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BMW E46 Lower Radiator Hose Leak at Temperature Sensor – M54 – E39 X5 X3 Z4 Z3

July 7, 2010

Replaced a bunch of parts, one is the lower radiator hose when doing a belt replacement. It is leaking at the sensor connector. Replaced the sensor and it is still leaking with new. Here is a reply from Mike at BMW CCA. Do you have some additional recommends? “The new style hoses have a plastic quick disconnect mechanism that uses an o-ring seal. This was in response to the fact that the engine compartments are incredibly tight nowadays, and there’s no room to get in there with a hose clamp tool. Of course, the new style means it’s easy to break the brittle plastic and you have to think about the o-rings. Anytime you see an o-ring, you should take it off, inspect it for cracks or deterioration under a light, and if none is found, lubricate it with Vaseline before reinstalling. If the o-ring breaks when you take it off, it needed to be replaced anyway. I doubt the problem is the hose itself. I would say the connector is either not snapped in completely, the o-ring is shot, or the plastic is cracked..

I have a new hose and the temp sensor, here at my desk. All appears to be as I would expect. This is actually a fairly common issue when either the hose or the sensor are replaced, on the M54 engine applications.  The o-ring compresses adequately as the sensor is installed in the plastic fitting at the hose end. During actual installation, I would lubricate the o-ring with some Vaseline. I agree with Mike, that the sensor is likely either not plugged in fully or that the o-ring is not seated properly or is damaged. Remove it one more time, inspect the o-ring, if it’s OK, lube it and re-install. Make sure that both snap clips are fully engaged. Please let us know what you find.

  1. Kamran permalink

    Can the sensor be changed without changing the lower Hose? How can I change the sensor? Do I need special tool or I just pull it up? It seems that the top part of the lower hose will be hard to pull apart. Should it not come appart after pulling the locking clip out and puling on the hose?

    • The sensor can be pulled out without removing the hose …. as long as you can physically get to it. Just pinch the two locking tabs on the sensor and pull it out. Now, with that said, you do have a couple issues to be aware of:

      * There is coolant in the hose and when you remove the sensor, the coolant will indeed gush out, until you install the new sensor. It would be far less messy and quite frankly, easier, to drain the radiator (via the drain plug), before removing the sensor.

      * The sensor may not just pull out. It may be stuck a bit and require some wiggling or even light prying to break it free.

      If access to the sensor is limited, you may need to remove the lower radiator hose. In disconnecting the hose ends, pull the securing clip out to its unlocked position and then pull the hose end off the nipple. As noted for the sensor, the hose end may be a bit stubborn and require a little persuasion.

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