expansion tank that sits next to the radiator developed a small hairline crack on the top surface causing the vehicle to overheat due to low antifreeze. No big deal, at around 100K miles this is one of the things to be replaced.

Had the part replaced at the mechanic, all was well until a few weeks later when the new tank developed a similar crack in the same general area. Had the vehicle towed home. First thing I checked was to see if something was fouling the tank causing friction and causing it to crack. Nope. Took back to mechanic, he replaced it for free and put it down to a defective tank.

Another three weeks go by, and I am sitting at the airport idling waiting for my parents flight to arrive, and all of a sudden i hear a POP sound and alot of steam from the engine. Get out, pop the hood, all the antifreeze is on the floor and upon further inspection I see the tank has come apart at one of the seams!!! It literally POPPED open. Had the vehicle towed AGAIN and back to the mechanic, who agreed it was strange that another tank was defective. He replaced it again at no charge.

Now fast forward to this weekend. We are in NYC, and at a TL car starts to overheat again!!! I couldnt beleive it. No flashlight so I couldnt see what was going on, but there was some antifreeze spewing out of somewhere so I knew I had to get towed again. I was pissed.

Next day checked it out, it was the tank AGAIN. This time, it had a crack down the side of the tank where the antifreeze leaked out under pressure.

Spoke to the mechanic again and he assured me he has been using authentic BMW parts. I went to another shop and they said the cap, which is supposed to auto-vent might be bad. So I bought a new cap and tank from the BMW dealer myself and had this new mechanic install the tank.

Anyone heard of anything like this? THREE replacement tanks all going bad within a month of installation??? Could the system be overpressurizing causing the tank to "rip"? What else could it be?

Fan clutch works, thermostat seems to be fine. What am I missing?

Cliffs:
Expansion tank keeps developing cracks
Keep replacing it, keeps getting cracks
What could be causing it?

Any feedback appreciated!

20 bucks says you overfilled it. The min-max lines are for when the car is hot... put in less coolant?

I was gonna suggest thermostat or fan, but that's apparently not it.

Hmm think its possible the mechanic overfilled it every time?? I also own a 740iL with similar engine and tank, and I keep the coolant level to the appropriate levels and always check it according to the book. I generally keep that red dipstick thing a little bit less than what is specified.

I dont think it was overfilled, and if so why would the cracks occur in different spots, and once it even burst??

 

[akshatp]

Originally posted by: Ronstang
Sounds like you have a slightly cracked head/block or a leaky head gasket. Believe it or not you can have this problem with no steam out the tailpipe and the engine seems to run fine....ask my how I know.

How would a cracked head/block or leaky gasket cause the radiator expansion tank to crack???

 

[Ronstang]

Originally posted by: akshatp

Originally posted by: Ronstang
Sounds like you have a slightly cracked head/block or a leaky head gasket. Believe it or not you can have this problem with no steam out the tailpipe and the engine seems to run fine....ask my how I know.

How would a cracked head/block or leaky gasket cause the radiator expansion tank to crack???

It lets combustion pressure enter the cooling system. Extra heat and extra pressure in the system can easily cause a failure. If you get another tank keep an eye on the radiator hoses. They can bulge significantly due to excess pressure in the system due to combustion gases getting in the system. You can also look at the coolant and see if there is any contamination.

The tank is plastic and obviously the weak point in the system. It seems you have too high a pressure in the coolant system. It is coming from somewhere. You can also check to make sure the radiator cap is keeping the system at the proper pressure. If it is failing it could lead to high enough pressure to cause a failure in your expansion tank.

 

[akshatp]

Originally posted by: Ronstang

Originally posted by: akshatp

Originally posted by: Ronstang
Sounds like you have a slightly cracked head/block or a leaky head gasket. Believe it or not you can have this problem with no steam out the tailpipe and the engine seems to run fine....ask my how I know.

How would a cracked head/block or leaky gasket cause the radiator expansion tank to crack???

It lets combustion pressure enter the cooling system. Extra heat and extra pressure in the system can easily cause a failure. If you get another tank keep an eye on the radiator hoses. They can bulge significantly due to excess pressure in the system due to combustion gases getting in the system. You can also look at the coolant and see if there is any contamination.

The tank is plastic and obviously the weak point in the system. It seems you have too high a pressure in the coolant system. It is coming from somewhere. You can also check to make sure the radiator cap is keeping the system at the proper pressure. If it is failing it could lead to high enough pressure to cause a failure in your expansion tank.

Okay thanks for that great info. Ill have the car checked out for those symptoms.

Also, the mechanic this time suggested the cap may be faulty, so I replaced that with a new one from BMW

 

[DivideBYZero]

Sounds like the cap to me. All caps should release pressure if it exceeds a predetermined level. The release valve on the cap should let go before the tank.

 

[MysticLlama]

The cap could make the problem go away if it vents, but I agree with the previous comments about possible headgasket.

Saw a similar thing in a race kart where we kept blowing the hoses off of the radiator completely or causing leaks at the seams. Ended up being a headgasket and we didn't see any of the common signs of it at first.

 

[dug777]

Sounds unpleasantly like a headgasket problem to me

 

[V00DOO]

I have a E39 540I which shares a similar cooling system as your 525. I replaced my expansion tank and radiator 2 weeks ago myself. Anyway, did your mechanic properly bleed the cooling system after he replaced the first expansion tank or the second one, etc? By bleeding the cooling system, I am saying did he get all the air out of the cooling system. BMW cooling systems always need bleeding after any cooling system repair. If your mechanic didn't properly bleed the cooling system, your car will overheat which caused the expansion tank to overflow. If you continue running this way you're going to need a new head gasket if it hasn't been damaged already. Good luck.

 

[LTC8K6]

"For those unfamiliar with BMW's, the radiator expansion tank on the BMW is the main filling point for antifreeze, and is under pressure just like a radiator."

My 1997 Chevy Z24 is exactly the same. That type of system is quite common.

 

[akshatp]

Update: Car overheated again with the new cap and new tank. Good news is that the tank is still intact, and the new cap vented as designed, meaning the previous cap was indeed defective. This still leaves the problem which is causing the overheat though. Unfortunately looks like a head gasket problem. Any idea how much something like this is going to cost me??

 

[DivideBYZero]

Could be a thermostat, could be 1000 things, of which some could cost you a metric ton of cash. Good luck! (i.e., time for a mech to look at it.)

 

[akshatp]

Originally posted by: DivideBYZero
Could be a thermostat, could be 1000 things, of which some could cost you a metric ton of cash. Good luck! (i.e., time for a mech to look at it.)

Well its going to BMW tomorrow for a diagnostic. I figure ill pay them $125 to figure out whats wrong and then have it towed to a german car shop to have it fixed.

 

[E39540i]

Hello, did you manage to find the issue for why the expansion tank kept failing? Thank you

 

[E39540i]

Well its going to BMW tomorrow for a diagnostic. I figure ill pay them $125 to figure out whats wrong and then have it towed to a german car shop to have it fixed.

Hello, did you manage to find the issue for why the expansion tank kept failing? Thank you

 

[repoman0]

Hello, did you manage to find the issue for why the expansion tank kept failing? Thank you

Cool username, love my E39 M5 still. This thread is 15 years old and no one here cares about real cars anymore. Try bimmerforums or m5board or something.

 

[E39540i]

Cool username, love my E39 M5 still. This thread is 15 years old and no one here cares about real cars anymore. Try bimmerforums or m5board or something.

Cheers mate will do, hopefully the owner of the thread does see it and shares their diagnostic

 

[Tech Junky]

Cheers mate will do, hopefully the owner of the thread does see it and shares their diagnostic

Doubtful. 15 years is a lifetime for a forum. Check for user activity.



In general though an expansion tank is cheap to replace under $30. It may or may not be the true issue though depending on if everything is OE or you upgraded things. On my Audi I swapped it out with all of the other stuff I was messing with in the same project. However, I made upgrades that lead later to the expansion tank as part of diagnosing what I perceived as a leak which could have been as simple as the cap to the tank being the issue. Figured though as it was OE ~10 years old might as well replace it for the $30 and not have to think about it. However my project went as deep as pulling my supercharger and swapping all of the gaskets and OAS under it due to a strange noise coming from under the SC. After swapping one of the O rings between the SC / OAS though I suspect that was another issue about to be a problem. The diaphragm on the OAS though was the indicated source of the sound I was hearing at startup until warm.

Anyway... the cooling system is a PITA to say the least. Especially when you make mods and expand the capacity for additional cooling.