General coolant systems discussion

[Tom_K]

Hi Guys and Gals,

 

I wanted to start this as a discussion point more than anything; firstly to see if my understanding is correct and secondly to gain input from those more knowledgeable if it's not or if they'd like to add anything. Oh and also, reason being I've recently replaced my coolant and it got me wondering.

 

So, what I did was:

- Drained the rad (took the cap off to drain it faster), drained the over-flow tank and then filled up both again as slowly as I could be bothered with. Oh and btw the front end was up high on axle stands for me to drain the rad.

- I then started the car, with the interior heater set to full heat to ensure full circulation, and let it warm up for a few minutes with the rad cap off (until I assumed the thermostat had opened).

- Then gave it some revs and held it about about 2.5-3k for another few minutes before putting the cap back on and letting it sit to warm up.

Interior heat was fine, all seemed good and I switched it off and let it cool down.

- The next morning I went to check and the overflow tank was pretty low, I assume the rad sucked any coolant it needed as it cooled down. So I it topped it back up.

 

So, I assume everything is fine but I'd like to understand a bit more about the technical side of things.

 

The 350z's coolant system uses a thermostatic radiator cap that releases pressure at 0.9 bar. This 'vents' any coolant into the tank, which is never pressurised, and when cold, this is how you gauge how much coolant is in the system.

Question:

-As the cap opens to circulate into the tank, would it naturally bleed air from the system?

-And would it suck the tank dry if there was a leak for example?

 

Also, on other cars I've had (like my Seat Ibiza), there is an expansion tank and no cap on the radiator.

This tank is the only access point to the coolant system and you fill up any coolant through it, not directly into the rad. This expansion tank is pressurised when the engine is up to temperature but there is no thermostatic cap like the Nissan that opens up at any point.

Can anyone explain the differences and why?

I know I could probably google this but I thought it would be good to get some discussion on the topic.

[Daisyduke]

From what I understand.

 

Q1

yes it would bleed but only if there is already sufficient coolant in the radiator to fill the space when expanded "swells". So only if there is only a slight amount of air.

 

Q2

No. The expansion tank will only allow coolant into the rad when the coolant "shrinks" to fill the space created. 

If you have a leak it creates a point where the coolant can expand rather than shrink. 

Like when you radiator is hot and you get some numpty take the cap off. It goes everywhere very fast as it swells. 

 

Eventually the level will drop and drain the exansion tank if it's a slow leak or closes when cold. But a large leak you will hopefully notice very quickly through coolant temp and steam loss.

 

Q3. 

Just different systems. 

1 uses rad cap to vent on overpressure and allow air to escape from expansion tank.

Other will normally have an expansion tank cap that can vent when hugely positive but also uses the head of pressure to control amount in the rad.

 

They aren't thermostatic caps. They are pressure. Pressure and temp in these circumstances have a fairly linear relationship.

Edited March 29, 2018 by Daisyduke
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[Tom_K]

Cheers for the reply.

 

So can anyone explain any pros or cons to each system? Why are they chosen?