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Discussion Starter · #1 ·
Had a look at the daughter's BF forrester that was apparently overheating.
Intersting design of cooling system - thermostat in radiator return - supposed to be a cause of k series HGF. Also, coolant expansion tank is not pressurised.

It turned out that the radiator cap has 2 valves, one for over pressure, where water passes into the expansion tank, the other for vacuum, where it sucks water out of the expansion tank as it cools down. great in theory BUT, the vacuum valve has a very light spring and seals over a big area so the sealing pressure is very low. So, any crud that is sucked up from the unseal expansion tank can cause the valve to crack open.

There was a huge amount of crud in the expansion tank, and a fair bit of it had caked up the vacuum valve preventing it from sealing.

result was dramatic water loss and overheating.

Much prefer the MGR sealed coolant system.

Sweet running engine though!
 

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zs rad cap has a vacuum and pressure valve in it too:) but im with you regard not liking that system, plus you cant really check the coolant level when hot
 

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We just bought a beaut of a wrx m reg with 90k on clock. Past owner had ideas head gaskets had failled. RESULT we found that it was purely down to perished rubber on rad cap just under £8 on ebay for a fix. It then went on to blow the top off the header tank due to a very very poor design on the early cars. A couple of metal lugs bent over the plastic top! So yes, I agree entirely.
 

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A boffin and proud of it
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Discussion Starter · #4 ·
Same "diagnosis" was made by the owner. (seems that any form of overheating is instantly HGF these days!)

Fortunately, on this forum, we soon learn all about what is/is not HGF or causes of coolant loss and overheating.

The problem with the Subaru system is that dirty water can be sucked up from the vented "header" tank as it is not really an expansion tank, just somewhere for water to go when vented due to overpressure. Unlike a pressurised expansion tank, there is no head of water to keep the system topped up in case of leaks so, in that way it is "worse" than the k series!.
But being a Subaru, of course, it does not matter.....
 

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Hi. Good Post. So what, in your experience, is the most likely cause of coolant loss in a zed if there are no obvious signs of leakage? Had to do a considerable top-up recently and have since noticed another small drop in the expansion tank. No Mayo in oil and no overheating.
 

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ah yes, i had realised this driving into work yesterday that of course the coolant is going through the pressure cap
 

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The rubber sealing ring around the top on the scooby was perished and bits missing so coolant not even under pressure, leaks past and into the expansion bottle which is down in n/s/f inner wing area under bonnet. It gives effect of overheating.
 
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