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Old 06-21-2021, 09:07 AM   #1
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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...
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Old 06-21-2021, 09:12 AM   #2
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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.
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Old 06-21-2021, 09:20 AM   #3
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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

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Old 06-21-2021, 11:14 AM   #4
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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.
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Old 06-21-2021, 11:48 AM   #5
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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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Old 06-21-2021, 12:33 PM   #6
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15-psi double-acting radiator caps and plastic coolant recovery bottles. .
I guess we need to make sure about the kind of radiator cap.

My Cat does not have a plastic reservoir either.
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Old 06-21-2021, 12:55 PM   #7
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Originally Posted by rgano View Post
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.
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?
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Old 06-21-2021, 01:05 PM   #8
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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?
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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Old 06-21-2021, 01:14 PM   #9
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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.....
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.
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Old 06-22-2021, 06:21 AM   #10
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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.
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Old 06-22-2021, 07:09 AM   #11
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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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Old 06-22-2021, 10:06 AM   #12
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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.
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.
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Old 06-22-2021, 06:01 PM   #13
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Originally Posted by rgano View Post
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.
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.
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Old 06-22-2021, 06:10 PM   #14
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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.
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.
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Old 06-22-2021, 10:53 PM   #15
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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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Old 06-22-2021, 11:24 PM   #16
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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.
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Old 06-23-2021, 10:05 AM   #17
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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.
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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Old 06-23-2021, 10:51 AM   #18
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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.
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.
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Old 06-23-2021, 11:42 AM   #19
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Was plastic invented after detroits??
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Old 06-23-2021, 12:30 PM   #20
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Was plastic invented after detroits??

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.
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