radiator pressure basics, and bubbles in the overflow

Mountainman · **Posted:** Fri Jan 06, 2017 7:07 pm

I'm hoping that one of you coolant system experts can explain what state of pressure the system operates in,and how this relates to bubbles rising to the surface in the overflow?

These bubbles often show me a leaky head gasket, or crack, but could they also come from a small leak in a hose?
Should there be zero bubbles in a well operating system?

I mean, if you're losing a tiny bit of coolant, does the system eventually become a vacuum and suck air in when it cools slightly?

In application; I am currently trying to decide if just a few bubbles are from a tiny coolant leak, or a combustion leak. I also want to be able to use this logic on diagnosing other crd's.

Thanks for any insight :JEEPIN:

flash7210 · **Posted:** Fri Jan 06, 2017 10:19 pm

Should there be any bubbles?
Sure, if right after a coolant flush and fill. Gotta give it time to get all the bubbles out.
But after 5000 miles, probably not.

Do bubbles indicate air leaking in?
Not usually.
As the coolant gets hot it expands and builds pressure. So any kind of leak will result in coolant leaking out.

The radiator cap is pressure releif valve.
If the hot pressurized water is too much, it will allow some to release into the overflow. Once cooled, the overflow will get sucked back in along with some air.

If you want to know where the coolant leaks are, get a cooling system pressure tester.
Pump it up to 16psi. Walk away and come back 30 min later.
Did pressure drop?
Check for drips at all the hoses and clamps.
If no drips, remove the glow plugs, unplug all the injectors, and crank the engine. Then check for water coming out the glow plug holes.

I was chasing coolant leaks and air bubbles for a long time before I finally figured out that my head was cracked.

flman · **Posted:** Sat Jan 07, 2017 8:23 am

Anything creating bubbles in a pressurized system, has to be created from a source of higher pressure.

mass-hole · **Posted:** Sat Jan 07, 2017 12:01 pm

Yeah my stock cap was 16 psi. Pretty sure that's what it will be at once the engine is warm and as the coolant expands. It just pushes some of the air from the overflow, which then makes it way back in through the check valve in the coolant cap once the engine cools and coolant shrinks again.

Mountainman · **Posted:** Sat Jan 07, 2017 1:42 pm

Thanks for all of the input! If anyone has a chance to put a bright light into the overflow under say 1500-2000 rpm, I'd be really curious if they see zero bubbles. When the leak is small, I have to get the light angled just right, and put my eye mighty close to the opening to see the tiny bubbles.

geordi · **Posted:** Sat Jan 07, 2017 2:36 pm

CRDs may or may not (according to Keith) operate with pressure in the cooling system - assuming there are no other sources of pressure. The temperatures really aren't high enough normally to cause a lot of pressure b/c the coolant isn't boiling. Steam is the major source of pressure in a hot system.

Now on the CRD if it isn't hot enough to have boiling (and the coolant raises the boil point way above what it would be with straight water) then you must be gaining pressure from somewhere else - this has to be a higher pressure and the only source of that will be the combustion jacket. 2500+ psi will easily leech into 16psi through the smallest gap.

Did you coat the head gasket and head / block in copper-kote before assembling? That is the only way (with studs) to ensure that the combustion system is 100% sealed IMHO. Tiny bubbles shouldn't be a huge issue, but if you want to have zero combustion leaking, you need more than the factory glaze on that gasket. The ones I have assembled with copper-kote have all shown zero pressure in the cooling system after running.

WWDiesel · **Posted:** Sat Jan 07, 2017 3:58 pm

geordi wrote:

(and the coolant raises the boil point way above what it would be with straight water)

Ture, and the pressurized system up to the 16 psig cap limits also raises the boiling and / or flash point of the coolant as well!!! :wink:

geordi · **Posted:** Sat Jan 07, 2017 4:41 pm

WWDiesel wrote:

geordi wrote:

(and the coolant raises the boil point way above what it would be with straight water)

Ture, and the pressurized system up to the 16 psig cap limits also raises the boiling and / or flash point of the coolant as well!!! :wink:

Yes, this is also true - but if there isn't sufficient heat to cause enough thermal expansion to generate that pressure... There probably isn't enough heat to worry about the boiling point either.

We shouldn't forget that even with the HDS thermostat (the hottest option available currently) the operating temperature is still lower than straight water's boiling point. I would expect that most CRDs are capable of operating with zero water bottle pressure even with the cap sealed. (Don't open it while hot anyway, unless you have one of the venting-caps with the red lever) I have one of those lever caps, and when I've pulled the switch... No sounds. There isn't pressure there.

flash7210 · **Posted:** Sat Jan 07, 2017 9:16 pm

geordi wrote:

WWDiesel wrote:

geordi wrote:

(and the coolant raises the boil point way above what it would be with straight water)

Ture, and the pressurized system up to the 16 psig cap limits also raises the boiling and / or flash point of the coolant as well!!! :wink:

Yes, this is also true - but if there isn't sufficient heat to cause enough thermal expansion to generate that pressure... There probably isn't enough heat to worry about the boiling point either.

We shouldn't forget that even with the HDS thermostat (the hottest option available currently) the operating temperature is still lower than straight water's boiling point. I would expect that most CRDs are capable of operating with zero water bottle pressure even with the cap sealed. (Don't open it while hot anyway, unless you have one of the venting-caps with the red lever) I have one of those lever caps, and when I've pulled the switch... No sounds. There isn't pressure there.

The boiling point of water is 212F at sea level.
With a 8psi radiator cap, the boiling point would be about 228F. (There are engineering charts for reference)
Adding coolant to mix raises the boiling point further.

The water coolant mix expands as it heats up, even when below boiling point.
And water starts to vaporize well below boiling point.
Expansion + vapor pressure + 16psi cap= pressure in the coolant tank well below boiling point.

tjkj2002 · **Posted:** Sat Jan 07, 2017 11:57 pm

Every liquid cooled engine,regardless of fuel burned, will have pressure in the cooling system as the engine temp rises.If you don't you got a non-sealed cooling system,IE you got something that is preventing pressure to build.Even without at the boiling point the coolant will expand enough to overflow out of the cooling system when it is at the correct fluid level(cold).

Anyone that says the CRD's cooling system should not have pressure in the cooling system at full operating temps is a complete idiot and knows nothing about automotive systems at all.

geordi · **Posted:** Sun Jan 08, 2017 1:05 am

Then I guess Keith of Green Diesel knows nothing, because he is the one that said first that it does not always have pressure. It might, it might not. Pressure by itself is not the indicator because yes, there can be thermal expansion.

tjkj2002 · **Posted:** Sun Jan 08, 2017 7:39 am

geordi wrote:

Then I guess Keith of Green Diesel knows nothing, because he is the one that said first that it does not always have pressure. It might, it might not. Pressure by itself is not the indicator because yes, there can be thermal expansion.

Then yes he is.

If the system is maintained there will always be pressure if the engine is ran close to or at operating temps.If not like I said you have bad parts,most likely a bad pressure cap.

lacabrera · **Posted:** Sun Jan 08, 2017 8:04 am

geordi wrote:

CRDs may or may not (according to Keith) operate with pressure in the cooling system - assuming there are no other sources of pressure. The temperatures really aren't high enough normally to cause a lot of pressure b/c the coolant isn't boiling. Steam is the major source of pressure in a hot system.

Now on the CRD if it isn't hot enough to have boiling (and the coolant raises the boil point way above what it would be with straight water) then you must be gaining pressure from somewhere else - this has to be a higher pressure and the only source of that will be the combustion jacket. 2500+ psi will easily leech into 16psi through the smallest gap.

Did you coat the head gasket and head / block in copper-kote before assembling? That is the only way (with studs) to ensure that the combustion system is 100% sealed IMHO. Tiny bubbles shouldn't be a huge issue, but if you want to have zero combustion leaking, you need more than the factory glaze on that gasket. The ones I have assembled with copper-kote have all shown zero pressure in the cooling system after running.

if no pressure try leaving the cap off and see what happens.

mass-hole · **Posted:** Sun Jan 08, 2017 1:19 pm

lacabrera wrote:

geordi wrote:

CRDs may or may not (according to Keith) operate with pressure in the cooling system - assuming there are no other sources of pressure. The temperatures really aren't high enough normally to cause a lot of pressure b/c the coolant isn't boiling. Steam is the major source of pressure in a hot system.

Now on the CRD if it isn't hot enough to have boiling (and the coolant raises the boil point way above what it would be with straight water) then you must be gaining pressure from somewhere else - this has to be a higher pressure and the only source of that will be the combustion jacket. 2500+ psi will easily leech into 16psi through the smallest gap.

Did you coat the head gasket and head / block in copper-kote before assembling? That is the only way (with studs) to ensure that the combustion system is 100% sealed IMHO. Tiny bubbles shouldn't be a huge issue, but if you want to have zero combustion leaking, you need more than the factory glaze on that gasket. The ones I have assembled with copper-kote have all shown zero pressure in the cooling system after running.

if no pressure try leaving the cap off and see what happens.

Seriously. Zerex G-05 claims 265F boiling point with a 15psi radiator cap(2 atms). Thats pretty close to what water is at at 2 atm. Being at 6500' mine will boil off the second I remove the cap at 95C/203F. When I had a faulty cap it was boiling over just driving around.

WWDiesel · **Posted:** Sun Jan 08, 2017 1:25 pm

tjkj2002 wrote:

If the system is maintained there will always be pressure if the engine is ran close to or at operating temps. If not like I said you have bad parts,most likely a bad pressure cap.

So very true! On any CLOSED (sealed) engine cooling system as the coolant absorbs heat up to and / or above the operating temperature, the liquid WILL expand in volume and thus pressurize the available air head space in the system up to the limits of the cap. If the pressure increases above the limits of the cap, it will bleed either the air or the liquid into the overflow tank...
That is how the system works on all closed cooling systems!!!
Don't believe me, look up volumetric effects of heat on liquids in a closed container....basic laws of physics!

From the Engineers Edge: wrote:

Effects of Temperature Changes on Fluid Properties
An increase in temperature will tend to decrease the density of any fluid. If the fluid is confined in a container of fixed volume, the effect of a temperature change will depend on whether the fluid is compressible. If the fluid is a gas, it will respond to a temperature change in a manner predicted by the ideal gas laws. A 5% increase in absolute temperature will result in a 5% increase in the absolute pressure. If the fluid is an incompressible liquid in a closed container, an increase in the temperature will have a tremendously greater and potentially catastrophic effect. As the fluid temperature increases, it tries to expand, but expansion is prevented by the walls of the container. Because the fluid is incompressible, this results in a tremendous increase in pressure for a relatively minor temperature change. The change in specific volume for a given change in temperature is not the same at various beginning temperatures. Resultant pressure changes will vary. A useful thumb rule for water is that pressure in a water-solid system will increase about 100 psi for every 1 °F increase in temperature.

mass-hole · **Posted:** Sun Jan 08, 2017 2:27 pm

I could see if the outside air temp was really high, like 100F, and the Jeep was not at operating temp or still had a stock, lower temp, tstat that it might not build much pressure. The absolute temperature at 100F is actually like 560R so the change in absolute pressure is small. If it went from 100F to 176F it would only be like a 15% increase, 15% of 14.7 psi is like 2 Psi.

In my case I'm pretty sure my coolant is operating at or close to the boiling point because of my higher temp tstat and 20% lower ambient pressure.

flman · **Posted:** Sun Jan 08, 2017 3:47 pm

lacabrera wrote:

if no pressure try leaving the cap off and see what happens.

Better yet pull the cap on a hot engine. Mine both have pressure when they are warmed up, if not why use a 16 PSI Cap?

tjkj2002 · **Posted:** Sun Jan 08, 2017 6:57 pm

WWDiesel wrote:

tjkj2002 wrote:

If the system is maintained there will always be pressure if the engine is ran close to or at operating temps. If not like I said you have bad parts,most likely a bad pressure cap.

So very true! On any CLOSED (sealed) engine cooling system as the coolant absorbs heat up to and / or above the operating temperature, the liquid WILL expand in volume and thus pressurize the available air head space in the system up to the limits of the cap. If the pressure increases above the limits of the cap, it will bleed either the air or the liquid into the overflow tank...
That is how the system works on all closed cooling systems!!!
Don't believe me, look up volumetric effects of heat on liquids in a closed container....basic laws of physics!

From the Engineers Edge: wrote:

Effects of Temperature Changes on Fluid Properties
An increase in temperature will tend to decrease the density of any fluid. If the fluid is confined in a container of fixed volume, the effect of a temperature change will depend on whether the fluid is compressible. If the fluid is a gas, it will respond to a temperature change in a manner predicted by the ideal gas laws. A 5% increase in absolute temperature will result in a 5% increase in the absolute pressure. If the fluid is an incompressible liquid in a closed container, an increase in the temperature will have a tremendously greater and potentially catastrophic effect. As the fluid temperature increases, it tries to expand, but expansion is prevented by the walls of the container. Because the fluid is incompressible, this results in a tremendous increase in pressure for a relatively minor temperature change. The change in specific volume for a given change in temperature is not the same at various beginning temperatures. Resultant pressure changes will vary. A useful thumb rule for water is that pressure in a water-solid system will increase about 100 psi for every 1 °F increase in temperature.

Yes I know,kinda part of my job and since every vehicle has a "closed" cooling system for the last what like 40 years your going to be hard pressed to sell me on the idea the CRD's should not have any pressure in the cooling system.

radiator pressure basics, and bubbles in the overflow

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