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505 Coolant bottle pressure cap replacement


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One of the most important parts of the cooling system is the pressure cap on the coolant bottle, and with our cars age finding ones in good shape is rare.

On a hunch I tried the cap off my Volvo and it fit perfectly! I also tried the cap off an Eagle Premier bottle and it too fit perfectly.

85coolta.jpg

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  • 4 weeks later...

Sorry Walter - I was about to respond, and then I had an epiphany with my own cooling system mods and I forgot to reply!

Our cooling systems use an expansion system where as the coolant expands the excess goes into the bottle and pressure on the whole system is kept via the cap. So in our systems the added pressure to the system allows for higher temperatures to be reached without expanding the coolant as much as if you were to heat the coolant at atmospheric pressure. (Also has a higher boiling point under pressure)

On cars with recovery systems - the cap plays a much more important role as it not only controls pressure in the system, it also controls the coolant being expelled and then sucked back from the coolant tank / bottle.

There have been a number of theories on our cars as to why the head cracks, and one of them that makes some sense to me is that both the expansion and the recovery systems have a flaw in them that if there is an air leak anywhere in the system that allows air into the system - the expansion and contraction of fluid that ensures the engine (and particularly the head in our N9T**'s) are kept right full of coolant fails. As the engine cools, it should draw the coolant from the tank back into the engine, and the cap should allow air into the tank. If there is a leak somewhere like a rad hose or even the rad itself - instead of drawing coolant into the cooling motor, it will draw in air and the engine will not be full of coolant. Not having the head full of coolant runs the risk if uneven cooling which could possibly aggrivate an existing head flaw causing the cracks we see in our heads.

I've been fighting my own cooling system issues and I've been trying to come up with a more robust system that will still ensure the system remains absolutely full even if there is the slightest leak that could prevent the cooling motor from drawing coolant back into the motor.

The epiphany I spoke of earlier was that not only do I need a high mount expansion tank, but I need to make changes to the system so that it self bleeds air out of the system, and always keeps the engine full of coolant as long as coolant is in the bottle. Still planning this out, but essentially the coolant expansion tank will be completely above the engine, with vent hoses from the rad and t-stat housings routed so that any trapped air has a path into the top of the expansion tank, and coolant "pressure" will always be on the system due to the slight pressure head from the slightly higher tank.

Playing around with a couple ideas right now - but once I get something concrete I'll post up.

Rabin

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