# Coolant overflow caps: rubber o-ring seal ... or not?



## sam_sheppert (Apr 19, 2010)

I have an aftermarket coolant overflow cap for my Mk III Golf that has a rubber o-ring. All of the original VW coolant caps I see in the salvage yard are missing an o-ring. The cap with an o-ring does not sit flush against the top of the overflow 'ball'. Without the o-ring, it does. Does the cap seal without the benefit of the o-ring?

Meanwhile, I may have found the source of my overflow/overheating problems of late:










Spot the o-ring - or what's left of one. It likely kept the cap from fully seating, letting coolant pour out when the engine overheated.


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## sam_sheppert (Apr 19, 2010)

I have two overflow tank caps for VW. They both fit the A2, A3 and probably other lines of VW cars. One has a rubber o-ring. The other has no rubber o-ring. The one with the o-ring causes the cap to stand about 1/4 inch above the deck of the top of the tank. The one without an o-ring sets it's bottom surface flush against the top of the tank. The question is, which seals best? Which is sanctioned by VW and works best with the car? I just had a mess trying to figure out why my car was overheating and I think a little piece of o-ring at the bottom of the overflow tank's fill-hole (area around the hole) may have been the cause. If an o-ring were not necessary, then maybe this could have been avoided. So I want to know. Does the overflow tank cap need an o-ring or not?


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## Guidofester (Aug 28, 2003)

*coolant cap*

Yes it needs an o-ring. It would be a good idea to take the cap to a mechanic that can test it with the 
special adapter,& the same pressure tester that is used to test the rad. The cap works like a pressure
relief valve. I think it releases at around 18 psi. When my cap failed it held zero pressure. So I 
guess if you have a good cap and o-ring you will notice the rad hoses holding pressure as 
Soon as the car starts to head up.
Hope this helps. Also,you can always look up the parts on bestpricecarparts to see if it is 
available. I know I got stuck once and bought a new o-ring at the dealer. I damaged my o-ring 
removing the cap when the system still had pressure.


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## sam_sheppert (Apr 19, 2010)

Thanks. I got a response in another thread where I was shown a photo of the underside of a cap shoeing the oring hugging the outside of the inside plug of the cap. So on this car someone had shoved a fat oring to the bottom of the outer channel of the reservoir. The oring rotted and the only plucked out 3/4 of the oring - leaving in a piece which kept the cap from bottoming out and sealing. Seeing it was leaking, instead of clearing out the piece of oring, they put an oring around the outer part of the cap. It's just one thing after another with this car.


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