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R33 cooling system questions

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Basically, at what sort of water temperature would you expect to see pressure in the cooling system?

I've got a nismo thermostat, stock, but 5 y/o radiator, stock clutch fan and rad shroud.

Remote oil cooler (keeping factory oil cooler/warmer under oil filter).


Ambient temperature of 22-25 and I see water temps of 71-75 and oil temp the same, I can open the rad cap at these temps and there is no pressure. Coolant level still at top of rad.

Recently changed water pump, and 0.9bar radiator cap and bled it correctly (car jacked up, heater on, bleed screw open, cut down bottle in top of rad, etc etc), took it for a drive up cambewarra mountain (basically a steepish, 6km hill) giving it a hard time, with air con on, and ambient temp of 42 (last weekend!) and it only just nudged 92 degrees at the top. 

By the time I'd got back home, it was at 75 deg, overflow bottle level went from half to 3/4. Opened cap and zero pressure.

Oil temps seem to mirror what the water temp is doing


Its also a drift car, and normally takes 5-7 passes before it hits 95ish, usually sitting in staging lane, it will fall to mid 80's within a few minutes.


From memory car has been like this for a long time, but as its never overheated and uses no coolant, runs perfectly fine, no oil/water mix in either oil or coolant side.

I haven't though too much of it, but having just done the water pump I was paying a bit more attention to it, and thought its strange.


Even at 70 degrees, I would have expected a bit of a pssht when opening the cap?




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On 03/12/2020 at 11:15 AM, CEF33Y said:

car jacked up, heater on, bleed screw open, cut down bottle in top of rad, etc etc

I've always done mine on the wheels, if it was jacked up at the front then maybe you created an airlock with the angle?  as Sir RB says, try bleeding it again.


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On 12/15/2020 at 6:45 PM, SiR_RB said:

I’ve had this happen to me in the past 

needed to be bled afew times and became good 


13 hours ago, tridentt150v said:

I've always done mine on the wheels, if it was jacked up at the front then maybe you created an airlock with the angle?  as Sir RB says, try bleeding it again.



Bled it one more time, on ground this time.

Then I drove it to work (50mins drive), then went to a mates place after work, (25 min drive through traffic), got to his place, opened bonnet, felt hose top hose which felt like it had some pressure, opened rad cap (carefully!) and sprayed coolant out. 


I'm guessing it just took another bleed/couple of drives to get all of the air out.


Tested it while hot and it holds pressure.


Thanks for the reply's guys



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17 minutes ago, Predator1 said:

Correct me if wrong, but if I was under the impression that if you open the cap when the thermostat is open, then there is very little, or no pressure?

No. The pressure is a function of temperature. The boiling point of water (at sea level) is 100°C. At that temperature the vapour pressure created is 1 atmosphere (101.325 kPa). It is at that point that the steam can jump out of the liquid phase. But at all temperatures below 100°C (and above 0°C if you want to be fussy about only talking about the vapour pressure originating in the liquid phase) there is a vapour pressure. It is very low at low temperatures and it rises to 101.325 kPa at 100°C. And, it increases above 101.325 kPa as the temperature exceeds 100°C, if the water is in a trapped volume (ie, it can't flash to vapour because there is no room for it to do so). That last bit describes a cooling circuit perfectly. Put a 20 psi radiator cap on the top, which will relieve at 20 psi absolute (about 5.3 psi above atmospheric pressure, or ~36 kPa above atmospheric) and it will hold pressure right up until it reaches that temperature and pressure and then the cap will open to relieve the excess pressure, venting hot arse water out into the overflow.

Open the cap when the engine is hot and the thermostast is open and you will instantly allow a quantity of water to flash to vapour, driving steam and hot arse water all over the place. This usually leads to 3rd degree burns and an extensive hospital stay, bill and use of morphine.

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