With certain cylinder heads, local steam pockets can form which will pressurize the system.
Some coolant systems are pre-pressurized in order to raise the coolant boiling point.
Depending on load, ambient temperature and duration of full throttle, the coolant may
boil, but at a much higher temperature. We have seen local coolant pressures over 75psi in
a system with a 40 psi cap. Even worse after the hot engine is shut off.
Those pressures are dwarfed by combustion gas pressure; otherwise coolant would appear in
the combustion chamber.
But local metal surface temperaures can be much higher than average coolant temperature
depending upon the measuring point.
Cylinder heads can be thermally distorted resulting in reduced clamping pressure thus allowing
high pressure combustion gas to enter the cooling system. Aluminum heads should always be
straight edged after gasket removal.
For a number of years we sold a coolant additive containing a pH indicator. When combustion
gas (acidic) entered the coolant, it would change from pink to clear. Of course, that was a bit
too late. We wound up inserting a clear tube into the coolant hose to observe the color and
bubbles. There were always bubbles.
Water Pressure Issue
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Re: Water Pressure Issue
Thank you for the reply. I understand all that. My question however was in regard to your statement "Gas in the cooling system can originate from local coolant boiling. Agreed,we've all seen thatAt some point the gas pressure can lift the head and allow the addidtion of combustion gases." Sentence underlined is what my question is referring to.I still do not see that happening. Please help me understand how steam in the cooling system can exert enough pressure to lift a head.
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Re: Water Pressure Issue
It is not the steam pressure, it is the heat in the head that affects the gasket seal (head warpage), allowing combustion gases to blow into the coolant. This is often encountered when the engine is operating under conditions of pre-ignition.fabr wrote: ↑Tue Jun 21, 2022 1:39 pm Thank you for the reply. I understand all that. My question however was in regard to your statement "Gas in the cooling system can originate from local coolant boiling. Agreed,we've all seen thatAt some point the gas pressure can lift the head and allow the addidtion of combustion gases." Sentence underlined is what my question is referring to.I still do not see that happening. Please help me understand how steam in the cooling system can exert enough pressure to lift a head.
Re: Water Pressure Issue
Thank you for taking the time address that point. I now understand and agree with what you meant to say. It is not the gas pressure but rather the gas bubble creating uneven cooling and distortion compromising the gasket/lifting the head.I think that is correct?David Redszus wrote: ↑Tue Jun 21, 2022 1:59 pmIt is not the steam pressure, it is the heat in the head that affects the gasket seal (head warpage), allowing combustion gases to blow into the coolant. This is often encountered when the engine is operating under conditions of pre-ignition.fabr wrote: ↑Tue Jun 21, 2022 1:39 pm Thank you for the reply. I understand all that. My question however was in regard to your statement "Gas in the cooling system can originate from local coolant boiling. Agreed,we've all seen thatAt some point the gas pressure can lift the head and allow the addidtion of combustion gases." Sentence underlined is what my question is referring to.I still do not see that happening. Please help me understand how steam in the cooling system can exert enough pressure to lift a head.
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Re: Water Pressure Issue
When the engine is running, there is some balance between combustion gas pressure and coolant steam gas pressure.fabr wrote: ↑Tue Jun 21, 2022 2:12 pmThank you for taking the time address that point. I now understand and agree with what you meant to say. It is not the gas pressure but rather the gas bubble creating uneven cooling and distortion compromising the gasket/lifting the head.I think that is correct?David Redszus wrote: ↑Tue Jun 21, 2022 1:59 pmIt is not the steam pressure, it is the heat in the head that affects the gasket seal (head warpage), allowing combustion gases to blow into the coolant. This is often encountered when the engine is operating under conditions of pre-ignition.fabr wrote: ↑Tue Jun 21, 2022 1:39 pm Thank you for the reply. I understand all that. My question however was in regard to your statement "Gas in the cooling system can originate from local coolant boiling. Agreed,we've all seen thatAt some point the gas pressure can lift the head and allow the addidtion of combustion gases." Sentence underlined is what my question is referring to.I still do not see that happening. Please help me understand how steam in the cooling system can exert enough pressure to lift a head.
But when the engine is shut off, the chamber pressure is well below coolant gas pressure until the coolant temperature is
reduced. That is why a cool down lap is often necessary. And sometimes needed to cool down the drivers as well.
Re: Water Pressure Issue
You are the expert for sure. I wasn't meaning to be argumentative,was just trying to understand. I'm glad we now agree.
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Re: Water Pressure Issue
Your combustion in the cooling system will be coming from combustion leaking past the head gasket. If the torque on your head studs has not relaxed ( re - check your stud torque ) then it is most likely from the surface finish on the head and / or block deck. MLS head gaskets require a very fine (smooth ) finish or the combustion will leak through the "machining grooves " on the gasket surfaces ( head or block ). Not all machines can achieve this smooth finish required for MLS head gaskets.
Allan.
Allan.