BMW X5 Expansion Tank

This covers information I found on the expansion tank used in the BMW X5 (also the BMW E39’s)

The following was done by a bimmerfest member known as Bluebee. She has pretty much disappeared from the net but she left us with a ton of good information.


Moving on to the expansion tank (aka surge tank) autopsy, it should be noted that I opted for a longitudinal autopsy because the only prior BMW E39 expansion tank autopsy I know of employed horizontal cuts. As such, that prior horizontal autopsy sliced directly into the level measurement pieces, complicating the analysis of the possible failure modes.

Vowing to not make the same mistake, and to further our knowledge, I bought a dremel tool and 200 dremel bits so that I could slice into the expansion tank without destroying the delicate level-sensing mechanism inside.

As luck would have it, I broke the only bits that cut into the expansion tank and had to return to the store to pick up just a set of the desired cutting tool.

Note: The “D” and the “M” are the markings for the distal and medial tubes respectively (the hoses are similarly marked so as to avoid confusion in replacement).

I carefully measured by eye a longitudinal slit that would bisect the expansion tank without damaging the coolant level sensing mechanism within. The second set of dremel tool bits easily cut into the expansion tank along that desired line.

At some points the plastic was thicker than others and melting occurred, but that was of no major consequence. Notice, by the way, that the level sensing bobber stick is outside the expansion tank. This is due to a failure previously noted about six months ago in this thread:
– My radiator floating level indicator seems to be MIA (where does it go?)

Soon I had the expansion tank longitudinally girdled. Notice the broken expansion tank nipple. Even though this Behr expansion tank nipple is reinforced with a tube of metal, this broke off when I removed the hose clamp from it in order to remove it from the car.

Once girdled, the expansion tank easily cracked open, revealing that I had cut it at the perfect junction of the inner bulkhead between the level-sensing compartment and the overflow relief compartment.

Placing the electrical sensor at approximately the position it would lie, I moved the float to the “full” position, with the broken-off bobber stick also placed in the correct OEM position on the side of the float.

It is important to notice the steel band of metal which I used to prove that the disc in the center of the float is slightly magnetic. It is clearly magnetic, but only slightly so. The white part of the float itself is made of a light plastic material and which is not magnetic.

Here I placed the float and bobber stick in approximately the coolant “empty” position. Notice that the magnetic disc now encircles the electrical sensor at the bottom of the expansion tank.

Here I’ve placed the float in the “too full” position. Notice also that there is an incomplete bulkhead on the other half of the expansion tank (much as was seen in the Titanic’s six front watertight bulkheads which were only watertight horizontally, not vertically). At the moment, I surmise this thin plastic bulkhead (incomplete at the top) apparently separates the overflow operation of the expansion tank with the level-sensing operation.

In my tentative summary, I assume the following operational explanation (which I invite others to help flesh out in detail).

Surmised operation:
– The expansion tank has at least three different operations, namely visual level sensing, electrical level sensing, and overflow operation (if there are more, let me know).

– The electrical level sensing is apparently triggered when a float containing a slightly magnetic disc with a hole in the center, envelopes the tip of the electrical sensor. This apparently triggers the “low coolant” warning.

– That same float also contains a loosely glued on bobber stick, which indicates visually the coolant level. A typical failure is for that stick to disengage from its float. This disables visual level sensing; but it does not seem to have any effect on electrical sensing unless the stick itself causes the float to hang up in operation.

– In practice, it is very difficult to remove an old expansion tank overflow hose clamp without breaking the nipple, even though that expansion tank nipple is metal reinforced against that happenstance. If you’re going to remove the expansion tank hose, you may as well assume you’re going to replace the expansion tank.

>> The above information came from this thread  on bimmerfest. The post has a ton more info than just what is posted above. Go read it for more detailed information. <<


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