PSI of pressure cap cooling system BMC 1.5

[Horsehorn]

Hi all,

 

What psi should you have for a pressure cap for the cooling system for a BMC 1.5? The spring on mine is bust and I foolishly forgot to check what the old one was and I'm not back for a couple of days now.

 

Cheers

Also what's the difference between short reach and large pressure caps, and blanking caps? Looking at the selection on ASAP here http://www.asap-supplies.com/engine-spares-gearboxes/pressure-caps

 

Thanks

[alan_fincher]

A blanking cap is exactly that, it just blanks the hole, but is NOT a pressure cap. You would use it, if you had a pressure cap elsewhere, say on a connected expansion bottle.

 

The long and short reach thing is how deep the hole is, so how far the spring loaded plate needs to reach into it. If you do'nt know what you have, I wouldn't guess, as the wrong one will not be suitable. (It will probably either not fit at all, or will appear to fit, but actually be sealing nothing, because the "innards" don't reach far enough.) You need to measure up.

 

Typically the cap will probably be 7 psi, but I chose to reduce mine to a 4 psi one, when we had one. The reduced pressure was fine for the maximum temperatures being reached, and it had the advantage that the nasty "rubber" end caps on the Polar heat exchanger stopped failing, as they had done repeatedly up until that point. Certainly I would say nothing MORE than 7psi.

[Horsehorn]

Thanks Alan. I'm guessing if I have a lower pressure rating then this may reduce water/coolant splurting out of the overflow valve a little when the engine is in full swing? But the downside is that it may struggle to get up to temperature, poss esp in the winter?

 

Lucy

[alan_fincher]

Thanks Alan. I'm guessing if I have a lower pressure rating then this may reduce water/coolant splurting out of the overflow valve a little when the engine is in full swing? But the downside is that it may struggle to get up to temperature, poss esp in the winter?

 

Lucy

 

No, I'm afraid both of those guesses are wrong.

 

The pressure cap should not really be opening unless the engine is getting up to full temperature.

 

If water is coming out, it may well be that you are over-filling it. Depending on the size of your skin tank heating the water can produce a lot of extra volume by expansion, so when looking into the header tank when it is old, the level needs to be well below the neck, (say about an inch, but it depends on exactly what you have), to allow for that expansion, or the excess water has nowhere to go but be expelled.

 

If your coolant contains anti-freeze, (as it should!), and this happens repeatedly, and you only top up with plain water, then over time the coolant anti-freeze percentage will get less and less, and may fall to a level where it is no longer able to protect the engine against sub-zero temperatures.

 

If you already know all this, I apologise, but it is probably still worth repeating, because it might save someone reading it a cracked cylinder block!

[David Schweizer]

The correct rating for a BMC 1.5 is 7lbs, but as Alan suggests some people choose to use 4lbs to protect the heat exchanger end caps. This may just be a problem with the Polar, as we have a Bowman heat exchanger with a 7lb pressure cap, and have never had an end cap fail.

 

With regard to water "spurting out" may i suggest a couple of things. It is only necessary to fill the heat exchanger until you can just see water in it, that way there is more room for expansion before loosing coolant, Alternatively, you could connect a small overflow container onto the neck of the pressure cap fitting and collect any expelled water which can be poured back into the heat exchanger when it has cooled down.

Edited August 22, 2016 by David Schweizer

[PaulG]

A blanking cap is exactly that, it just blanks the hole, but is NOT a pressure cap. You would use it, if you had a pressure cap elsewhere, say on a connected expansion bottle.

 

The long and short reach thing is how deep the hole is, so how far the spring loaded plate needs to reach into it. If you do'nt know what you have, I wouldn't guess, as the wrong one will not be suitable. (It will probably either not fit at all, or will appear to fit, but actually be sealing nothing, because the "innards" don't reach far enough.) You need to measure up.

 

Typically the cap will probably be 7 psi, but I chose to reduce mine to a 4 psi one, when we had one. The reduced pressure was fine for the maximum temperatures being reached, and it had the advantage that the nasty "rubber" end caps on the Polar heat exchanger stopped failing, as they had done repeatedly up until that point. Certainly I would say nothing MORE than 7psi.

I had similar experiences with my boat whan I first bought it. Going through Polar heat exchager caps like there was no tomorrow.

Discovered that the cap fitted by the previous owner was a 13 psi one.

Changed to 7psi and the problem has not recurred.