Radiator still has pressure when cold

[BushG8]

2002 KZN165 since new only 230K on ODO
1KZte engine

The radiator is retaining pressure overnight i.e the coolant overflow bottle is filling up over a period could be days but also depends how long its driven on the hi way if I do 500ks it will get to 3/4 full and not return to the radiator as it cools down as it is supposed to.

Cap replaced with Toyota Genuine no change.

When the engine is cold overnight and turn the cap 1/4 turn to the first stop when removing it will vent air into the bottle i.e blow a few bubbles.

Never been overheated owned since new and only driver.
Temp Gauge sits at near horizontal and has not moved in 20 years.
No water in the oil.
Does not use water as i just remove the bottle pour it back into fill the radiator to full and the bottle will then be close to the full line of coolant.

Noticed it 18 months ago but should do something about it.

So I assume a gas leak into the water jacket somewhere which could be Head Gasket, or a crack in the head or Block, Water pump?
any pointers appreciated and what to do first?

[Gipsy]

Possible leaking head gasket.

[Rob_Wood]

Possible leaking head gasket.

Or like bush already said could also be cracked head. I'd like to look on the positive side though. You mentioned something about emptying the reservoir back into the radiator occasionally. How often do you do this? The reason for asking is that the time it takes the system to level out is not likely to be one or two drives down the road. I'd suggest driving daily for a week or two (up to temp each time) and check levels. Do the same for another week or two and check again. If there's any considerable disparity then look more closely. If no considerable change then have a beer & stop stressing.

[astromo]

Possible leaking head gasket.

Isn't the typical symptom for this oil in the coolant?

Certainly, if the coolant system is leak tight, cooling will make the liquid more dense and its specific volume will decrease. In short it should tend to pull a vacuum.

[BushG8]

Possible leaking head gasket.

Isn't the typical symptom for this oil in the coolant?

Certainly, if the coolant system is leak tight, cooling will make the liquid more dense and its specific volume will decrease. In short it should tend to pull a vacuum.

As no oil in coolant or coolant in oil and not losing coolant other than into the overflow tank which is there to capture the expanding coolant as it gets hot anyway, seems only a small amount of gas under pressure getting into the cooling system is my best guess. And once shut down and pressurised residual pressure is still greater than the vacuum of the cooling liquid so it can’t suck the coolant from the overflow tank back into the system hence the bubbles into the tank when the cap is turned. But why won’t that pressure go out of the system the way it got in once the engine is off and cooled to ambient if there was a head crack or gasket blown it should get into the crank case and dissipate.

[Rob_Wood]

Possible leaking head gasket.

Isn't the typical symptom for this oil in the coolant?

Certainly, if the coolant system is leak tight, cooling will make the liquid more dense and its specific volume will decrease. In short it should tend to pull a vacuum.

As no oil in coolant or coolant in oil and not losing coolant other than into the overflow tank which is there to capture the expanding coolant as it gets hot anyway, seems only a small amount of gas under pressure getting into the cooling system is my best guess. And once shut down and pressurised residual pressure is still greater than the vacuum of the cooling liquid so it can’t suck the coolant from the overflow tank back into the system hence the bubbles into the tank when the cap is turned. But why won’t that pressure go out of the system the way it got in once the engine is off and cooled to ambient if there was a head crack or gasket blown it should get into the crank case and dissipate.

It actually sounds like it's all working as designed. Once cooled, the coolant system in the.engine will retain a small amount of pressure.as.the relief valve in the radiator cap will stop releasing coolant into the reservoir at a certain (cool-down) temperature. This is how the system works.

[Mudgee mud]

I'm guessing it is not heat related to the cap temp, but air pressure entering the radiator tank. And air pressure is forcing the coolant out.
I can only imagine it to be on the pressured areas of the motor. ... head gasket.. etc.
A cheap simple radiator leak down test would be best.
A tiny leak I'm guessing that only leaks when the motor is hot / expanded. But closing up when cold. Shrinking.
Other possibility is turbo pressure leak.

[Philos]

I don't see what the problem is. If there was a leak of some kind, there'd be no pressure build-up.

[Mudgee mud]

I don't see what the problem is. If there was a leak of some kind, there'd be no pressure build-up.

If it was an old Holden red 308, one could quickly diagnose the fault.
These days, with so many variables due to new gizmo's fitted has led to many more possibilities.
BT Mazdas are notorious for their EGR coolant systems failing/ leaking.
Symptoms are loss of coolant, ( with no visible signs of leak point) , hard to start after sitting/cooling. (Hydro-lock)
Because exhaust gases are "pressurising" the cooling system with exhaust gases from the EGR unit.
Once the engine cools, Coolant enters rear piston pots once the engine is off, Thus, "air pressure" in the radiator forces it back into the intake recirculation system to the piston pots.
Thermostat/ head gasket/ radiator cap/ over flow bottle are all irrelevant of faults previously commonly targeted for such faults.
This "pressure" accumulated in the radiator to force coolant out to the over flow bottle, may not always be from heat expansion of coolant.