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E46 (1999 - 2006)
The fourth generation 3 Series (E46 chassis) was introduced in 1999 and set the standard for engineering and performance during it's years of production including being named to Car & Driver's 10 best list every one of those years! ! -- View the E46 Wiki

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  #1  
Old 02-01-2009, 07:34 AM
rjh325 rjh325 is offline
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Coolant temp sensor known to leak?

I searched but didn't find a plague of posts regarding leaky sensors. While changing my belts last night, I noticed a small area on the splash guard where coolant had recently leaked. It was obvious that it was from the sensor area. I was planning on doing a flush in a couple months but I may have to do it sooner now. Is it typically just a sensor replacement or the lower house that it is house in?

2005 325i Sport 78k
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  #2  
Old 02-01-2009, 08:06 AM
desertrat desertrat is offline
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Mine started leaking after I replaced the lower hose. It wasn't the new hose it was the little green O ring around the sensor, it was to loose. You can't buy the O ring without the sensor. I went to the pluming section at Ace Hardware and found one that was just about the same size, the ring part was just a bit fatter. It fit nice and snug and hasn't leaked a drop since, total cost 28 cents. I think you could change it without loosing much coolant, just find something the right size to stick down in the hole, something tapered like maybe a pencil with some tape around it.

Last edited by desertrat; 02-01-2009 at 08:10 AM.
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  #3  
Old 05-14-2009, 12:59 AM
e46_platform e46_platform is offline
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good hint, desertrat!

i'm hitting the same problem now....

anyway, i am wondering if anyone else did it in the way desertrat described.
i know the temp sensor is only 20-30 bux even at the dealer.... but mine just looks fine (even the o-ring looks ok on it), and changing it wouldn't cost any coolant loss when it's completely cooled, so i am seriously considering finding a "replacement o-ring" from a hardware store.... but still wondering what you guys' opinions are, and if there are more experienced examples
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  #4  
Old 05-14-2009, 04:24 AM
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elhombre elhombre is offline
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Quote:
Originally Posted by e46_platform View Post
good hint, desertrat!

i'm hitting the same problem now....

anyway, i am wondering if anyone else did it in the way desertrat described.
i know the temp sensor is only 20-30 bux even at the dealer.... but mine just looks fine (even the o-ring looks ok on it), and changing it wouldn't cost any coolant loss when it's completely cooled, so i am seriously considering finding a "replacement o-ring" from a hardware store.... but still wondering what you guys' opinions are, and if there are more experienced examples
I did a coolant sys overhaul a couple months ago and the temp sensor(old) didn't seal in the(new) lower hose, so I replaced it. I'm an A&P and routinely change o-rings on components that require them. A primary consideration is material compatability to avert deterioration and subsequent leakage. In this case I think it's commendable to seek alternatives to oem requirements. Be creative, yea stick it to the "stealer"!
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  #5  
Old 05-14-2009, 07:27 AM
desertrat desertrat is offline
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Update-a year and 15K miles later and my 28 cent O-ring is still working great. Now I think I need a new coolant level sensor, mine is starting to alarm for no reason at all (coolant level normal) when it's cold. I'll stick a new one in next oil change.
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  #6  
Old 05-14-2009, 08:31 AM
e46_platform e46_platform is offline
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el******, thanks for the professional tip!

desertrat, thanks for the update!
the M.T.C. expansion tank i got from autohausaz.com comes w/ a new coolant level sensor, which i didn't expect.... so i am keeping the old one for backup
but when i look at this M.T.C. expansion tank, the quality looks worse than the original OEM one... (the seals between the main body and the most upper part didn't look as perfect) i am afraid that it will last even less than as if i had kept using the original
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  #7  
Old 05-14-2009, 08:36 AM
rjh325 rjh325 is offline
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When I pulled the sensor, I noticed a "film" around the o-ring and some oxidation around the metal which probably compromised the seal. It may have been possible to clean off the gunk and re-install it without it leaking but I didn't want to take the gamble on a $15 part and buying another $20 jug of coolant. I replaced the lower hose, sensor and coolant last month..all is good
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  #8  
Old 05-14-2009, 01:25 PM
e46_platform e46_platform is offline
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spent 79 cents (+ some tax of course) at OSH for a pair of rubber o-rings that work fine on the coolant temp sensor... two rings in a box

the closest one i can find is BrassCraft 0567... it is a little fatter as desertrat mentioned, but it won't make it much harder to push the sensor on the lower hose... and it's pretty tight when it gets in there... and a slightly fatter o-ring makes me feel more secured.


in the picture, the green one is the original o-ring on the sensor, i broke it when i used a small flatbed screw driver to take it off... but it was much easier to put the new one on and no tools required... just used my fingers to stretch it a little bit...

just that after the new black o-ring is on, since it was stretched a bit, some surface turned white... but when i gently rubbed it w/ my finger, the white went away
is this ok, el******?

and i still have one spare one left over
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  #9  
Old 05-14-2009, 02:34 PM
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elhombre elhombre is offline
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Quote:
Originally Posted by e46_platform View Post
spent 79 cents (+ some tax of course) at OSH for a pair of rubber o-rings that work fine on the coolant temp sensor... two rings in a box

the closest one i can find is BrassCraft 0567... it is a little fatter as desertrat mentioned, but it won't make it much harder to push the sensor on the lower hose... and it's pretty tight when it gets in there... and a slightly fatter o-ring makes me feel more secured.


in the picture, the green one is the original o-ring on the sensor, i broke it when i used a small flatbed screw driver to take it off... but it was much easier to put the new one on and no tools required... just used my fingers to stretch it a little bit...

just that after the new black o-ring is on, since it was stretched a bit, some surface turned white... but when i gently rubbed it w/ my finger, the white went away
is this ok, el******?

and i still have one spare one left over
Cool beans e46! for future reference try "lubing" o-rings with a little of the coolant, (oil, grease, fuel, or whatever is being sealed) prior to installation. The lube aids in installation and helps to protect the seal from damage. Also, ask your dentist for his old peridontal pics as they make great o-ring removal tools. Have fun - Mike
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  #10  
Old 05-14-2009, 03:25 PM
e46_platform e46_platform is offline
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ah~ i did put coolant on the o-ring prior to installation of the sensor to the hose, but i didn't put coolant to do the same prior to installation of the o-ring onto the sensor!
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  #11  
Old 05-15-2009, 04:31 PM
jjtap jjtap is offline
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Did you lose much coolant when you replaced the O-ring???
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  #12  
Old 05-15-2009, 06:11 PM
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Did you lose much coolant when you replaced the O-ring???
As long as the car is cold you should loose none.
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  #13  
Old 05-16-2009, 01:13 AM
e46_platform e46_platform is offline
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As long as the car is cold you should loose none.
yes. and make sure that the coolant filler cap and the bleeder screw are closed.

when i had my leaky sensor on the lower hose, it keeps losing coolant, but after i pulled it out, none is coming out anymore.


btw, smolck... it was amazed how easily you pulled out the old expansion tank in your video... i did realize that you took it out one time in the previous night...
so was it not so much harder to pull that out for you for the first time?

i first tried one of the DIYs' method about hammering from the bottom. it didn't seem to work at all since the piece of plastic that attached to the expansion tank moves along with it.
then i tried to copy how you did it in the video... but no luck either...
i finally inserted inserted a screw driver (the arm needs to be not too thick) from the passenger side towards the driver's side direction at the bottom of the expansion tank (the side close to the front of the car), and used that leverage to pop it out

Last edited by e46_platform; 05-16-2009 at 01:18 AM.
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  #14  
Old 05-16-2009, 07:01 AM
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smolck smolck is offline
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Quote:
Originally Posted by e46_platform View Post


btw, smolck... it was amazed how easily you pulled out the old expansion tank in your video... i did realize that you took it out one time in the previous night...
so was it not so much harder to pull that out for you for the first time?
Great observation. I thought I would be accused of doing it "horsepower TV style" with some editing. But for whatever reason (maybe prior replacement) mine came out by hand BOTH times. I did break my trans t-stat, but overall, it came with little problem. On other cars I have had to fight tooth and nail. Weird. I guess it is just good living on my part!
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  #15  
Old 11-16-2009, 11:17 AM
nupraiz nupraiz is offline
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Thanks for the info on the O Ring. Went to Lowe's and found the same size. They only came in a 10-pak. I guess that means I can change it 9 more times. Extremely easy fix and took only 5 minutes total.
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  #16  
Old 11-16-2009, 01:30 PM
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How did you guys do that? Obliviously, you need o-ring with you at hardware store to find right replacement, but then what did you stick to the sensor hole?
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  #17  
Old 11-16-2009, 01:44 PM
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Well you'll probably want to get a ride or borrow a car to the store......... driving without the o-ring in there is bad news.
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  #18  
Old 12-01-2009, 08:39 PM
bshade1 bshade1 is offline
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I just replaced the o ring today. Thanks for all the info. For those of you wondering, it seems an o ring with dimensions 5/16" ID 9/16" OD and 1/8" wall works well. At least it did for me. Hardware stores though don't carry o rings that tell you what size they are and Autozone only had ones with a much thinner wall. I found another post on here that gave me the dimensions. I just bought an assortment pack and measured to find the right one.
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  #19  
Old 08-06-2010, 12:54 AM
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Originally Posted by bshade1 View Post
it seems an o ring with dimensions 5/16" ID 9/16" OD and 1/8" wall works well.... I found another post on here that gave me the dimensions
In the alternator & cooling system thread ... I measured a new FAE coolant temperature sensor o-ring at 0.330" ID by 0.115" thick (i.e., 8mmIDx3mmThick) which is the same 8x3 size as the BMW PN 13621743299 intake manifold air temperature sensor. It apparently fits all E39 and E46 vehicles; I'd recommended adding it to your cooling system overhaul recommended parts list.



Last edited by bluebee; 08-06-2010 at 01:07 AM.
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  #20  
Old 01-19-2011, 08:24 AM
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romeomike romeomike is offline
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Hi:

Did the o-ring replacement using the part number listed above this morning on a 2004 325i. As compared to the original instructions, the metal retaining clip needs to be pushed in before you can lift off the electrical connector. I also needed to use a small set of vice-grips to grasp the sensor so I could remove it after pressing in the release tabs. No coolant loss with a cold engine and radiator cap and bleeder screw closed. All straightforward after that.

I was a bit surprised to see how soft and brittle the coolant level sensor in the original expansion tank has become. The Germans don't always know how to make good stuff.

Again, thanks for all the contributions here. We'll see in the next few days how well the new o-ring seals things up.

Edit: after two days, the garage floor is still bone-dry. Praise God.

Adios for now,

RM
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Last edited by romeomike; 01-22-2011 at 12:27 PM.
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  #21  
Old 04-11-2011, 01:11 PM
barroyo barroyo is offline
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leak

mine didnt had any big problems but the stealer mention the small leak and i decided to
replace the o ring by myself and prevent any future leaks

my problem was when i open the sensor ( 4 hours after last drive ) and i sprayed coolant to my face and everywhere haha is this normal ?

small leak do i replace this with preston any color coolant ? or should i leave i it like that ?
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  #22  
Old 01-25-2012, 11:13 AM
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By way of cross reference, this thread over in the E39 side of the house contains more ways to obtain the M54 engine aux thermoswitch lower-radiator hose (aka temperature sensor) green o-ring than you can imagine:
- Dimensions, material, part numbers, suppliers & prices for the aux fan thermoswitch (sometimes mistakenly called the coolant temperature sensor) o-ring (1)
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See also: E39 Bestlinks & How to easily find what you need
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  #23  
Old 04-02-2012, 11:46 AM
EconoBox EconoBox is offline
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Anyone have a picture of where this sensor is?
I'd like to add o-ring to my DIY list.
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  #24  
Old 04-03-2012, 02:54 AM
shanneba shanneba is offline
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Originally Posted by EconoBox View Post
Anyone have a picture of where this sensor is?
I'd like to add o-ring to my DIY list.
http://www.bimmerfest.com/forums/att...9&d=1248503490
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  #25  
Old 04-03-2012, 09:11 PM
EconoBox EconoBox is offline
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I can't tell what I'm looking at. What's the general area?
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