Have you noticed that the coolant level seems to slowly drop but all the hoses look clean and dry? You may be suffering a small leak in the temperature sensor located in the lower left radiator hose. This sensor uses an O-ring that can shrink and harden with age, allowing small amounts of coolant to ooze when the engine is hot and the radiator is under pressure, but the amount evaporates or blows off, without leaving much of a trace. In this DIY, we'll see how to replace the part to neutralize the leak. No tools are needed and the only part you have to get is a new sensor, P/N 13621433077, $32 MSRP, can be found in the market in the $17-$40 range. You may be able to get a replacement O-ring and change only that for less. As a side note, this sensor is used to control the electrical fan in front of the radiator to maintain coolant temperature.
First off, let the car be totally and absolutely cold. Do the repair next morning. Not only you won't be scalded, but you will lose a lot less coolant. If you were planning on flushing the cooland as regular maintenance, you can go ahead and drain the old coolant now, but you can change the sensor without doing that.If you are not flushing the old coolant at this time, make sure the pressure cap on the expansion tank is screwed on tight; do not remove it. The sensor location is shown in pic. #1, you may see a slight ooze on the pressure relief holes on both sides.
You are replacing the old sensor, #2, with a new one, #3. Note how the new O-ring is much rounder. Sensor is held in place by the two side tabs. If you're doing a "live exchange" (no flush), you can make the new sensor slide in easier by pre-lubeing the O-ring with undiluted BMW coolant.
The connector needs to be removed first, the sensor cannot be removed otherwise. As shown in #4, pull out and hold the chrome retaining clip and pull the connector straight up. If it resists, that means you are not pulling the clip out enough.
Now comes the quick part if you don't want to lose too much coolant. Place the new sensor in an easy and quick to reach location. As shown in #5, pinch the two side tabs on the old sensor together and pull gently but firmly straight up. The old sensor may be somewhat stuck in there, but do not go Neanderthal on it cause the hose fitting can be damaged and then you'll have much bigger problems. When it releases, quickly pull it out, set aside, grab the new sensor and slide it in until it clicks in place, noting that it is keyed. Don't overdo the pressure for the same reason as before.
Before reconnecting, I like to daub a smidge of dielectric grease on all my connectors to keep the moisture out and to ease the next disconnect, #6. Just press on till the retaining clip clicks on. Now unscrew the pressure cap on the expansion tank and fill up the missing coolant. Giv'er a whirl to get everything nice, hot and pressurized, to make sure the leak is fixed? Time for a beer or whatever fluid suits your fancy after this extenuating repair job?
Next time around, you could change the O-ring on the old sensor you removed, use that and save a bit.