Differences Between Expansion Tanks & Overflow Tanks in Automotive Cooling Systems

Overflow tanks are not pressurized, and simply hold what overflows from the pressurized side of the system when pressure exceeds what it noted on the radiator cap, typically 16PSI. An expansion tank expands the amount of coolant that the pressurized system can hold, and the top of the tank takes on the role of the previous design's radiator cap (expansion tank caps and radiator caps are sometimes the same exact part) for venting excess pressure.

What is a expansion tank in a car? How does a car expansion tank work?
The expansion tank takes in excess coolant and minimizes excess pressure in the engine cooling system. An expansion tank helps maintain a minimal pressure increase during the expansion of heated water, helps reduce coolant hammer, and helps protect the engine cooling system from stress, typically brought on by pressure and temperature changes.

What is the purpose of a coolant expansion tank?
The expansion tank is always located higher than the radiator and functionally becomes the top of the radiator, providing additional fluid capacity, with plenty of room for coolant expansion. This method allows the hood line to be low, as the radiator can be a bit shorter in stature and doesn't need to have it's own cap.

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[QuigzDrifts]

Great video, no intro, jumped right into it, explained it concisely. Learned more in 3 mins then I did in 10 mins of google/forum searching lol

[ashleysnyder]

I just want to say thank you. I bought a car with an expansion tank and NEVER could find anything like your video to explain WHY I didn't have a radiator cap and what more, how each was used. Bravo! 👏❤

[tonyvang]

Good job on the explanation. Keep note that the bigger spring on a pressure cap is named pressure spring. Then, the smaller spring is named vacuum spring.

[Gyppor]

Awesome, thank you for the description. I've worked on cars for 2 decades but never had to do much on coolant systems, this helps a lot!

[JeffSmith-dvzb]

Great explanation. I had wondered by my BMW could have small hole built into the top of the coolant tank, when most websites I saw said that the coolant tank is pressurized and part of a closed system

[stevelong]

wow thank you for the valuable info, please consider explaining the internal chambers to expansion tanks as I have a 2018 Nissan Frontier v6 crew cab and noticed after playing with the levels in radiator and expansion tank that there are 3 different levels, or chambers in my whitish semi-transparent tank, first closest to line from radiator, a small rubber hose about 3/8 inch ID was about 2 inches above max line and only about 1/4 of tank area, then near the middle of tank a 2nd chamber that was about 1.5 inches above max line covering another 1/4 of the tank area, then a 3rd chamber in the remaining part of tank, the back half that was about 1 inch above max line. I have search high and low on the internet for this info, breakdown but have failed. I think you are more in tune with the proper resources to supply the explanation, I hope.

[cbxjlsx]

Marvelous way to break it down and to explain it

Numbers 6:24-26
24 “The Lord bless you and keep you
25 The Lord make His face shine upon you,
And be gracious to you;
26 The Lord lift up His countenance upon you,
And give you peace.

God bless

[jettisonyurei]

very informative, thank you. i was playing "The Long Drive, " a driving survival game and was just curious how my coolant tank was working just by putting a liquid in it. i will be watching more of your videos, sir.

[Mr_BUSINESS__]

your expansion tank explanation isn't correct. There is only (2) hoses on the expansion tank and one goes to the engine and the other goes to the top of the radiator (bleeder tube). Having (2) hoses going to nowhere makes no sense. And there is no expansion tank that leads to an overflow tank.

[labradormcgraw]

Just found this channel - not too sure how I've missed it. This was very clear, thank you. Subbed from London.

[jhtprojects]

Very clear, the difference between the two designs. Thank you. Question, with an expansion tank design, can I gradually "change" the coolant by siphoning off the contents of the expansion tank and pouring in new coolant, say, once a week or per month? Thank you

[jpleroux]

Is there a difference in pressure between these two systems? I know that the cap determines the pressure in the radiator but can you adjust your pressure with the expansion tank setup?

[steveh]

Very nice clear explanations, thanks for posting!

[madetoremain]

HI -
i have Chevrolet epica 2009 with 2.4 6 cylinders engine - i remove the old engine and install new Toyota Camry engine every thing is great but i want to know
Do i have to change the RAD system from the original one ( expansion tank ) to Over flow type ?
Thanks

[VC]

Hey, an you explain why there is a return line on a expansion tank that sends coolant back to the pump? My tank has 3 connections, 1 to the radiator, 1 to engine outlet (behind the thermostat) and 1 connect just before the waterpump. Isn't there a bypass to the radiator in such a design? I'd like to understand why it was build that way and whats the purpose. Thanks!!

[lruiz]

Upon coming back to my car (did a 10 min. hywy run to local store) noticed Prestone near and under the driver's side of my 1976 Lincoln Continental Coup. I lifted the 900 lb. hood expecting to find a wet or steamy radiator, but lightly touched the unpressurized cap and accessed it was safe enough to touch. When I unscrewed the cap, Prestone gushed out of the coolant overflow tank - making me think that the plastic OEM container must have seen it's last day. After days of searching for a replacement, I decided to remove mine and see if I could "repair" it with GOOP. What I discovered was that the tank on this model is a "purge" tank. The coolant enters the overflow and purges it out of the car entirely! I had never heard of such a thing, but sure enough when I got under the car saw how the tank has a nipple which sits in a groove designed to allow the coolant out - right onto the road or wherever. Mind that it's not daily. Perhaps it's running hotter than when I drove it late winter.
Have you seen this before? I think you may have stated that some cars 60's - 70's may have not even had overflow reservoirs.
This experience also made me think about how or where would I ever get a replacement. Meaning, is there something universal?

[seemann]

Thanks for that. Now I understands the difference.

[tradeswithjohnnie]

Perfect explanation. Thank you for making this video

[vallan]

Do you know why my nissan frontier has a pressure cap on the radiator and the overflow reservior? How necessary is the overflow pressure cap?

Reason: I wanted to change my reservoir to the aluminum reservoir with the vertical tube that clearly shows fluid level.

[davemcguigan]

And one of the things people take for granite. I remember the days in the 60's and 70's where you didn't have the expansion tank. You strictly add coolant to the radiator cap and there was a hose next to the cap when there was expansion and the excess would fall to the ground. Hopefully no animals like dogs would be around to drink that green sweet Preston antifreeze!