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Why overflow tanks

diver_dave

diver_dave

Advanced Contributor
"I'm wondering why coolant

"I'm wondering why coolant overflow tanks exist. Non of my FWC marine engines have had them, and I'm not sure why many new designs have them installed. They do allow easier visual detection of low coolant, and also furnish some extra coolant to account for small leaks. Any other reasons?"
 
"Those plastic coolant tanks o

"Those plastic coolant tanks on closed cooling systems allow easier visual cues as to the quantity of coolant in your engine. They normally have a "Hot" line and a "cold" line drawn on them and these two lines are how you ascertain the fluid level without removing the rad cap. Unless you are a young whipper-snapper, I am sure you can remember when we used to pull a radiator cap off while protecting our hand with a big rag and ducking in case the rad "blew" when the pressure was relieved.
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The coolant tank also allows for expansion as the antifreeze expands the volume of fluid in the system as the engine heats up.

With FWC there is no need to for a tank as the engine contains no fluid that is "captured" or any antifreeze which would expand as the engine gets hot.

My Westerbeke Gen-Set has a plastic coolant tank and is a closed system that uses an FWC heat exchanger. The hot coolant is cooled by the cold FW that is pulled through the heat exchanger."
 
"dave:

i added them when i

"dave:

i added them when i switched from factory hx to the mercrusier versions - they were in the kit and the instructions called for their installation. As best as i can tell, you get them/omit them based on the details of the cap on top of the HX. I haven't had the stock stuff for going on 20 years so I can't tell you what they have or don't but I'm sure you can check.

On the merc caps, they are equipped like a car. they have an inner and an outer seal (or upper and lower). on the inner or center, there is a little relief valve that works (opens) when things cool off. This opening allows the fluid from the overflow connection to 'fill' the hx - it gets sucked in instead of the hoses collapsing.

When it heats up, the coolant expands and any that goes thru the relief before it closes is captured in the bottle for 'reuse'. On my big blocks, this is less than a pint. If it overheats a little, the bottle will catch it; if it is bad, the bottle usually overflows thru its vent - really no benefit in this case.

So, I'd say the tanks provide a means to ensure the HX is always full and to provide a means to inspect coolant level while keeping the system closed.

stant, the radiator cap people, used to have a great tech description on their www site but I haven't been there in a few years. I learned all this stuff by reading the manualin the radiator pressure tester kits and inspecting everything i touch."
 
"Crusader actually did away wi

"Crusader actually did away with them several years ago. They chose instead, a pressurized plastic coolant reservoir, like what is currently used on automobile cooling systems. As with the old overflow bottle, it allows visual inspection of fluid condition and level."
 
"Much better design, for the c

"Much better design, for the coolant doesn't evaporate. Had one on my old Mercury and never neeeded a drop of coolant for the life of the car (which was considerable).

Jeff"
 
"evaporate.....ethylene glycol

"evaporate.....ethylene glycol....not at any temp a human body tolerates well.

I'll bet the change was driven by the auto market...even today, the ratio still exceeds 500:1 (auto vs marine)."
 
"Yes, I also see the auto base

"Yes, I also see the auto based trend to use overflow plastic bottles at the radiator pressure. I recall my dad's 1969 or so Volvo that used a vertical tank next to the radiator with the pressure cap on it. I suppose the bottom line is that it lets you check the coolant level without removing the cap. I'm not inclined to make that addition, considering the space required and the increased points of failure."
 
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