roadmax

Had hard time finding out where is coolant leaking from, every 3-4 months have to add about 1-2 quarts of coolant.
Finally located, with engine hot removed the expansion tank mounts and noticing under the tank dripping coolant from the bottom hole.
Last year I replaced the expansion tank cap just as preventive measure (so cap is fairly new) but it does not appear to be normal having this much coolant loss that happens at engine normal operating temps.
Would it be safe to conclude the tank needs replacement?

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seanjordan20

The tank is a weakness in these cars. Everyone has had to replace it at one point in time. Replace it and be done with it

roadmax

I just ordered the tank, will use the almost new cap. I thought it is only the cap that deals with pressure sensitive coolant release adjustment but based on the amount of coolant I had to add this does not appear to be normal and somehow has to do with the tank itself.
Still not clear how does this process work in terms of releasing coolant at high pressure levels, between the cap and the tank?
Is it possible that even the cap is good the tank can be defective causing coolant loss via that hole?

I can say this was kind of pain to detect as it leaked under the tank (not visible) and every time coolant would dry very quick from heat.

dingolfing

If your tank was original from new, it was due for a replacement. The cooling system is pressurized while the engine is running, and when the tank is nearing the end of its life, around 10 years, the plastic cannot withstand the high pressure anymore, and will develop cracks. Most of the time, they are on the side and is visible with dried blue coolant stains. Yours happened to crack on the bottom.

Ideally, this part should have been made out of metal, but beans have to be counted.

FormerE30Owner

Actually 10 yrs without a leak on a plastic expansion tank is pretty good and normal. A different plastic material could have been used for the tank, but what for? I don't know if metal expansion tanks even exist. The expansion tank is part of the cooling system, but we don't expect more than 10 yrs from the hoses, which could also be made of a more "durable" material.

dingolfing

Mine lasted 10 years, but others might have shorter lifespans, depending on how much mileage they drive. And believe it or not, but all of my rubber cooling hoses are still in good condition. When I replaced my second water pump six months ago, I checked all the main radiator hoses. The only hoses I replaced so far are both tank return vent lines, which only failed from the plastic tube connectors on them disintegrating, the rubber tube portions were still useable.

FormerE30Owner

I think that you just "disproved" your contention about "the expansion tank should have been made of metal".

roadmax

Just to clarify, there is no crack at the bottom, coolant is dripping from very top point through that bottom hole. When checking with light it appears there is a small hole at the top, maybe that is the same valve hole that releases coolant at very high pressure levels in case of overflowing the tank.
Currently have 130K and this is original expansion tank, 2008 550i model.

FormerE30Owner

At any rate, you must replace the tank - it is leaking, one way or another!. A hairline crack is not visible when the tank is cold. The cap is what relieves the pressure, not some orifice in the tank itself.

roadmax

Tank is to be delivered tomorrow and I should replace it by this weekend.
The cap relieves the pressure but is that extra coolant also dripping via same hole as shown in my pictures?