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Posted: Tue Jun 04, 2013 10:59 pm
by Denis247
ChrisJC wrote:The rad cap is not the normal type - I don't know what it is called.
But the important point is that it seals to the outer part of the neck (whereas a normal one doesn't seal that part). The inner part is normal, i.e. it is sealed to the inner part of the neck with a spring loaded seal. So excess pressure will pass the inner part, and will pass down the overflow pipe to the expansion tank. This only happens when the pressure exceeds the rating of the cap. When the engine cools, it will generate a vacuum, and suck water from the expansion tank back into the engine. A normal radiator cap will let air back in here because the outer part of the cap is not sealed, but this kind of radiator cap means the vacuum will definitely suck from the expansion bottle and draw in water.
So it temperature cycling will purge any air, and keep it purged.
Chris.
Am I using the wrong cap then?, I cant see how water can be drawn back into the system from the overflow bottle with the cap closed and sealed at point B. Vacuum in the system would only increase this seal. I thought this was just to expel excess coolant and the only reason for the overflow bottle was to stop it going over the engine/road.
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Posted: Tue Jun 04, 2013 11:42 pm
by katanaman
Its a common system on bike cooling systems. The caps release both ways, one under pressure and the other under vacuum. There is also a double seal so the coolant goes via the pipe rather than out the neck of the radiator and cap.
Posted: Wed Jun 05, 2013 8:08 am
by DaveEFI
I bought a pressure test set some time ago - it is near essential on my BMW to bleed the residual heater on it after changing the coolant.
My SD1 had always lost a small amount of coolant - not enough to really matter, just annoying. Turned out to be a small leak at the pressure cap which was new. A good clean and polish of the seat in the expansion tank sorted it. I had to make up an adaptor to pressurize the system via the filler plug to find this, as using the supplied one to pressurize it via the filler showed no leaks.
Posted: Wed Jun 05, 2013 5:45 pm
by ChrisJC
Denis247 wrote:
Am I using the wrong cap then?, I cant see how water can be drawn back into the system from the overflow bottle with the cap closed and sealed at point B. Vacuum in the system would only increase this seal. I thought this was just to expel excess coolant and the only reason for the overflow bottle was to stop it going over the engine/road.
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So, at point B, there would be a small valve & rubber seal to permit a vacuum to draw into the cooling system.
And at the top, the underside of the cap would have a rubber seal against the top of the neck.
Chris.
Posted: Thu Jun 06, 2013 9:36 am
by Denis247
ChrisJC wrote:Denis247 wrote:
Am I using the wrong cap then?, I cant see how water can be drawn back into the system from the overflow bottle with the cap closed and sealed at point B. Vacuum in the system would only increase this seal. I thought this was just to expel excess coolant and the only reason for the overflow bottle was to stop it going over the engine/road.
[/img]
So, at point B, there would be a small valve & rubber seal to permit a vacuum to draw into the cooling system.
And at the top, the underside of the cap would have a rubber seal against the top of the neck.
Chris.
Thanks Chris..and are such caps available in the normal car size, and for different pressures? What are they called and any clues of stockists?
Posted: Thu Jun 06, 2013 11:46 am
by JSF55
I got interested in this and had a google, I though all caps had this
http://www.mgb-stuff.org.uk/radcap.htm
the only other type I've seen have been flat ones, excluding the mickey mouse(small) ones fitted to our Nissan forklifts
Posted: Thu Jun 06, 2013 5:00 pm
by Denis247
JSF55 wrote:I got interested in this and had a google, I though all caps had this
http://www.mgb-stuff.org.uk/radcap.htm
the only other type I've seen have been flat ones, excluding the mickey mouse(small) ones fitted to our Nissan forklifts
Great stuff, thanks very much. Sorry to hijack the thread.
Posted: Thu Jun 06, 2013 7:49 pm
by ChrisJC
That is exactly what I am talking about - excellent find.
If you just have a header tank, the main seal doesn't need to be present. This is because the cooldown water is drawn from the header tank, not up the overflow from the expansion bottle.
Chris.
Posted: Fri Jun 07, 2013 6:26 am
by unstable load
If you have that style of cap on the radiator, then you should not pressure seal the header tank, a simple vented cap will be all you need.
Posted: Sun Jul 28, 2013 3:08 pm
by Camo
To put an end to my original problem, I have purchased a bigger radiator, added an expansion tank, checked the thermostat and water pump... and I think my main problem was the gauge needed a voltage stabiliser as it is showing hotter than the engine actually is
So after spending a few hundred pounds more than I needed I guess I have found my issue, at least I have a better system and a better understanding of it !
Thanks for the help.
Stu
Posted: Fri Aug 02, 2013 5:32 am
by unstable load
Glad you got it sorted, Stu. Pity about the extra costs, but now you KNOW it is good and even if the gauge fails, you should be safe as houses.