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Leaking Coolant Temp Sensor?

42K views 34 replies 27 participants last post by  leblancrene 
#1 ·
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.

AM.
 
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#2 ·
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)...
 
#4 ·
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.
 
#6 ·
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.

 
#10 ·
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.
 
#11 ·
Excellent thread, subscribed. I love seeing threads that have solutions and not just complaints.
 
#14 ·
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?
 
#15 ·
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.
 
#19 · (Edited)
Here are the inputs used for the electric fan:

The fan is activated based on the ECM calculation
(sensing ratio) of:
***8226; Coolant outlet temperature
***8226; Calculated (by the ECM) catalyst temperature
***8226; Vehicle speed
***8226; Battery voltage
***8226; 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"
 
#20 ·
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..).
 
#25 ·
BMW part number for the O-ring is below in my signature.
 
#28 · (Edited)
I spotted a leak in my wifes E46, found the sensor to be the culprit and looked here for advice finding this thread. I had an O-ring that would fit already in my hardware supplies, and I really appreciate the OP posting this, as I was planning a trip to the dealer to replace the entire sensor. Thanks.
 
#30 ·
Thanks to everyone who replied on this thread.

Especially the guy who posted the O-ring dimensions! I was stuck like a truck and through a google search your reply got my 2006 E60 back on the road and for $0.81 at Lowes!!!!

I came and registered just to say thanks. Always share your findings, DIYs, and mistakes - you never know who it will help and where!

Aloha from Honolulu!
 
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