Coolant Expansion Tank Behavior

[Syndrome]

Hi all. My E28 M5 is still relatively new to me, so I don’t know how it’s supposed to behave. I have replaced every piece of the cooling system and the car runs at what seems like a nice temperature. That gauge shows it’s at about 3/8 of the way when warm, or just under that. However…. If I fill the expansion tank (with BMW blue fluid) to the MAX line, I will have coolant flowing out of the vent hose as the car warms up until it finds a level where that stops. I think it’s when the expansion tank is about 1/2 full.

I’ve always driven air cooled cars, so I am learning here. I don’t think this is normal behavior, but don’t have a clue what to do. I could leave the coolant in the expansion tank at about 1/2 full and it doesn’t seem to mind, but that seems weird to me. Can anyone advise? Thanks,

[Aldo525]

Hi, it is supposed the max level is when coolant is cold....after driving wait at least overnight to check the level again, when cold. Level should be max again.

If not, my guess is (assuming there is no water leaks, broken hoses, etc) is a bad expansion tank cap, as the coolant system is pressurized when hot, maybe the cap "releases" pressure (bad spring)

[Syndrome]

Aldo, Thanks. I can fill the tank when cold, then run the car and watch it drain out of the vent tube. The next morning, the tank is about half full. It’s weird. So it could be as simple as the cap on the expansion tank being bad? Thank you!

[Aldo525]

Aldo, Thanks. I can fill the tank when cold, then run the car and watch it drain out of the vent tube. The next morning, the tank is about half full. It’s weird. So it could be as simple as the cap on the expansion tank being bad? Thank you!

Have you seeing the same medium level day after day without refilling???...or every day the level decreases and decreases???

[Syndrome]

I don’t actually know. I will monitor it and let you know. I wonder if the capo on the expansion tank is simply not the right one. I wish I had a minimal clue! Hahaha. Thanks

[Aldo525]

I don’t actually know. I will monitor it and let you know. I wonder if the capo on the expansion tank is simply not the right one. I wish I had a minimal clue! Hahaha. Thanks

I suppose your car is 87 model (e28 M5), if so, the right expansion cap is this OEM 17112227683. Note the mark "140" on one ear...that means 1,4 BAR cap....this is very important.

[Syndrome]

And thar she blows! I have the wrong cap on the expansion tank. No markings at all! Now I have to find the right cap for my car. Thank you!!!

[Syndrome]

I got a new cap from the BMW dealer today, using the part number of your cap. The new cap has 140 on it, so I hope it will eliminate the issue. The one thing I want to correct is that upon cleaning the cap that was on my car, it does have markings under the corrosion. It is all in German but has 120 on it, not 140. Does this sound like it would cause the problem I’ve seen? I’m really hoping this will be a $16 fix for the coolant venting problem. Thank you and HAPPY NEW YEAR!!!

[turbodan]

The only cause for concern would be coolant loss through the overflow in conjunction with high system pressure. If you are seeing the overflow and the hoses are soft then it would indicate a bad cap. If it's purging and the hoses are very hard to squeeze it indicates excessive cooling system pressure, usually due to a compromised head gasket. You usually only see this occurring after overheating.

New cap should sort it out.

[Syndrome]

Yup, thanks. And thankfully, the new cap fixed the issue. No more coolant coming out now!

[Aldo525]

Yup, thanks. And thankfully, the new cap fixed the issue. No more coolant coming out now!

Good news!!!

[Etip]

Glad to hear that it was a simple fix!!

[eastside_m5]

This is good info. I need to check mine too. MIne seems to lose coolant also.