Expansion Tank Cap Replacement

[BCS_ZHP]

Thought I'd post this up since I just did it a few minutes ago and its an easy, cheap first step if you're losing a little bit of coolant and can't find a leak.

So this car had a complete cooling system overhaul about 2 years ago but I noticed the coolant was about 8-12 ozs low the other day. When adding the coolant, the 2 rubber o-rings on the cap appeared a little flattened out to me and the sealing gasket at the top of the cap was definitely flattened out, thus ordered a new cap.

Left side is the old one, right side the new one.
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Also notice how there is some white residue on the old cap between the upper o-ring and the rubber sealing gasket. My theory is when the coolant get hot enough, some of it is vaporizing as steam and is leaking past these slightly flattened rubber o-rings & gasket. So I believe once the steam gets outside to the atmosphere it stays as vapor for a few seconds and is carried away, this would explain why some of us are low on coolant this winter with no visible signs of leaks anywhere. I'm not sure if the extremely cold winter temperatures are contributing to this but I suspect so. (Where's our thermodynamics engineer to validate or shoot down my theory?)
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Anyway, the cap is only a $15 part so its worth a try if you have no visible signs of a coolant leak but you had to add some recently. Hope this helps someone.

[Avetiso]

Once the gas gets out of the tank, I don't think the engine bay is nearly hot enough to keep it in the form of a gas. It would condense almost instantly, I would imagine.

Also, if you boil coolant, I believe you only boil away the water from it, which is why there are no signs of coolant. Just like if you boil sugar (covalent) or salt water (ionic), only the water goes away.

[BCS_ZHP]

Dario, have had really bad ET caps on a couple of cars I bought that required replacement immediately, and you could see the white residue drips on the sides of the ET. None of that evidence here or anywhere in the engine bay.

[ZHPRegistry.net]

Replace the cap at 50K intervals. This little, cheap part has been responsible for falsely accusing many blown expansion tanks.

[Avetiso]

Dario, have had really bad ET caps on a couple of cars I bought that required replacement immediately, and you could see the white residue drips on the sides of the ET. None of that evidence here or anywhere in the engine bay.
If it was really bad, then that means the coolant could have escaped in liquid form, which would leave residue because it didn't vaporize.

If I took salt water and splashed it over something, then let it evaporate, it would leave residue wherever it touched. If I boiled the salt water and collected the vapor, then let that vapor condense over the same area, it would leave nothing (or very little) behind.

This is just going off my physics/chem knowledge. If there is some sort of exception to that, then I may be wrong, but that's what I'm thinking is happening. My ET actually had white residue when I got it. I cleaned it up and installed the G.A.S. cap, and it hasn't returned.

[johnrando]

Good tip to remember to replace that cap.

[NorCalZman]

the GAS cap is the 1.2 bar cap right? Lower than the stock 2 bar. You subscribe to the theory that it might help prevent ET explosions?

I have been thinking of putting a stock E30 1.4 bar cap on. The threads on the forums are pretty passionately split about doing something like that.

[johnrando]

No sure. Just to be clear though, the G.A.S. cap is over the actual radiator, not the expansion tank.

[Avetiso]

No sure. Just to be clear though, the G.A.S. cap is over the actual radiator, not the expansion tank.

Uhhh... Mine is on the expansion tank. :dunno

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[ELCID86]

Uhhh... Mine is on the expansion tank. :dunno

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+1 and my gas cap is on the fuel filler neck behind a little door. #smartass

[ELCID86]

Thought I'd post this up since I just did it a few minutes ago and its an easy, cheap first step if you're losing a little bit of coolant and can't find a leak.

So this car had a complete cooling system overhaul about 2 years ago but I noticed the coolant was about 8-12 ozs low the other day. When adding the coolant, the 2 rubber o-rings on the cap appeared a little flattened out to me and the sealing gasket at the top of the cap was definitely flattened out, thus ordered a new cap.

Left side is the old one, right side the new one.



Also notice how there is some white residue on the old cap between the upper o-ring and the rubber sealing gasket. My theory is when the coolant get hot enough, some of it is vaporizing as steam and is leaking past these slightly flattened rubber o-rings & gasket. So I believe once the steam gets outside to the atmosphere it stays as vapor for a few seconds and is carried away, this would explain why some of us are low on coolant this winter with no visible signs of leaks anywhere. I'm not sure if the extremely cold winter temperatures are contributing to this but I suspect so. (Where's our thermodynamics engineer to validate or shoot down my theory?)


Anyway, the cap is only a $15 part so its worth a try if you have no visible signs of a coolant leak but you had to add some recently. Hope this helps someone.

Did you go with the standard 2.0 bar?

[BCS_ZHP]

Stock OEM cap from Fairfax BMW.

[danewilson77]

No sure. Just to be clear though, the G.A.S. cap is over the actual radiator, not the expansion tank.

Huh? No comprendo Amigo.

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[johnrando]

Did I say that? LOL. I was thinking of my other car that has a radiator cap and an expansion tank cap separate, and you can replace them both separately. Sorry. I've had that car on my mind.

[BCS_ZHP]

Bartender, I'll have what the gentleman on the floor (JR) is having.

[johnrando]

One for everybody, on me!

[aurelius]

I have the G.A.S. cap on mine, no issues.

And the "late E30" 1.4-bar cap on our other E46 (325i). The part number for which is 17111742232. BEHR version for $9.