Help explain coolant expansion tank??

Hey all...

Wondering if someone could set me straight on the proper function of my cooling system, specifically as it relates to the reservoir, cap, and expansion tank?

Particulars are diesel, 480hp Volvo TAMD74-PA engines in a 43' Ocean.

This is probably a dumb question but I'm working on improving my understanding of all things engine related and my mechanic's explanation isn't making sense to me...

My engine has a radiator style cap at the top of the engine it's self on, what I would call, the coolant reservoir. There is a small clear 1/4" tube that leads from a nipple just below this cap, up and across my engine room ceiling to a plastic bottle mounted on a bulkhead that I would call the expansion tank. The tube attaches to another nipple on the bottom of this plastic bottle. The plastic bottle also has a cap to allow the bottle to be filled. It has lines for HOT and COLD fill levels.

In a healthy functioning diesel engine such as mine, what is the purpose of the expansion tank? Should coolant go into the expansion tank when engine is up to operating temp if everything is working correctly? AND, if so, can coolant be drawn back into the reservoir FROM the expansion bottle/coolant tank when the engine cools?

My mechanic said that normally, coolant should not go into the expansion tank. It will only go to the expansion tank if the coolant pressure exceeds the cap pressure rating. And that the coolant can't be "drawn back" from the bottle. I was under the impression this "flow back and fourth" was normal and, that when adding coolant, you would add it to the expansion tank, not at the cap for the reservoir on top of the engine.

I'm assuming my mechanic is right or I am misunderstanding him somehow. Just hoping for some clarification. Thanks in advance...

In most cases, when everything is good and warm, you'll hit cap pressure and push a bit of coolant out to the expansion tank. That's why it's there (so the coolant doesn't just puke into the bilge). And on shutdown, as it cools, it should pull some coolant back from the expansion tank. Generally, the tube between the 2 will be full of coolant.

With a fully functioning coolant and expansion tank system, at cold the radiator is full and the expansion tank is at the cold level. When the engine starts and the coolant warms up, it expands and pushes some coolant out of the radiator due to the cap lifting and sending coolant to the reservoir. When the engine is hot and running at a good load enough coolant has been sent to the reservoir to reach the hot level.

When the engine is shut down, often a bit more coolant is forced through due to the heat soaking effect: the coolant is no longer circulating and removing heat but the engine is still hot so it pushes more coolant out.

As the engine cools it sucks back coolant from the reservoir into the radiator. When the engine is finally cool the reservoir should be at the cool level.

So while cold, check the level on the reservoir and it should be at the cool mark. Open the radiator cap and coolant should be at the top- there should be no air space. You are good to go then.

David

David

I think I'd find a new mechanic.

Modern coolant systems almost all have expansion tanks, which have some amount of coolant in it (often marked as yours is). It operates as David says above. You normally never open the cap on the engine, if coolant needs to be added, add it to the expansion tank. If you do open the engine cap - even when the engine is cold - prepare for and expect some to spill out, as the coolant should be right up against the cap and perhaps under some pressure.

Firstly, the plastic tank of which you speak is a coolant recovery tank/bottle, just like your car has. The place where the radiator caps sit atop your engines is the expansion tank. I had a pair of 120-HP Ford-Lehmans which were built with single acting 4-psi radiator caps and overflow pipes which just shot the excess coolant into the bilge as the engine heated up. I learned to leave the coolant an inch or so below the full level when checking the level. Later, I discovered American diesel, had a kit which consisted of a new filler neck to be fitted to the engines' expansion tanks along with new 15-psi double-acting radiator caps and plastic coolant recovery bottles. Now I could fill the expansion tanks full of coolant, slap the radiator caps on, fill the recovery bottles to the "cold" line and monitor the engine coolant without having to pull the radiator cap every time before starting. Engine room checks underway always showed the coolant in the bottles at the "hot" level and after cool down at the "cold" level. Any other condition was cause for some concern Based on NO coolant in a bottle one time after cool down, I diagnosed a leaking circulating pump which could not be seen while underway because it was steaming as it exited. And the bilge no longer emitted the sweetish smell of ethyl glycol coolant. Pay close attention to your coolant recovery bottles.

rgano said:

15-psi double-acting radiator caps and plastic coolant recovery bottles. .

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I guess we need to make sure about the kind of radiator cap.

My Cat does not have a plastic reservoir either.

rgano said:

Firstly, the plastic tank of which you speak is a coolant recovery tank/bottle, just like your car has. The place where the radiator caps sit atop your engines is the expansion tank. I had a pair of 120-HP Ford-Lehmans which were built with single acting 4-psi radiator caps and overflow pipes which just shot the excess coolant into the bilge as the engine heated up. I learned to leave the coolant an inch or so below the full level when checking the level. Later, I discovered American diesel, had a kit which consisted of a new filler neck to be fitted to the engines' expansion tanks along with new 15-psi double-acting radiator caps and plastic coolant recovery bottles. Now I could fill the expansion tanks full of coolant, slap the radiator caps on, fill the recovery bottles to the "cold" line and monitor the engine coolant without having to pull the radiator cap every time before starting. Engine room checks underway always showed the coolant in the bottles at the "hot" level and after cool down at the "cold" level. Any other condition was cause for some concern Based on NO coolant in a bottle one time after cool down, I diagnosed a leaking circulating pump which could not be seen while underway because it was steaming as it exited. And the bilge no longer emitted the sweetish smell of ethyl glycol coolant. Pay close attention to your coolant recovery bottles.

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Thanks for this and ALL the responses which seem to jibe. I feel like I was on track with my understanding and perhaps my mechanic was was describing something more along the way your old set-up was?

Is a double acting cap what allows the coolant to move back and forth between the bottle and engine during heating/cooling? Is it the radiator cap that allows the coolant out of the reservoir/expansion tank? And then is it just suction cause by the contraction of cooling that draws it back from the bottle?

Prospective said:

Is a double acting cap what allows the coolant to move back and forth between the bottle and engine during heating/cooling? Is it the radiator cap that allows the coolant out of the reservoir/expansion tank? And then is it just suction cause by the contraction of cooling that draws it back from the bottle?

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And Bob's you uncle! IOW, correct, you have it. I was thinking your mech may have been trying to tell you how a single acting cap works, but I wasn't there.....

rgano said:

And Bob's you uncle! IOW, correct, you have it. I was thinking your mech may have been trying to tell you how a single acting cap works, but I wasn't there.....

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Yes, thank you. I believe that is where the miscommunication is. And I've found some stuff on line explaining double acting caps, which is indeed what mine are. Mystery solved.

The purpose of the system is to get rid of any air that is in the cooling system.

Air is an insulator and circulating coolant with no air can make the cooling system 25% more efficient.

The car % truck builders make cooling systems 25% smaller , saving big bucks.

A boat built with just a puke overflow can add the system form most auto supply stores for low cost , as insurance against an overheat.

Lots of good explanations here.
What helped me understand the common 2-way cap is understand it has 2 valves, with different cracking pressures. The “outbound” high pressure spring wants to keep coolant IN the engine under 7 to 15 psi to raise the boiling point. The “inbound” spring/valve allows the cap to suck back in coolant that was expelled during engine warmup.

diver dave said:

Lots of good explanations here.
What helped me understand the common 2-way cap is understand it has 2 valves, with different cracking pressures. The “outbound” high pressure spring wants to keep coolant IN the engine under 7 to 15 psi to raise the boiling point. The “inbound” spring/valve allows the cap to suck back in coolant that was expelled during engine warmup.

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Yes, this two way or double action cap is what I think I was wrestling with. But totally makes sense.

Thanks to everyone for all of the responses. One more thing I understand a little better now.

rgano said:

Firstly, the plastic tank of which you speak is a coolant recovery tank/bottle, just like your car has. The place where the radiator caps sit atop your engines is the expansion tank. I had a pair of 120-HP Ford-Lehmans which were built with single acting 4-psi radiator caps and overflow pipes which just shot the excess coolant into the bilge as the engine heated up. I learned to leave the coolant an inch or so below the full level when checking the level. Later, I discovered American diesel, had a kit which consisted of a new filler neck to be fitted to the engines' expansion tanks along with new 15-psi double-acting radiator caps and plastic coolant recovery bottles. Now I could fill the expansion tanks full of coolant, slap the radiator caps on, fill the recovery bottles to the "cold" line and monitor the engine coolant without having to pull the radiator cap every time before starting. Engine room checks underway always showed the coolant in the bottles at the "hot" level and after cool down at the "cold" level. Any other condition was cause for some concern Based on NO coolant in a bottle one time after cool down, I diagnosed a leaking circulating pump which could not be seen while underway because it was steaming as it exited. And the bilge no longer emitted the sweetish smell of ethyl glycol coolant. Pay close attention to your coolant recovery bottles.

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Good description. A coolant recovery system allows you to overfill the coolant. The standard Leyman fill tank needs about an inch of head space. This gives the coolant room to expand, but doesn't overflow. If filled to the top, it spits out the excess into the bilge. If overfilled with a conversion to the reservoir system, it spits out and can then suck the excess back in. The excess in only used if you have a problem in your coolant system and are losing coolant somewhere else. Since I bleed any accumulated air out of the petcock every day prior to running, and have to open the radiator cap to do so, I just use my homemade Popcicle dip stick and make sure that the level is down an inch (i.e., at the same level). I've had no change in 200 hours of run time. Cheaper and simpler than installing the overflow kit, but the reservoir may provide some benefit.

Marco Flamingo said:

Since I bleed any accumulated air out of the petcock every day prior to running, and have to open the radiator cap to do so, I just use my homemade Popcicle dip stick and make sure that the level is down an inch (i.e., at the same level). I've had no change in 200 hours of run time. Cheaper and simpler than installing the overflow kit, but the reservoir may provide some benefit.

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Is this a Ford-Lehman? If so, you'd love the filler neck/radiator cap/recovery bottle modification offered by American Diesel. It truly changed the way I monitored the coolant levels and usage in my FL 120s.

Just in case.... the old Detroits did not come with an expansion tank. I fact the book says to keep the coolant level about an inch below the fill cap. (cold). The cap is a 7psi cap. My PO had expansion tanks added and had the reservoirs full. I took them back down to the recommended level. Now if I see pink in the plastic I have problems.

My mechanic fitted locally available Mercruiser overflow bottle systems to my FLs. Seemed to work, saved importing the ADC product across the Pacific on that occasion.

oscar said:

Just in case.... the old Detroits did not come with an expansion tank. I fact the book says to keep the coolant level about an inch below the fill cap. (cold). The cap is a 7psi cap. My PO had expansion tanks added and had the reservoirs full. I took them back down to the recommended level. Now if I see pink in the plastic I have problems.

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Doesn't leaving the air gap in the expansion tank defeat the ability to effectively monitor the ebb and flow of your coolant between KNOWN levels in the recovery bottle? I consider knowing the low and high levels accurately the first line of defense in monitoring for any issues with the coolant system.

rgano said:

Doesn't leaving the air gap in the expansion tank defeat the ability to effectively monitor the ebb and flow of your coolant between KNOWN levels in the recovery bottle? I consider knowing the low and high levels accurately the first line of defense in monitoring for any issues with the coolant system.

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Maybe the Detroit engineers have a reason for what the book says..... I am not going to second guess them. I check the level cold before start. If any of it escapes I will hopefully smell/see it.

Was plastic invented after detroits??

diver dave said:

Was plastic invented after detroits??

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First real plastic was invented in 1907 -- bakelite. PVC was first made in the mid 1800's. Various other types really took off post WWII.

Not to be too pedantic, but terminology is important.

Expansion tanks are pressurized. They will have a pressure rated vented cap on them. It is also possible to have a remote expansion tank when the engine has a water heater or "bus" heater attached that is above the elevation of the engine. The original expansion tank would then have a non-vented cap. Expansion tanks and remote expansion tanks are part of the pressurized cooling system, and thereby increase the cooling systems' overall volume.

Recovery tanks (or overflow tanks) are not pressurized. They rely on the vented pressure cap on the expansion tank or the remote expansion tank to function. Recovery tanks do not increase the overall volume of the system, but merely prevent coolant from going into the bilge, and allow it to be recovered back into the system when the system cools back down.

Prospective,
I have the same Volvo engines as you except mine are the 430HP 74 LB. My set-up does not have the overflow bottles with any excess draining into the bilge. My owners manual is very specific regarding the fluid level in the expansion tank. The level should be at the second knuckle on your index finger, which I check on a regular basis. I wonder if your manual indicates something different. Thanks

Coolant expands when it gets hot. With no expansion/recovery tank, you need air space at the top of the pressure tank or it will make its own space by blowing some coolant out. However an expansion/recovery tank can be added to most any engine and will only improve it.

Almost any modern engine (as distinct from a DD ) has a recovery tank. The trend is very much towards an expansion tank as defined by Northern above. Though in common usage any plastic tank attached by a tube to the main cooling system is called an expansion or recovery tank interchangeably.

What does the engine manual say?

DDW said:

However an expansion/recovery tank can be added to most any engine and will only improve it.

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I tend to disagree that every added accessory on a boat is an improvement, so I'll play Devil's advocate.

The excess coolant is only pushed into the expansion tank as it heats up, thus slowing the heating of a diesel engine. Improvement? The excess is pushed into the expansion tank where it does nothing and sits until the engine is shut off and cools down. Improvement? It does allow one to routinely overfill the system and not make a mess because of improper filling of the system. Improvement?

It is an improvement in the same way as idiot lights over analog gauges. An improvement for some.

Overflow tanks keep the bottom of the pressure cap wet with coolant, thereby reducing corrosion.
Its happened to my heat exchangers, and it may be a thin argument, but the poly tanks are cheap and reliable.

Marco Flamingo said:

I tend to disagree that every added accessory on a boat is an improvement, so I'll play Devil's advocate.

The excess coolant is only pushed into the expansion tank as it heats up, thus slowing the heating of a diesel engine. Improvement? The excess is pushed into the expansion tank where it does nothing and sits until the engine is shut off and cools down. Improvement? It does allow one to routinely overfill the system and not make a mess because of improper filling of the system. Improvement?

It is an improvement in the same way as idiot lights over analog gauges. An improvement for some.

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Your example of an expansion tank slowing the heating of an engine is flawed;
the thermostat controls that.
I think that adding one to older engines designed without a tank is not likely to
improve much.

JSK55 said:

Prospective,
I have the same Volvo engines as you except mine are the 430HP 74 LB. My set-up does not have the overflow bottles with any excess draining into the bilge. My owners manual is very specific regarding the fluid level in the expansion tank. The level should be at the second knuckle on your index finger, which I check on a regular basis. I wonder if your manual indicates something different. Thanks

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When I had TMD40s, they would slobber excess coolant into the bilge, leaving telltale tide lines down the side of the engines. My mechanic got me recovery bottles and changed the caps, so you could likely do the same.

Marco Flamingo said:

I tend to disagree that every added accessory on a boat is an improvement, so I'll play Devil's advocate.

The excess coolant is only pushed into the expansion tank as it heats up, thus slowing the heating of a diesel engine. Improvement? The excess is pushed into the expansion tank where it does nothing and sits until the engine is shut off and cools down. Improvement? It does allow one to routinely overfill the system and not make a mess because of improper filling of the system. Improvement?

It is an improvement in the same way as idiot lights over analog gauges. An improvement for some.

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I would say mainly that. In the olden days, you had to underfill the system to allow for expansion. Exactly how much you could arrive at by making a mess several times. And then, you could not add coolant when it was warm, only when stone cold. But then you never knew what the level was until you opened it up, right? EVERY car has this now as it makes maintenance less frequent and easier. Pretty much every new boat too.

I would agree with you that not every gadget is an improvement, but overflow tanks are. In my boat, factory installed on engine, genset, and hydronic system, for good reasons. If you overheat your engine, the overflow tank will catch the mess rather than you mopping it up out of the bilge later, you can check the level at a glance, and add coolant whenever you wish.

DDW said:

I would say mainly that. In the olden days, you had to underfill the system to allow for expansion. Exactly how much you could arrive at by making a mess several times. And then, you could not add coolant when it was warm, only when stone cold. But then you never knew what the level was until you opened it up, right? EVERY car has this now as it makes maintenance less frequent and easier. Pretty much every new boat too.

I would agree with you that not every gadget is an improvement, but overflow tanks are. In my boat, factory installed on engine, genset, and hydronic system, for good reasons. If you overheat your engine, the overflow tank will catch the mess rather than you mopping it up out of the bilge later, you can check the level at a glance, and add coolant whenever you wish.

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Agreed. It’s really an enhancement in
maintenance, both in monitoring coolant level, and in replenishment. And there isn’t much to wrong with the setup.

If you have a 1930s Detroit, you can continue to start every diagnostic with an analysis of the puddle on the floor, and go from there.