Just to correct you on a technicality Bryan, increasing the cap presure doen not lower the boiling temp, it increase the boiling point. As for running a higher 1.3bar cap, I don't see the point. The system was designed to vent at 1.1 bar TT and 0.9 bar NA. You really won't see the effect of a higher pressure cap in normal conditions and as chrispy said, if you need a higher pressure cap in these cars to prevent the coolant boiling you have another issue. A car like Rob260's that is making an ass tonne of under bonnet heat may need this cap when he is really on it, but for us average schmo's stick with what nissan screwed on there. The other thing to check with over heating issues, and I mentioned it in an earlier post above, make sure the cap seals on the bottom of the neck. There are several different reach caps available to suit different depth necks. Put the cap on but do not turn it, press down on it, it should push down a few millimeters and spring back up. If it doesn't push down, the cap is too short and if you really have to push it down hard to twist it on, it is probably too long.
Just to correct you on a technicality Bryan, increasing the cap presure doen not lower the boiling temp, it increase the boiling point Thanks Eric thats what I meant but good u picked up on it... I think for most going out to buy a new rad cap if needed still better to buy from nissan dealer. cost is not bad and its going to fit properly... Others from supercrap/ repco and alike can look like they fit but leak forever and most never seal...For ease and peace of mind your best chance is nissan who offer a 1.2bar cap that fits. bought mine 3 years ago mine also has a metal stop cock.. not sure if the recent ones are plastic Yes all the new asi rads have a plastic stop cock
Oks. Boiling point is when the vapour pressure of a fluid equals the atmospheric pressure. So atmospheric pressure at sea level is ~101.3kPa. The rad cap increases the pressure to the atmospheric pressure plus the pressure noted on the cap. The temperature at which a pure substance at a fixed pressure changes phase (solid to liquid, liquid to gas etc) is called the saturation temperature. So for water that is 100degC @ 101.3kPa. 1.1Bar is equivalent to 110kPa, so that gives a total pressure of 211.3kPa. 0.9Bar is 90kPa, and that gives a total pressure of 191.3kPa. Now looking at our steam tables (big lists of properties of water and other gases/fluids, which show saturation temperatures, pressures, enthalpy, entropy, density, internal energy and all that fun stuff) we can see that those pressures aren't exactly noted. Closest we have is: 0.175MPa with a Sat. Temp of 116.06 0.200MPa with a Sat. Temp of 120.23 0.225MPa with a Sat. Temp of 124.00 Interpolating those points to get our desired pressures gives: Saturation temp @ 211.3kPa = 128.6degC Saturation temp @ 191.3kPa = 122.4degC So there you go, 6deg difference
A bit heavy for a bloke who's not too bright. I think a new cap is the go! That's about my limit. I've made sure the cap seals and took it for a casual spin again keeping more of an eye on the temp gauge than the road and all's well, still. I'll pick up my $38 cap tomorrow and let you's know how it went. By the looks of it temps sure are important, especially in a zed.
Nice chrispy cheers for that info im still pretty happy that it gives a 6deg difference from .9bar to 1.1bar so that makes going from a .9bar to 1.3bar still a good way to keep cool.. Im not suggesting this will fix overheating issues but on a good working zed this is another thing to help keep temps down... Cheers Bryan
Another thing to look at is the radiator neck where the cap goes on. Over time the plastic round ridge flattens out so the cap can't seal properly.
Thanks Chrispy! Interesting that in theory as long as nothing bursts or leaks the coolant should not actually boil until well over 120 degrees.
...And knowing that the cooling system is actually a mix of ethylene glycol and water (the former having its own saturation temp). But Chrispy's explanation is good enough for me
Yeah, I think glycol has a higher boiling point than water so the water calculations seem safe to use to me.
Just consulted my Si Chemical data Book... Ethylene Glycol (1,2-ethanediol) has a boiling point of 198 Degrees Celsius. This is assuming standard atmospheric pressure and temperature (1bar pressure at 25 degrees celsius).
There would be some way to test it based on the mix of the water and ethylene glycol. You can use a refractometer to measure the mix quite accurately. But in the end it will just change the final value by a poofteenth of a few degrees, so why bother. Love your work chrispy
Got my new cap on, took her for a good drive and all's normal. Amazing how a simple cap can cause so much grief. Thanks for all your help guys. Finished up an interesting thread on temps.
I think you will find the addition of glycol to the water actually LOWERS the boiling point of water. The Glycol is there as antifreeze and anticorrosion additive.
So how can a solution of liquids that boil at 100* and 198* boil at <100* ??? Chemical engineers, Unite. Its called antifreeze/antiboil for a reason. I would think that it does BOTH. Corrosion inhibitors are relatively inert as far as the temperature goes.
