Coolant expansion tank boiling

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Originally Posted By: Emanuel
Originally Posted By: zeng
How old (in weeks or months) is this 'new' radiator cap ? ...... I mean why and when did you have this 'as is new' cap installed ?

- Less than 3 months but I don´t trust this radiator cap, I had to modify it in order to be able to use it, I need a new one.

- I replaced it because the previous one wasn´t holding any pressure, this one does hold pressure but I think it has problems with the upper seal and this causes the coolant to not return from the overflow tank into the radiator when the engine is cold.

Coolant not returning from overflow/reservoir tank into the radiator when the engine is cold likely is caused by mal-functioning check valve within the cap .......
in this scenario your engine cooling system is operating under perfectly normal conditions as long as full coolant level at cold is satisfied before every start, granted with absence of thermostat.
OP, as long as there is full coolant level before every start, you may get away with immediate replacements of cap (and thermostat)in your context until such time when you're ready.

Note-
 
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On most street driven engines some restriction is necessary in the system to allow enough dwell time for radiator to dissipate heat.
As the coolant heats up it begins to expand and generate pressure long before 100c/212f anyone who has owned a model steam engine can relate to this.

In the most simple terms the engine is nothing more than the heat source, the thermostat a variable restriction and the radiator core a big heat sink, the fins between the tubes are what dissipates most of the heat.
At higher speed the air flow through it will probably be enough to keep the fluid cool enough but not always, a nascar engine for example is creating too much heat and will overheat without some small amount of restriction.

In traffic that's another story, the anemic fan cannot provide enough flow for the cooling fins to dissipate the amount of heat being absorbed by the fluid if it is not held in the core longer, that's where the variable restriction comes into play.

http://www.hotrod.com/articles/ctrp-0910-critical-engine-cooling-technology/

In engines that see WOT most of the operating time, something like this were commonly used, today they lower the flow rate of the pump mostly through pulley size..

https://www.summitracing.com/parts/mor-6...AiABEgLc8PD_BwE

I want to point out that thermal engineers will probably claim that the more flow the better the cooling, that is true but not under all conditions. On old cars the heat sink was usually much larger and cooling system capacity was greater holding as much as 20qt+ with a low rpm engine.
Modern cars can have as little 5-6 qts total capacity with less than 3 qts in the much smaller radiator meaning less surface area to dissipate heat if the entire contents of the system are heat saturated.

If the heat sink which in this case is the radiator has the same capacity are the heat source then sure the more flow the better.

My worthless
49.gif
and $2.25 will buy me coffee
 
Originally Posted By: Trav
On most street driven engines some restriction is necessary in the system to allow enough dwell time for radiator to dissipate heat.
As the coolant heats up it begins to expand and generate pressure long before 100c/212f anyone who has owned a model steam engine can relate to this.

In the most simple terms the engine is nothing more than the heat source, the thermostat a variable restriction and the radiator core a big heat sink, the fins between the tubes are what dissipates most of the heat.
At higher speed the air flow through it will probably be enough to keep the fluid cool enough but not always, a nascar engine for example is creating too much heat and will overheat without some small amount of restriction.

In traffic that's another story, the anemic fan cannot provide enough flow for the cooling fins to dissipate the amount of heat being absorbed by the fluid if it is not held in the core longer, that's where the variable restriction comes into play.

http://www.hotrod.com/articles/ctrp-0910-critical-engine-cooling-technology/

In engines that see WOT most of the operating time, something like this were commonly used, today they lower the flow rate of the pump mostly through pulley size..

https://www.summitracing.com/parts/mor-6...AiABEgLc8PD_BwE

I want to point out that thermal engineers will probably claim that the more flow the better the cooling, that is true but not under all conditions. On old cars the heat sink was usually much larger and cooling system capacity was greater holding as much as 20qt+ with a low rpm engine.
Modern cars can have as little 5-6 qts total capacity with less than 3 qts in the much smaller radiator meaning less surface area to dissipate heat if the entire contents of the system are heat saturated.

If the heat sink which in this case is the radiator has the same capacity are the heat source then sure the more flow the better.

My worthless
49.gif
and $2.25 will buy me coffee




Thanks for the links. Good article
 
Originally Posted By: SilverFusion2010
Originally Posted By: Trav
On most street driven engines some restriction is necessary in the system to allow enough dwell time for radiator to dissipate heat.
As the coolant heats up it begins to expand and generate pressure long before 100c/212f anyone who has owned a model steam engine can relate to this.

In the most simple terms the engine is nothing more than the heat source, the thermostat a variable restriction and the radiator core a big heat sink, the fins between the tubes are what dissipates most of the heat.
At higher speed the air flow through it will probably be enough to keep the fluid cool enough but not always, a nascar engine for example is creating too much heat and will overheat without some small amount of restriction.

In traffic that's another story, the anemic fan cannot provide enough flow for the cooling fins to dissipate the amount of heat being absorbed by the fluid if it is not held in the core longer, that's where the variable restriction comes into play.

http://www.hotrod.com/articles/ctrp-0910-critical-engine-cooling-technology/

In engines that see WOT most of the operating time, something like this were commonly used, today they lower the flow rate of the pump mostly through pulley size..

https://www.summitracing.com/parts/mor-6...AiABEgLc8PD_BwE

I want to point out that thermal engineers will probably claim that the more flow the better the cooling, that is true but not under all conditions. On old cars the heat sink was usually much larger and cooling system capacity was greater holding as much as 20qt+ with a low rpm engine.
Modern cars can have as little 5-6 qts total capacity with less than 3 qts in the much smaller radiator meaning less surface area to dissipate heat if the entire contents of the system are heat saturated.

If the heat sink which in this case is the radiator has the same capacity are the heat source then sure the more flow the better.

My worthless
49.gif
and $2.25 will buy me coffee




Thanks for the links. Good article


Except it contains this:-

" Conversely, if your pulley is too small, water will be pushed through the system so fast it won’t have enough time to collect heat from the motor or dissipate that collected heat."

which is not believable.
 
Originally Posted By: Trav
I'm no thermal engineer just a mechanic so I cant give any explanation. I will let the egg heads argue over that.

I've tried to follow this thread and make sense out of it but I can't. So his "boiling" coolant in the expansion tank is just bubbling, not liquid that is above the boiling point? If it was actual boiling liquid then that would surely soften and destroy the reservoir.
 
His coolant may well be over the boiling point and pressure is escaping into the overflow producing the bubbles but like you I am just as confused, he said he had a bad cap which would also reduce the boiling point, who the heck knows whats really going on with that car.

If someone called me and started describing this I would just say look bring it down so I can see whats going on. I am just waiting for the almost inevitable omission of something like eg "it was leaking out the side of the head a while ago" or some such thing to come up. You know how it goes.
 
Originally Posted By: Trav
I'm no thermal engineer just a mechanic so I cant give any explanation. I will let the egg heads argue over that.


Long, long time since I did any thermodynamics and I don't think I was very good at it then, so I can't formulate a full scientific explanation, but that just isnt how things work.

Heat tranfers from the hot thing to the colder thing based on the temperature differential.

Thats it. Thats what happens.

If the things are fluids and moving, a higher temperature differential is maintained because hot and cold fluids are continually replaced, so the rate of heat transfer is higher.

(I think he's got a bad head gasket and the thermostat thing is collateral noise, but I'm guessing)
 
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Originally Posted By: Ducked
Originally Posted By: SilverFusion2010
Originally Posted By: Trav
On most street driven engines some restriction is necessary in the system to allow enough dwell time for radiator to dissipate heat.
As the coolant heats up it begins to expand and generate pressure long before 100c/212f anyone who has owned a model steam engine can relate to this.

In the most simple terms the engine is nothing more than the heat source, the thermostat a variable restriction and the radiator core a big heat sink, the fins between the tubes are what dissipates most of the heat.
At higher speed the air flow through it will probably be enough to keep the fluid cool enough but not always, a nascar engine for example is creating too much heat and will overheat without some small amount of restriction.

In traffic that's another story, the anemic fan cannot provide enough flow for the cooling fins to dissipate the amount of heat being absorbed by the fluid if it is not held in the core longer, that's where the variable restriction comes into play.

http://www.hotrod.com/articles/ctrp-0910-critical-engine-cooling-technology/

In engines that see WOT most of the operating time, something like this were commonly used, today they lower the flow rate of the pump mostly through pulley size..

https://www.summitracing.com/parts/mor-6...AiABEgLc8PD_BwE

I want to point out that thermal engineers will probably claim that the more flow the better the cooling, that is true but not under all conditions. On old cars the heat sink was usually much larger and cooling system capacity was greater holding as much as 20qt+ with a low rpm engine.
Modern cars can have as little 5-6 qts total capacity with less than 3 qts in the much smaller radiator meaning less surface area to dissipate heat if the entire contents of the system are heat saturated.

If the heat sink which in this case is the radiator has the same capacity are the heat source then sure the more flow the better.

My worthless
49.gif
and $2.25 will buy me coffee




Thanks for the links. Good article


Except it contains this:-

" Conversely, if your pulley is too small, water will be pushed through the system so fast it won’t have enough time to collect heat from the motor or dissipate that collected heat."

which is not believable.


It makes sense to me that there would be an optimum coolant flow rate that maximizes heat transfer based on a number of factors. Single or double pass radiator, total system capacity, size of coolant passages in the block, amount of air flow through the radiator.

As with so many other topics, the devil is in the details. What works for a street machine may blow up on the track, and a track car may not do well on the street.

I think where the disagreement is forming is coolant temp drop vs energy removed in one pass.

Follow me here. I’m assuming a fixed airflow and a fixed amount of heat being generated by the engine. We are also going to hold the air temperature fixed. Identical radiator for both scenarios.

Here is what I would expect to see by monitoring the inlet and outlet temps on the radiator.

Slower flow will yield a higher temp drop. Let’s say we’re at 2GPM and we see a 10C drop between the inlet and outlet radiator temperature.

Now let’s double the flow of coolant 4GPM. I would expect between 5-7C inlet outlet temp difference due to less dwell time.

Here’s the key... both cooling circuits are removing about the same amount of heat from the system. The difference will be where the mean coolant temperature lies. I would expect circuit 1 to be a bit cooler over all because it’s radiator is more effectively dumping heat to our fixed amount of airflow

Circuit 2 with it’s faster flow has less time to shed heat to the fixed amount of airflow, so the delta isn’t as great. With half the delta and twice the flow the math for heat exchange comes out the same, the key here is that the circuit that’s flowing faster than the radiator is designed for is running a higher mean temp.

Make sense?
 
Originally Posted By: SilverFusion2010
Originally Posted By: Ducked
Originally Posted By: SilverFusion2010
Originally Posted By: Trav
On most street driven engines some restriction is necessary in the system to allow enough dwell time for radiator to dissipate heat.
As the coolant heats up it begins to expand and generate pressure long before 100c/212f anyone who has owned a model steam engine can relate to this.

In the most simple terms the engine is nothing more than the heat source, the thermostat a variable restriction and the radiator core a big heat sink, the fins between the tubes are what dissipates most of the heat.
At higher speed the air flow through it will probably be enough to keep the fluid cool enough but not always, a nascar engine for example is creating too much heat and will overheat without some small amount of restriction.

In traffic that's another story, the anemic fan cannot provide enough flow for the cooling fins to dissipate the amount of heat being absorbed by the fluid if it is not held in the core longer, that's where the variable restriction comes into play.

http://www.hotrod.com/articles/ctrp-0910-critical-engine-cooling-technology/

In engines that see WOT most of the operating time, something like this were commonly used, today they lower the flow rate of the pump mostly through pulley size..

https://www.summitracing.com/parts/mor-6...AiABEgLc8PD_BwE

I want to point out that thermal engineers will probably claim that the more flow the better the cooling, that is true but not under all conditions. On old cars the heat sink was usually much larger and cooling system capacity was greater holding as much as 20qt+ with a low rpm engine.
Modern cars can have as little 5-6 qts total capacity with less than 3 qts in the much smaller radiator meaning less surface area to dissipate heat if the entire contents of the system are heat saturated.

If the heat sink which in this case is the radiator has the same capacity are the heat source then sure the more flow the better.

My worthless
49.gif
and $2.25 will buy me coffee




Thanks for the links. Good article


Except it contains this:-

" Conversely, if your pulley is too small, water will be pushed through the system so fast it won’t have enough time to collect heat from the motor or dissipate that collected heat."

which is not believable.


It makes sense to me that there would be an optimum coolant flow rate that maximizes heat transfer based on a number of factors. Single or double pass radiator, total system capacity, size of coolant passages in the block, amount of air flow through the radiator.

As with so many other topics, the devil is in the details. What works for a street machine may blow up on the track, and a track car may not do well on the street.

I think where the disagreement is forming is coolant temp drop vs energy removed in one pass.

Follow me here. I’m assuming a fixed airflow and a fixed amount of heat being generated by the engine. We are also going to hold the air temperature fixed. Identical radiator for both scenarios.

Here is what I would expect to see by monitoring the inlet and outlet temps on the radiator.

Slower flow will yield a higher temp drop. Let’s say we’re at 2GPM and we see a 10C drop between the inlet and outlet radiator temperature.

Now let’s double the flow of coolant 4GPM. I would expect between 5-7C inlet outlet temp difference due to less dwell time.

Here’s the key... both cooling circuits are removing about the same amount of heat from the system. The difference will be where the mean coolant temperature lies. I would expect circuit 1 to be a bit cooler over all because it’s radiator is more effectively dumping heat to our fixed amount of airflow

Circuit 2 with it’s faster flow has less time to shed heat to the fixed amount of airflow, so the delta isn’t as great. With half the delta and twice the flow the math for heat exchange comes out the same, the key here is that the circuit that’s flowing faster than the radiator is designed for is running a higher mean temp.

Make sense?


No. More flow will remove more heat, I think it really is that simple.
 
Originally Posted By: SilverFusion2010
You’re wrong about that. For more flow to be colder you would need an increase in airflow through the radiator.


Not as long as there's a temperature difference between the ambient air and the coolant.

Plus you seem to be saying that more airflow is better but more coolant flow is worse. Isn't there a contradiction there?
 
I’m not going to argue it with you anymore. I have experience with water cooling circuits in high performance PCs where I could control airflow, and pump speed and thermal input. and the article cited examples of racers having problems when the pulley or pump was sized improperly.

Water and air transfer heat at different rates. That’s why you would need more airflow to maintain the same delta with increased coolant flow.

The radiator has a fixed amount of surface area which can transfer x amount of heat per second to the air from the coolant. Shoving more coolant through means the coolant transfers less heat per unit volume because the radiator still can only transfer x amount of heat to the air at a given air flow rate.

And I’m done
 
I THINK what MIGHT be happening here is that people are confusing temperature with heat, and taking the exit temperature from the radiator as an indicator of cooling system efficiency.

A slower coolant flow rate will produce a lower exit temperature from the radiator, and vice versa, but what is important is the rate of total heat removal from the system, and, more specifically, the rate of heat removal from its hottest point, probably in the cylinder head near the exhaust valves.

A slower coolant flow rate will produce a lower exit temperature from the radiator, but less heat has been removed since the mass of fluid flow is less, (and the average temperature differential between the air and the radiator is less too) This reduced mass of coolant has increased residence time in the radiator, so it cools down more, but it also has increased residence time in the cylinder head, where it heats up more. Keep reducing the flow rate and eventually you'll have the coolest coolant exiting the radiator and your cylinder head coolant will boil.

Which isn't, like, cool.

Does seem to be a very widespread misunderstanding if it is one though.
 
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I think the most relevant point to all this is what is a thermostat. Nothing more than a variable restriction.
Engine cold, thermostat closed, as coolant gets up to temp thermostat begins to open and opens fully when thermostat rated temp is achieved.

Car driving good air flow across the radiator thermostat makes minor adjustments in open to maintain rated temp.
Car stopped in traffic no air flow, fans come on to provide air over the radiator. No fans means no air flow, thermostat wide open, engine continues to generate heat and overwhelms the dissipation abilities of the radiator, temps continue to rise resulting in boil over.
Increasing the flow of coolant through the radiator will not improve its heat dissipation ability only the flow of air will do that.

No thermostat can result in overheating if the heat sink (radiator) has a lower capacity to dissipate heat than the amount being generated by the engine then it will boil over, restricting the flow slightly to increase the dwell time so the air can remove more of the heat will prevent it.

I remember some cars years ago when electric fans were just making an appearance that had a huge radiator and small displacement engine that even with a failed fan it didn't bother them enough to boil over although temps did rise.
Heat output on these vehicles was mediocre at best in winter, covering the grill was a common way to restrict air flow across the radiator.
 
The main problem isn´t the overflow tank boiling, is the fact that sometimes there are bubbles on the upper radiator hose. I have read a lot and I think that the lack of thermostat isn´t causing this problem nor a head gasket leak, I filled the coolant to the top in the radiator and drove two days 150 miles, one day I even drove at 90 mph and the coolant is still full in the radiator.

The problem is that sometimes the coolant doesn´t stays full and this is what causes to air enter in the cooling system and cause the overheating even without the coolant gauge noticing it. I´m pretty sure the real cause of the problem is the radiator cap that sometimes fails to let the coolant from the overflow tank return into the radiator, I´m gonna be monitoring the coolant level periodically to see when the coolant level drops again to see if I buy a new radiator cap.

If the new radiator cap solves the problem it´ll be great, if not I need a backup radiator cap in the case the one that I have fails to mantain pressure in the system someday in the future. My next step would be to put a new upper radiator hose, the one that I have has lost flexibility and had some troubles sealing after I did a coolant change, the area where the clamp seals to the tube in the engine has shrink, I put the clamp in other place and tightened harder in order to make it seal. Maybe the coolant is leaking there but I can see the leak because the place where it could be leaking is very close to the exhaust manifold so the heat evaporates the coolant. Though I can´t see any traces of coolant there, and coolant leaks leave orange traces in this case which I can´t see there, but I would change that hose anyway in that case.
 
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