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DIY: Do It Yourself
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Old 01-08-2014, 11:31 AM   #1
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Join Date: May 2013
Location: DE, USA
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My Ride: 2001 BMW 325xi
Coolant Expansion Tank Woes!

Hey all,

After reading through pretty much every DIY i could find, I'm still in the dark about my coolant expansion tank. Yesterday I left my 2001 325xi (automatic, there are some significant differences in the area around the expansion tank between auto and manual) running in the driveway for 5-10 minutes to warm up. When I came back out to leave for the morning, there was a big ole puddle of coolant in the driveway (not unbelievable, it was 5 Fahrenheit that morning). Lo and behold, I get the car jacked up and look around, its coming from the coolant expansion tank, or so I think. The leak was originating from where the tank connects to the two lines leading into the bottom of the tank, next to the expansion tank sensor.

So, the tank and all the parts you need to remove to get to it have been removed (the bracket for the tank is still in the car), and upon inspection I can't find any hairline cracks on the tank. Which leads me to believe that the cracks are either so small they can't be seen, the cracks are further into the line inputs on the tank than I can see, or the lines leading into the bottom of the tank are bad.

I was wondering if anyone could offer any advice here? I know these tanks are infamous for cracking, and at 121k miles it doesn't surprise me that it would crack, i just can't find any with the naked eye. I don't know if any previous owners have replaced the tank before, but it certainly doesn't look new. Really just need any tips on where exactly to look for cracks or how to tell which was faulty, the lines or tank.

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Old 01-08-2014, 07:31 PM   #2
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Location: VA/DC, USA
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I'm thinking you might not be able to see them as water under pressure can come out of a very small crack.

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Shawn in VA/DC.

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Old 01-08-2014, 11:34 PM   #3
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Honestly at this point I would take a flashlight and some time. My suggestion is to inspect all lines going into and out of the expansion tank. Sometimes water especially under pressure and heat will do strange things making it seem like it's coming from somewhere it isn't. I did a complete coolant system overhaul on my 328ci and ran into a couple of those hiccups myself. Another thing to keep in mind is to take your TIME the last thing you want to do is think you found the problem and try a quick fix only to have it bite you in the ass later. Best of luck.
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Old 01-09-2014, 10:08 PM   #4
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It needs to be replaced, so replace it. The automatic I believe has other piping running by the expansion tank for the transmission fluid cooler (i think, I have a manual). But the thing will be a pain in the butt to take off if it's 121k old. Mine was tough to yank out and it had only been on there for 50k. The plastic pieces will sieze to each other with all the heat. But the job can be done, you'll just need to set aside 4 hours. Good luck!
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Old 02-07-2014, 02:27 PM   #5
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This is an old thread so hopefully you got it fixed. Don't know if you realize it or not but automatics have a thermostat between the expansion tank in the mounting bracket, often times that thermostat fails, cracks, and forces a leak at the bottom of the expansion tank. If you replace the expansion tank you should also replace the thermostat.
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