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General E46 Forum
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Old 10-07-2009, 12:14 PM   #1
Attacking Mid
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Leaking Coolant Temp Sensor?

Apparently the coolant temp sensor must be replaced (about 30 bones) whenever you replace the lower radiator hose. I didn't realize this until I had a leak following a complete cooling system overhaul. For some reason, the o-ring must shrink, as it is a very loose fit in the new lower hose fitting.

By luck, I had a package of cheap o-rings in the garage which were slightly oversize compared to the old one. I changed the o-ring out and end of leak. I can't say if this o-ring will last forever, but if it does fail, it's a pretty simple fix down the road.

Anyway, if you want to save the 30 bones, here's the details:

"Brasscraft" brand o-ring twin pack from Home Depot or Lowes (not sure)

Package number 0567

Dimensions: 5/16" I.D. (9/16" O.D.) x 1/8" Wall

This o-ring is a snug fit, so it takes a good push to get the sensor seated in the hose fitting.

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Old 10-07-2009, 12:29 PM   #2
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Yeah, good tip. I replaced everything except for the damn sensor, so I also found this out the hard way (and I didn't have spare o-rings sitting about)...
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Old 10-21-2009, 10:45 PM   #3
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Interesting, I wonder if this is where my coolant has been going. How fast was your coolant leaking out?
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Old 10-22-2009, 02:05 AM   #4
Safar
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In my case, fast enough that coolant levels would be too low after a couple of days...

But you can easily check; feel around the sensor location to see whether it's wet or not.
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Old 10-22-2009, 02:39 AM   #5
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I am going to, just trying to get a little background info to see how similar the situation is
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Old 10-22-2009, 12:05 PM   #6
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I did the same thing - used one from a pack of assorted O-rings that I had in the garage. Ideally, you should use a brown or green O-ring, as those will withstand the harsh environment better. but, I used a black one almost 50k miles ago, and haven't had a leak since.


Green = old hardened O-ring = leaks in new hose.
Black = new soft rubbery O-ring = no leaks.

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Old 10-29-2009, 02:44 PM   #7
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I am glad someone started this thread.

The problem has always been finding the right o-ring for the coolant temp sensor and most folks ended up buying a new sensor.

Actually there is a BMW pn: 13.62.1.743.299

It's the same o-ring used for IAT sensor on the older E46 models.
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Old 10-29-2009, 09:30 PM   #8
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you can also try flipping the oring around backwards....it gets a flat spot on it from how it is installed and turning in around puts a
'fresh" part of the oring up against the seal area. Worked on my car.
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Old 10-31-2009, 05:32 PM   #9
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good stuff guys...i just found the leak from my sensor this afternoon and logged on to see if anyone else had the same issue...bingo! thanks!
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Old 11-02-2009, 08:02 AM   #10
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I just replaced the hoses, thermostat, and expansion tank hoping not to need a new temp sensor. It leaked pretty good at the sensor. Tried different o-rings till it stopped leaking. So far so good. Saved a few bucks but drove myself crazy trying to stop the leak.
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Old 11-02-2009, 08:27 AM   #11
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Excellent thread, subscribed. I love seeing threads that have solutions and not just complaints.
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Old 08-27-2010, 08:37 PM   #12
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is that the sensor? i just replaced the lower radiator hose too.

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Old 08-27-2010, 09:18 PM   #13
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The sensor is in the lower radiator hose about a 8 to 10" from the radiator, on the left side bottom as you face the front of the car.
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Old 12-02-2011, 10:59 PM   #14
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replacing coolant temperature sensor

I am trying to replace the coolant temperature sensor in my 2000 328i which is located in the lower radiator hose. As many other posts have stated, mine is leaking. However, when I went to do the "easy" swap, the old sensor won't come out. I pinched the 2 little levers on the side to release it and pulled but it won't budge. Any ideas how to get it loose? Also, does this sensor have any control over the electric fan cooling the ac?
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Old 12-02-2011, 11:13 PM   #15
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It does indeed just pull straight out. Use gloves so the "ears" don't cut into your fingers and you can tug harder. Rock it back and forth (it will only move a little each way) while pulling it out. Keep at it and the plasticized o-ring will eventually give.

I don't know about the AC thing.
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Old 12-02-2011, 11:14 PM   #16
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Try squeezing the two tabs in and then push the sensor down before pulling it out.

Squeeze
Push
Pull

Out!
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Old 12-03-2011, 06:16 AM   #17
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Quote:
... Also, does this sensor have any control over the electric fan cooling the ac?
Yes, that's one of the inputs the system uses to determine when to turn the electric fan on.
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Old 01-07-2012, 02:21 AM   #18
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Bump!!

Quote:
Originally Posted by Sansho View Post
Yes, that's one of the inputs the system uses to determine when to turn the electric fan on.
Is there another input..??


I mean is that the only temp sensor or is there another also on the engine..?? (that gives info to the fan)
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Old 01-07-2012, 07:27 AM   #19
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Here are the inputs used for the electric fan:

The fan is activated based on the ECM calculation
(sensing ratio) of:
Coolant outlet temperature
Calculated (by the ECM) catalyst temperature
Vehicle speed
Battery voltage
Air Conditioning pressure (calculated by IHKA
and sent via the K-Bus to the ECM)

Page 36 of this document- http://www.bmwmotorsports.org/BMW_docs/m54x5.pdf

There is another coolant temp sensor in the block under the intake manifold (rear) it may be used in the ECM "calculations"
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Last edited by shanneba; 01-07-2012 at 07:29 AM.
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Old 01-07-2012, 09:40 AM   #20
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Quote:
Originally Posted by shanneba View Post
Here are the inputs used for the electric fan:

The fan is activated based on the ECM calculation
(sensing ratio) of:
Coolant outlet temperature
Calculated (by the ECM) catalyst temperature
Vehicle speed
Battery voltage
Air Conditioning pressure (calculated by IHKA
and sent via the K-Bus to the ECM)

Page 36 of this document- http://www.bmwmotorsports.org/BMW_docs/m54x5.pdf

There is another coolant temp sensor in the block under the intake manifold (rear) it may be used in the ECM "calculations"
Thanks for the reply..

That's the reason that i asked if there is another sensor..

Actually, i have a faulty (99%) fan (works all the time, even with A/C off) and i want to disconnect the sensor and see what happens..

And.. iam ready for some preventative maintenace and iam thinking of changing almost all sensors (cam, crankshaft, temp..).
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