Club 80-90 Forums

Ok a physics lesson
as water gets hot it's volume expands
when it cools it contracts
if there is no expansion there is no contraction

therefore you cooling system will push water in your expansion tank out into the top up tank as the engine heats it up and it reaches the release pressure the cap is designed for ... once there it cools.
when your engine cools the pressure (created by the cap) reduces and water flows back into the expansion tank.


if there is no retraction vacuum due to leaks it will remain in the top up tank and the system will draw air into it from the leak

And of course if your cap isn't working correctly the water isn't drawn back into the main tank.

Ok. But Kev, wouldn't you expect, after 70 miles at 70mph, with a very hot engine, the coolant in the expansion tank to even a little bit warm?

Maybe, like Oldie says, I have a dodgy cap on the tank by the engine?

isaune wrote: Maybe, like Oldie says, I have a dodgy cap on the tank by the engine?

That is a common cause of this phenomenon. When getting a replacement, try to get a genuine VW one (Brickwerks = http://www.brickwerks.co.uk/shop?page=s ... ory_id=118" onclick="window.open(this.href);return false; ) as some of the others (GSF and JK ones) are poor quality.

You can sometimes revive an old one by blasting water through it via the little stub pipe. A bit of "packing" (insulation tape) to make it a tight fit onto a hose pipe and turn the tap on. Repeat a few times and they often start to work again (it blows debris out of the one way valve system).

As said though, if you have a leak, such as a poorly fitting water level sensor on top of the expansion tank, as the water cools and contracts, it will suck air in through the leak place, instead of water back from the top up tank.