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[TheBoy]

Trying to work out what can cause this.

I know a leak can, though don't understand how (surely coolant at higher pressure?). Anyone know why?

What else can cause cause it?

[Kevin Wood]

It doesn't seem logical that a leak can. I can't get me head around that. OK, if water leaks out under pressure when the engine's hot it will draw some air into the system as the engine cools again but modern systems are largely self-bleeding so that air would find its' way to the expansion tank (if it didn't enter through the cap in the first place) and displace a bit of water. No change in system pressure. I seriously doubt any part of a pressurised cooling system is below ambient pressure when running. It would take a powerful water pump!

Only thing I can think is that it's the first symptom of overheating due to too much air / not enough water in the system, that the leak is severe enough to reduce the pressure of the system so much that the coolant boils or that topping up has reduced the coolant concentration. Probably a combination of the above.

Other cases:

Expansion tank cap doesn't relieve pressure.
Expansion tank cap doesn't seal, insufficient coolant pressure which then causes coolant to boil causing a swing to overpressure.
Too weak an antifreeze mixture, allowing coolant to boil at "normal" coolant temperatures.
Coolant boiling due to general overheating e.g. blocked rad, thermostat, pump
Coolant boiling due to localised overheating (airlocks, poor coolant circulation, gauge reads normally)
HGF or other leak of combustion products to water jacket (cracked head, etc.)  

Kevin

[TheBoy]

It doesn't seem logical that a leak can. I can't get me head around that. OK, if water leaks out under pressure when the engine's hot it will draw some air into the system as the engine cools again but modern systems are largely self-bleeding so that air would find its' way to the expansion tank (if it didn't enter through the cap in the first place) and displace a bit of water. No change in system pressure. I seriously doubt any part of a pressurised cooling system is below ambient pressure when running. It would take a powerful water pump!

Only thing I can think is that it's the first symptom of overheating due to too much air / not enough water in the system, that the leak is severe enough to reduce the pressure of the system so much that the coolant boils or that topping up has reduced the coolant concentration. Probably a combination of the above.

Other cases:

Expansion tank cap doesn't relieve pressure.
Expansion tank cap doesn't seal, insufficient coolant pressure which then causes coolant to boil causing a swing to overpressure.
Too weak an antifreeze mixture, allowing coolant to boil at "normal" coolant temperatures.
Coolant boiling due to general overheating e.g. blocked rad, thermostat, pump
Coolant boiling due to localised overheating (airlocks, poor coolant circulation, gauge reads normally)
HGF or other leak of combustion products to water jacket (cracked head, etc.)  

Kevin

Thanks.

I think the current cap may have a suspect pressure relief.  Must try yet another....   ....putting off the inevitable for another week

[Markie]

It doesn't seem logical that a leak can. I can't get me head around that. OK, if water leaks out under pressure when the engine's hot it will draw some air into the system as the engine cools again but modern systems are largely self-bleeding so that air would find its' way to the expansion tank (if it didn't enter through the cap in the first place) and displace a bit of water. No change in system pressure. I seriously doubt any part of a pressurised cooling system is below ambient pressure when running. It would take a powerful water pump!

Only thing I can think is that it's the first symptom of overheating due to too much air / not enough water in the system, that the leak is severe enough to reduce the pressure of the system so much that the coolant boils or that topping up has reduced the coolant concentration. Probably a combination of the above.

Other cases:

Expansion tank cap doesn't relieve pressure.
Expansion tank cap doesn't seal, insufficient coolant pressure which then causes coolant to boil causing a swing to overpressure.
Too weak an antifreeze mixture, allowing coolant to boil at "normal" coolant temperatures.
Coolant boiling due to general overheating e.g. blocked rad, thermostat, pump
Coolant boiling due to localised overheating (airlocks, poor coolant circulation, gauge reads normally)
HGF or other leak of combustion products to water jacket (cracked head, etc.)  

Kevin

Thanks.

I think the current cap may have a suspect pressure relief.  Must try yet another....   ....putting off the inevitable for another week

got a "known good" cap if you want/ need it first classed tomorrow.

Let me know  

[Cav9_4]

When mine overpressurised the small brass tube in the expansion tank (which feeds to plenum) was blocked with some grey sealant,what i later found to be from oil cooler plate!...Wayne

[TheBoy]

Markie - I think I should have some good ones, as I have a garage full of them!

Cav - the TD one is different, and doesn't have that tube. Ta anyway


Cheers guys