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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > Mechanical Maintenance: Break-in / Oil & Fluids / Servicing / Warranty > Random coolant leaks even though the system holds pressure



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      01-03-2021, 10:38 AM   #1
Jay1101
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Random coolant leaks even though the system holds pressure

I have a 2007 335i and I just replaced the intercooler and radiator, I took it for a test drive and everything was normal even after the car got hot for about 15 minutes, then I got the low coolant light and when I got home and turned the car off all the coolant dumped out of the drivers side. I ran a pressure test to 20 psi and it held it for about 30 mins before I removed the tester. I refilled the coolant and did the bleed procedure again and drove around and got the car hot again with no leaks. Later that night I went for a cruise and after about 30 minutes I stopped to idle for a few minutes and when I started driving again the coolant light came back. I came home again to see it slowly dripping from the drivers side behind the lower far left grill. The top of the engine is dry and I can't seem to locate the leak underneath the car because by the time I get it jacked up to look a majority of the coolant is already gone. I am baffled as to why it's leaking even when it can hold perfect pressure. Any help? (This is my first post so if I put it in the wrong spot I apologize in advance)
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      01-03-2021, 07:57 PM   #2
bluewater328
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Did you replace the hoses or the o-rings for the hoses when you did it? Those o-rings aren't very good the second time around.

Next point would be to check the plug (the one that holds the drain plug). If that was over-tightened or if it wasn't put in completely it can leak. Also, what radiator did you use? Some have two plugs and require different lengths for different applications: ie. For an automatic 328i the Nissens requires long plugs inserted on both sides vs the long and short that's installed by default.
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      01-04-2021, 10:55 AM   #3
Jay1101
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Originally Posted by bluewater328 View Post
Did you replace the hoses or the o-rings for the hoses when you did it? Those o-rings aren't very good the second time around.

Next point would be to check the plug (the one that holds the drain plug). If that was over-tightened or if it wasn't put in completely it can leak. Also, what radiator did you use? Some have two plugs and require different lengths for different applications: ie. For an automatic 328i the Nissens requires long plugs inserted on both sides vs the long and short that's installed by default.
I did not replace the O-rings. That would make sense as to why its leaking only on cool down. Where could I buy new O-Rings, do the harbor freight metric O-ring kits work? Or is there a kit I can buy, I can only find kits on ebay, not sure if I should go with that. Also my radiator is the Duralast one from autozone and had the drain plug preinstalled when I opened the box. I loosened and retightened it before installing it just in case it wasn't done properly and I'm pretty sure the long one was installed but since the coolant is out and Ill have to jack the car up anyway I'll recheck that. Do I just need to check the plug or pull the whole drain plug assembly out?
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      01-04-2021, 11:04 AM   #4
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I would try to avoid pulling the whole assembly out if you cant determine if the leak is from there. That tab that most of them have to install is wildly fragile - basically its there to guide you with that quarter turn at the end and there is a slot you can see it go into place. However, turn beyond that point, and the tab breaks off and you may be in hot water. Not sure if the Duralast plugs are like that, but most of the others are.

As for the o-ring, yeah, the best ones I found were on ebay as BMW doesn't really sell them for most of their clip hoses. Mine happened to include that one with the radiator and I was grateful. There was a guy on YouTube who happened to find some aftermarket o-rings, but his channel escapes me. Worse case, you're buying that one hose which I think is only about $22 or so. I don't have any experience with the Harbor Freight sets but I can tell you that the o-rings at that size have very slight differences in size and it makes a difference.
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      01-04-2021, 12:09 PM   #5
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Ok I found this website with the exact part number,

https://www.realoem.com/bmw/enUS/sho...diagId=17_0377

and I found the oem one for 52 cents on FCP Euro. I'm going to buy all of the o rings off of FCP. I will reply back when I get the new O rings installed to say if it worked or not, thanks a lot for your help.
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      01-04-2021, 05:33 PM   #6
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Quote:
Originally Posted by Jay1101 View Post
Ok I found this website with the exact part number,

https://www.realoem.com/bmw/enUS/sho...diagId=17_0377

and I found the oem one for 52 cents on FCP Euro. I'm going to buy all of the o rings off of FCP. I will reply back when I get the new O rings installed to say if it worked or not, thanks a lot for your help.
Yeah that's the one that came with my Nissens. I don't think there are many others listed. Most are just part of "kits" by aftermarket companies. (Nissen provided that one, and the o-ring for upper and lowe rad). But even the dealer couldn't find me those in their catalog.
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      01-04-2021, 07:14 PM   #7
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If you pull the intercooler out you can get a really good look under there. My drain plug was leaking but it was so hard to see from where until I pulled out the Intercooler. Pulling it out is very easy you'll spend more time removing the under tray
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      01-05-2021, 03:49 PM   #8
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Quote:
Originally Posted by Luis561 View Post
If you pull the intercooler out you can get a really good look under there. My drain plug was leaking but it was so hard to see from where until I pulled out the Intercooler. Pulling it out is very easy you'll spend more time removing the under tray
Did you pressure test it before you found out? I pressure tested mine and it holds pressure when it's cold which is why I didn't think it could have been a split hose or a leaky drain plug. It only starts to leak when it gets hot and cools down and also doesn't hold pressure then either. This is why I thought it had to have been the old O-Ring excessively shrinking.
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      01-05-2021, 05:03 PM   #9
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Quote:
Originally Posted by Jay1101 View Post
Quote:
Originally Posted by Luis561 View Post
If you pull the intercooler out you can get a really good look under there. My drain plug was leaking but it was so hard to see from where until I pulled out the Intercooler. Pulling it out is very easy you'll spend more time removing the under tray
Did you pressure test it before you found out? I pressure tested mine and it holds pressure when it's cold which is why I didn't think it could have been a split hose or a leaky drain plug. It only starts to leak when it gets hot and cools down and also doesn't hold pressure then either. This is why I thought it had to have been the old O-Ring excessively shrinking.
No I did not because when I pulled my Intercooler out, everything is pretty visible I was able to find the leak easy. Try jacking the car up and remove the under tray but only loosen the Intercooler, turn the car on and as soon as it starts leaking pull it out and inspect where the leak comes from
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      01-05-2021, 08:52 PM   #10
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Quote:
Originally Posted by Luis561 View Post
Quote:
Originally Posted by Jay1101 View Post
Quote:
Originally Posted by Luis561 View Post
If you pull the intercooler out you can get a really good look under there. My drain plug was leaking but it was so hard to see from where until I pulled out the Intercooler. Pulling it out is very easy you'll spend more time removing the under tray
Did you pressure test it before you found out? I pressure tested mine and it holds pressure when it's cold which is why I didn't think it could have been a split hose or a leaky drain plug. It only starts to leak when it gets hot and cools down and also doesn't hold pressure then either. This is why I thought it had to have been the old O-Ring excessively shrinking.
No I did not because when I pulled my Intercooler out, everything is pretty visible I was able to find the leak easy. Try jacking the car up and remove the under tray but only loosen the Intercooler, turn the car on and as soon as it starts leaking pull it out and inspect where the leak comes from
Ok I'll give that a shot tomorrow, the O-rings are supposed to be delivered tomorrow so I'll install that and look for any more leaks by removing the IC if I see any coolant coming out. I've noticed though just loosening the IC bolts down and having it drop a few inches has been enough to see the radiator drain plug and the heat exchanger hose that plugs in above the drain plug.
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      01-07-2021, 10:18 PM   #11
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It ended up being the short plug installed on the radiator by default, I installed the O-ring from FCP and it still leaked so I checked the drain plug housing piece itself and it was the short one instead of the long. Didn't realize it could be that because on autozone it said it was for auto trans and everything came preinstalled. Should have checked before installing the radiator anyways, oops.
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      01-07-2021, 10:26 PM   #12
Luis561
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Quote:
Originally Posted by Jay1101 View Post
It ended up being the short plug installed on the radiator by default, I installed the O-ring from FCP and it still leaked so I checked the drain plug housing piece itself and it was the short one instead of the long. Didn't realize it could be that because on autozone it said it was for auto trans and everything came preinstalled. Should have checked before installing the radiator anyways, oops.
Ok glad you found it, I'll keep that in mind if I ever change my radiator. I'm currently dealing with a leak on my water pump, not sure if it's cracked or a hose is loose, I swear if it's not one thing it's the other lol
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      01-08-2021, 03:41 AM   #13
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Quote:
Originally Posted by Jay1101 View Post
It ended up being the short plug installed on the radiator by default, I installed the O-ring from FCP and it still leaked so I checked the drain plug housing piece itself and it was the short one instead of the long. Didn't realize it could be that because on autozone it said it was for auto trans and everything came preinstalled. Should have checked before installing the radiator anyways, oops.
Glad you're sorted.

For others, here's a good thread for explanations.

e90 proper sized drain plug on the passenger side for an automatic?? https://www.e90post.com/forums/showthread.php?t=1097628
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