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2004 325xi automatic
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41 Posts
Discussion Starter · #1 · (Edited)
One night last January on a busy highway I rear ended a black car stopped dead with no lights or hazard flashers on. Grrr. I was hard on the brakes coming down from about 70mph, probably needed just 5 or 10 feet more, but didn't have them so the nose of my car absorbed the impact. No air bag deployment, fortunately. I thought the car was damaged beyond practical repair, but upon examination everything damaged was bolt-on. The fenders escaped damage, the frame was fine. But that's a story for a different thread. I bring it up only because during the course of the accident repair I did a 100% overhaul of the cooling system. All the plastic pieces, all the hoses, pump, t-stat, radiator, tank, cap... the whole thing.

Fast forward to a few weeks ago. I look at the dash and I'm shocked to see the temperature gauge rising fast. I parked it, walked to my destination (which was not home), did my errands for an hour, bought some bottles of water and waited for the engine to cool down some more. Poured the water in and it ran out just as fast. Fck. AAA, tow truck, home hours later.

When I was able to get under the car I found the oil cooler no longer in place in the bottom of the expansion tank. Hmmm. It had been fine for over a year since the accident. I put it back in, made sure the retaining clip was good, and refilled and bled the system. Drove it around a bit, everything seemed fine. Next day was hot and I drove a little farther (maybe 5 miles?). I parked the car only to see coolant streaming down the street. Now it was leaking around the bleeder screw! Tried tightening it to no avail. I loosened the radiator cap (gingerly) to relieve the system pressure and limped home with the needle making a beeline for the red zone.

My conclusion is that the radiator cap was no longer releasing pressure at the rated level (2 bar, I think). The first time it managed to push off the oil cooler. The second time it found a weak spot at an overly exercised bleeder o-ring. I replaced the o-ring and got a 1.7 bar cap (I think... it's a standard BMW cap with a lower pressure). I ran the car on pure water for a few days just to be certain before investing in my third gallon of BMW juice in a year and a half. After I was sure the cap fixed the issues I changed out the coolant and it's been perfect ever since. Now... why didn't my low coolant light ever come on? Yet to be determined!
 

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Yup,
they're a maintenance item.
I don't change them often enough, either.

t
 

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2004 325xi automatic
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41 Posts
Discussion Starter · #3 ·
Yup,
they're a maintenance item.
I don't change them often enough, either.

t
It was less than two years old! Less than 10,000 miles! (I don’t drive much in general and even less during the pandemic.)
 

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2003 M3 6MT Slicktop
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did you use OE parts when you did your cooling system overhaul? aftermarket parts have issues not fitting tightly or fully.

I’m not sure what you explained shows the radiator cap has failed.

somewhat unrelated but curious if you have considered updating to a lower pressure cap that many do.
 

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My conclusion is that the radiator cap was no longer releasing pressure at the rated level (2 bar, I think). The first time it managed to push off the oil cooler. The second time it found a weak spot at an overly exercised bleeder o-ring.
No way that the properly fitted tranny cooler can be pushed off the radiator ports with the metal clips fitted on, regardless if the tank cap was bad.
No way that the bleeder O-ring - properly installed -- can be leaking regardless of the tank bad cap. It has too little surface area for the pressure to create enough force.

If anything would be busted it should be the coolant tank.
 

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2004 325xi automatic
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Discussion Starter · #6 ·
did you use OE parts when you did your cooling system overhaul? aftermarket parts have issues not fitting tightly or fully.

I’m not sure what you explained shows the radiator cap has failed.

somewhat unrelated but curious if you have considered updating to a lower pressure cap that many do.
All parts were sourced from FCP. I didn’t skimp. I can’t recall the exact brands of each item, but everything was OE quality. I did go to a lower pressure cap as noted in my original (overly lengthy) post.
 

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2004 325xi automatic
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Discussion Starter · #7 ·
No way that the properly fitted tranny cooler can be pushed off the radiator ports with the metal clips fitted on, regardless if the tank cap was bad.
No way that the bleeder O-ring - properly installed -- can be leaking regardless of the tank bad cap. It has too little surface area for the pressure to create enough force.

If anything would be busted it should be the coolant tank.
I was surprised myself that the oil cooler was blown off and that the tank didn’t crack. The ports on the oil cooler are roughly 3/4 square inch each, maybe a bit less. I didn’t measure them, just eyeball estimate. With a normal system pressure of 30psig, there will be 45 pounds of force that the retaining clip has to hold. If the cap decided not to vent and went up to say 40psig, then there will be 60 lbs against the clip. Enough to overcome the clip? I don’t know, but it’s plausible. The oil cooler was fine for over a year prior to the event and had no indication of impact.
As for the o-ring, I think it was tired and not sealing very effectively, and if there was 30 or 40 psig in the system it would certainly piss out at that point, which it clearly did because I saw it happening. I wish I had a way to test the cap to confirm that it is no longer venting at the design pressure, but having two consecutive coolant losses that resolved with a new (albeit lower pressure) cap supports the possibility of a failed cap.
 

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2004 325xi automatic
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Discussion Starter · #8 ·
Oh, and I believe it’s the engine oil cooler and not the tranny cooler in question. It’s fitted to a plate attached to the expansion tank with only one clip on an aluminum boss between the two coolant ports.
 

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2003 M3 6MT Slicktop
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Oh, and I believe it’s the engine oil cooler and not the tranny cooler in question. It’s fitted to a plate attached to the expansion tank with only one clip on an aluminum boss between the two coolant ports.
your car doesn’t have an engine oil cooler. that should be the automatic transmission cooler.
 
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2003 M3 6MT Slicktop
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also there are specific tools/ways to check the cap and pressure. even the pressure of your system for that matter. but do you really need to go that route.
 

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Discussion Starter · #11 ·
your car doesn’t have an engine oil cooler. that should be the automatic transmission cooler.
Thanks. I wasn’t certain. For some reason I thought engine, but it’s a while since I looked over the whole system function.
 

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Discussion Starter · #12 ·
also there are specific tools/ways to check the cap and pressure. even the pressure of your system for that matter. but do you really need to go that route.
Of course not. Just out of curiosity.
 

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I didn’t measure them, just eyeball estimate. With a normal system pressure of 30psig, there will be 45 pounds of force that the retaining clip has to hold.
Calculate the force of that same pressure on the gigantic tank. The tank wasn't ruptured because the pressure was not 30 psi or higher, I guarantee it. I think the original cap was OK too.

The cooler was pushed off because the clips were not fully in the grooves.
 

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Discussion Starter · #14 ·
Calculate the force of that same pressure on the gigantic tank. The tank wasn't ruptured because the pressure was not 30 psi or higher, I guarantee it. I think the original cap was OK too.

The cooler was pushed off because the clips were not fully in the grooves.
I’m not guaranteeing anything, and I’d be foolish to do so without measurements. It’s possible the clip (singular) wasn’t properly seated, but unlikely that it would take 14 months to come loose. The force on the tank isn’t a relevant parameter since it’s a pressure vessel designed to contain pressure, with a certain factor of safety. The safety factor is apparently marginal based on how many tank failures people report, but that doesn’t mean that every tank will fail when overpressurized to some degree. Every 17 year old car has its own unique weak spots. Mine was apparently the cooler retention, for whatever reason. Anyway, I’m not here to argue. Overpressurization due to a failed cap fits the failure modes I experienced. So does a poorly seated clip and bad o-ring, as you’ve suggested. I believe the cap theory to be more likely, you believe the opposite, and that’s fine. We’ll never know unless I get my cap tested which I’ll do if I can find someone with the tool who won’t charge an arm and a leg.
 

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2003 M3 6MT Slicktop
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Thanks. I wasn’t certain. For some reason I thought engine, but it’s a while since I looked over the whole system function.
yeah no worries.
only the cool kids (M3) got the engine oil coolers 😎😉
 

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JonL, I am impressed you got those repairs squared away. Bravo!
 

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Please look at the cooler clip/barb and you will agree that no way it can be pushed off if the clip snapped over the barb.
Somewhat agree. What we don't know (!) is if the "mounting plate" is genuine, how old etc... Are the O Rings genuine and provide not only a seal, but are still supple and able to maintain a "square" so that the cooler can't possibly rock side/side.
Has this happened before and is the clip and/or the slot (where the clip locks into) compromised and rounded.

These cars are getting old and I'm starting to notice many outside the box failures (both here and in my bays) owing to age. One such ask by me; Have the transmission cooler lines dried (age) and have shrunk somewhat? If so (?) pulling aft/sideways onto the cooler.

(Sidebar: I was just under a newer BMW 4WD drive car yesterday. The automatic transmission cooler lines are a nylon/plastic ((thin and flimsy IMO)) tubing. WHAT COULD GO WRONG???)

Too many unknowns at this point.
 
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