Aguila, thanks for the leads. Yes, I have not seen Behr tanks yet, though I've seen a reference or two to them (Behr Hella Service, apparently made in China) over at fanatics.

TX5, here's my leak, sounds like we have the same problem:
http://i239.photobucket.com/albums/f...psf520de5c.jpg
I am pondering on the posibility of resealing as well...maybe filling the seam with a sealant/epoxy or something?

QS, thanks for the EPC tip. I kinda figured it over time as this truck has me constantly looking up parts, lol! Yes my coolant level sensor is still working, but now as many of us have discovered with BMW's, it's better to just replace almost everything going in, as what's left will break soon after...and you break more parts dissasembling ang reassembling! I'm not sure if the level sensor is one of those coolant system overhaul must-changes or one that is reasonably safe to re-use.

couldn`t you bring it to a shop that welds plastic. I know when I was doing bodywork a few years ago we had a plastic welder.

Interesting! I wouldnt mind trying this if I had access to one. Not aware of any local though.

looks like its just leaking at that one area is that right?

check this out theres a video Plastic Radiator Tank Repair Kit | Plastic Radiator Welder

Ok, so I am overhauling the system, I dont wanna have to touch this area for another few years!

The expansion tank is a two piece part, the upper portion where the radiator cap screws on to, and the tank body.

It has a gasket between the two parts. Mine seems to have partially blown out. See pic where there is gasket material and dried coolant at portions of the seam:
http://i239.photobucket.com/albums/f...ps4e8632c5.jpg

Otherwise it's fine, no sign of cracking and does not seem brittle yet. I can confirm it was previously replaced, it is not a BMW original part.
http://i239.photobucket.com/albums/f...psd01c58bb.jpg

Puddinboo, thanks for the link. It seems plastic welding will not work here though.

looking at the seal can you just use a heat gun to split the 2 parts apart clean up the sealant and reseal the unit ? looks like it could be done just get a sealer like permatex that you can use on plastic. how much is the part anyways?

The expansion tank is $80 or so for an original BMW one. Yeah it does look like a permatex-like sealant. I'll be keeping this leaking tank for further study but for now I will be replacing it with a BMW OE tank.

i paid $45 for OE BMW part, brand new in a box. it seems to be a sealant and not a gasket - i wonder if i got my "new" tank out of an old stock where the tank was laying around warehouses for a decade and the seal just dried out... nevertheless, i would have expected more than 3-4 months out of an original factory piece... it never occurred to me to read the date stamp on the new tank... maybe i should have...

anyways, if we could bake the headlights open - what can preclude us from opening the tank?? what pressure values are inside the cooling system? anyone has an idea?