- Probably Behr Chinese reservoir that fails in 3 months.

- Best is to go to ebay, at this moment, there is a seller "cenauto" selling OEM reservoir for $55 (instead of the usual $95 price).

https://www.ebay.com/itm/Genuine-BMW.../122630659270?

I just bought one for my 2004 X3 for $55, and it is genuine BMW reservoir.
There are only a few left at that price.

When I bought my reservoir it came with the thermostat. On an aged car the tstat will self-destruct upon disassembly

Aghhh that's exactly what I think I have is a Hella Behr, I guess is the chinese one because it failed in less than 3 months...

Until you see the crack, and see the coolant leaking out of it, I would not assume the tank has failed yet.

If there is an overpressure condition, the cap on the expansion tank will vent coolant, allowing it to flow onto the tank, etc.

If your engine overheats, you might warp the head or damage the head gasket. That would allow combustion gases to enter the coolant, raising pressure beyond the 2-bar cap release setting, allowing coolant to exit through the underside of the cap.

And I know all this because I went through it all on my 2001 3.0i earlier this summer. In my case it was a slightly warped head.

Here's a post on how you can pressure test your cooling system, if you can rig this thing up:
https://xoutpost.com/bmw-sav-forums/...ystem-e53.html

For me, doing that test ruled out any cooling system leaks, so I moved onto the next test, which was a Lisle combustion gas detector, giving me the bad news.

If all of this testing is beyond your limit of tools availability, you could try this as a test:
Clean and hose everything off around the expansion tank. Roll up a paper towel and wrap it around, under the expansion tank cap. That is where the pressure-released coolant comes out - between those ribs on the bottom of the cap. Then run the engine / drive around until it leaks. If you see leaks below, and on the expansion tank, but the paper towel is dry, then it is not the expansion tank releasing. If the paper towel has coolant on it, it could be what I'm suggesting above, or it could be a failing cap (o-rings, etc.) that is leaking even though the coolant is not over-pressurized.

Thank you, I don't see any liquid stains close to the cap, but I guess is possible that the tank is not leaking, I saw your post on how to test the system pressure (genius), but I don't have the bleader or a spare cap.

But I will definitely get a combustion gas detector kit to make sure is not the head gasket and ruin the engine while focusing on the cooling system.

Drive until hot (to pressurize) and if leaking from the seam you'll see a drip forming. Alternatively, stuff paper towel where you see the leak stain and after a drive or two the evidence should be clear.


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I edited my previous post, adding the last paragraph, which maybe you missed. It would be a way to do a simple test before buying that Lisle tester.

The Lisle tester is about $40 and your local PepBoys, etc. might have them. I paid more for mine there, vs. waiting a couple of days from Amazon (which was cheaper).

In my post on the pressure tester, there is a link to my warped head problem and solution.

If you've got a warped head or head gasket, I doubt you're going to hurt it too much by driving it until you eventually fix it. But you can at least save yourself from diagnosing and replacing a ton of other parts you don't actually need to replace. In fact, if you do have that problem, you may have had it for a while and not known it.

BTW, I don't know if this is what happened to you, but I tell you, from going through this experience on my car, I now look at used car ads for the X5's much differently. MANY of them are offered as needing a new radiator, coolant leak, etc., passing off a possibly major issue as a minor one. That's not what happened to me - I've owned for 4 years now. And actually, I think that most of my warped head was probably caused when the PO had a major problem at 158k a year or so before I bought it ... and then a recent problem put it over the edge.

So maybe everyone should buy that Lisle kit and take it with them whenever they go used car shopping.

Too late..
I already ordered a Combustion Leak Detector kit, Funny because I got the reservoir and the liquid separate and end up paying 27 instead of almost 40.
I really hope I find that was an unnecessary buy, but I'm not looking forward for a head gasket project, and better be safe than sorry
Anyway, I think is worth having it around.

This whole problem started with getting random warnings for a second or two about low coolant level, but everytime I open the cap, the little rod pups up.
So I guess there's something funny going on with the expansion tank anyway.

Reservoir full and low coolant warning indicates a problem with the sensor


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Agreed. That's another problem you should get figured out. Probably pretty easy. There is a little electrical connector at the bottom of the tank (bottom, rear, driver's side) with two thin wires going to it. It could be that leaked coolant residue is interfering with the connection there at the plug/socket. First thing to do is to unplug, clean, etc.

And of course, if the sensor is saying one thing and the floating rod is saying another, believe the floating rod.

If the problem is not at the connection, that magnetic float sensor has been known to be flaky (or even non-existent) for some non-BMW brands.

BTW, if you do end up having an issue with your sensor (i.e., not the connector, your tank is good, etc.), this thread has a ton of info on it. Pics of it cut open, etc. so you can see what that float looks like, the connector location, etc. After all the mysteries about its workings are discussed, the answer is given (I think) in post #16.
https://xoutpost.com/bmw-sav-forums/...l-message.html

But the sensor problem is separate from the leaking coolant problem - solve them both.