Crossflow Cooling System -Overflow Tank or Expansion Tank

[San Augustine]

Probably used the wrong terms here, the car has an overflow tank and I want to convert it to an expansion tank. 

[MV8]

Then all you need is a tee in the heater circuit (or the one you ordered to tee a radiator hose) and an expansion tank.

 

Examples:

https://www.ebay.com/itm/256717068259?_trkparms=amclksrc%3DITM%26aid%3D1110006%26algo%3DHOMESPLICE.SIM%26ao%3D1%26asc%3D20240415110138%26meid%3Db75df59a2efe47d8be0a0a2eabdf5a5d%26pid%3D101875%26rk%3D2%26rkt%3D4%26sd%3D365014549820%26itm%3D256717068259%26pmt%3D1%26noa%3D0%26pg%3D2332490%26algv%3DSimplAMLv11PairwiseFitmentPromotionV2WithCassiniQueryEmbRecallKatReco&_trksid=p2332490.c101875.m1851&itmprp=cksum%3A256717068259b75df59a2efe47d8be0a0a2eabdf5a5d|enc%3AAQAKAAABQCggmqi%2BjGBvS8WShKTeLW5J2g6qXnzgH61T5t8ujPfO4ZJHWSDajmSreRV%2BXNto1elxKNVVM64kNmwVgiMN2szhK0tlR1011FXMzSAB1PUgkXdlAtp1SJ2P1Ia246YdDc6Lz8RqY5J3Udbo2ifiNz9Ty%2F0PHH1s8EKnHfOcCBzmEgRxCIN2Zn9HgmCS1k6AZeBNTMKlaZRZ2WNDksfpX44iTQnZvnI4Dt722Qdc8VyykIRbEgO5GisZJbzb%2B4%2B7btodHPi%2FWbaXOQkCse4E2DR5GNj%2FM8xvWUmB%2FJ%2BYhG2ozgYXZwg9jwK34x2WrP0aKibkYvLCbiZ37z%2FbLlNt3Na19qY6Y%2FHye68UsDQ9KEquaE22WvyPZmoo9fQfGWT7%2B6E4V9CTXmflBAIL0Q%2BeNeJ8GMtxahjs4rOKTrX8go4k|ampid%3APL_CLK|clp%3A2332490&itmmeta=01KBJQWRCSVA0EGAMN5S2HW764

 

A tee (assumes 1/2 inch hoses and fitting for expansion tank): https://www.ebay.com/itm/267284759799?_skw=heater+hose+tee&epid=230386134&itmmeta=01KBJR1N5F8BM5KNMCHP85WEX1&hash=item3e3b69c4f7:g:rcQAAOSwwt5oRyTA&itmprp=enc%3AAQAKAAAA0FkggFvd1GGDu0w3yXCmi1cwvKWLr3m5skZUhd7IzrdGPI5hLO6qnV81qhoM62naBqKkKn8KMIXKQ8JyDjr2v68JiSRbCJIhGEEvJ8IZY6OlP3hMT4r6RLSDdFuNO9MDivKi8zmuwWUcSrQqr0uNWRswARDTgDL8Gaxge%2BwLP8%2FRDMXxDCUeseGXbKziP6wcNhAZRBrsY4d5GytIEVmXFq1fNW9UtaMtPEX%2FlKzREgSb2LuZY0i%2FpMax%2FMh2eMXaP359dWD6ia6pK4Ur66mF5zI%3D|tkp%3ABk9SR4jThtjcZg

Edited December 3, 2025 by MV8

[San Augustine]

Right, but you see I am a rather visual learner, that is why I need pictures.  

[MV8]

ok. I'll post a pic shortly.

[MV8]

You can tee this hose anywhere between the engine and the heater. If the expansion tank you select has a small port next to the filling cap, then it can be capped with fuel hose and clamps or tee'd into the overflow tank up front. The other heater hose could be used also even if the heater valve were left closed.

Edited December 3, 2025 by MV8

[NSXguy]

for your visual interest… 

Sharing some photos from when I did a closed system. I have since removed as I wanted to declutter the engine bay and simplify. Either system (open or closed) works well if properly installed. 
 

as MV8 mentioned, Tee off the heater hose circuit or if no heater then just connect to the water pump neck  (reference the photo with the arrow ) and route the line (larger ID) to the bottom of the expansion tank. 
 

Smaller ID hose will go from nipple on existing thermostat housing to small nipple on expansion tank
 

you’ll want to cap off the existing thermostat housing with a non pressure rad cap and then use a ~13lb rated cap at the expansion tank. 

some expansion tanks have a purge/air bleed nipple as well.  

 

[San Augustine]

Is it possible to have an interruption if I go off the heater inlet when the water valve is shut off since there is no continuous flow?

[MV8]

No interruption but why not use the other hose?

[San Augustine]

Sorry, I am an idiot. Got my hoses mixed up. Didn’t realize the hose you marked went to the h2O pump.

[MV8]

What is the issue with the cooling system as-is? The filler should stay as full as it will hold under the cap and the level is maintained by the overflow. Minimum cold engine level in the overflow covers the hose end inside the overflow.

 

If the filler neck is not kept full, then the cooling fan switch (unplugged in your picture) does not pickup the engine temp and the cooling fan doesn't come on when it should. I believe one wire goes to a chassis ground and the other to the fan motor to provide ground.

[San Augustine]

As you know I bought the car on BAT last week. I knew that I would need to check over the car before doing any driving, one of which was the cooling system.

As to your question- the fan wires were plugged into the housing. But what got my attention was that the cap on the catch can was askew and the can was empty and dirty. The hose itself was dry and needed replacement. 
 

When I removed the radiator cap no coolant was visible. I then removed the housing to inspect the thermostat and found it cruddy but functional in the boiling water test. The coolant under the housing was clear and tested to be almost 100% h2O.

 

So after these findings I knew that at a minimum I was replacing the coolant, the old thermostat, and radiator cap. And after reading this topic I thought that maybe it would be prudent to upgrade the cooling system.
 

 

Edited December 4, 2025 by San Augustine

[NSXguy]

2 hours ago, San Augustine said:

As you know I bought the car on BAT last week. I knew that I would need to check over the car before doing any driving, one of which was the cooling system.

As to your question- the fan wires were plugged into the housing. But what got my attention was that the cap on the catch can was askew and the can was empty and dirty. The hose itself was dry and needed replacement. 
 

When I removed the radiator cap no coolant was visible. I then removed the housing to inspect the thermostat and found it cruddy but functional in the boiling water test. The coolant under the housing was clear and tested to be almost 100% h2O.

 

So after these findings I knew that at a minimum I was replacing the coolant, the old thermostat, and radiator cap. And after reading this topic I thought that maybe it would be prudent to upgrade the cooling system.
 

 

there’s really no need to “upgrade” the cooling system especially for a mostly road driven 7; unless you just want to, of course. It works well in current configuration even in a track situation- I can attest . For your cooling system baseline I would say:

-replace the thermostat

-new gasket on the housing

-new cap (I believe 13lbs)

-new 50/50 mix ford green coolant,

-the overflow hose to the overflow tank

- if overflow tank holds, no need to replace unless for aesthetics

-probably won’t hurt to flush the rad (and heater heatexchanger) while everything is off

-water pump is fairly inexpensive; would throw a new one in as well (while you’re in there)

-make sure you put enough coolant and bleed system thoroughly

 

should provide lots of trouble free miles and smiles, at least for the coolant side.

 

I have a more comprehensive checklist for a “new to owner” analog car baseline process. 

 

 

 

 

 

Edited December 4, 2025 by NSXguy

[MV8]

The previous owner(s) may have used water without antifreeze for superior cooling but added a rust preventative. If not, the radiator core may be partially plugged with rust particles. I'd use a garden hose to flush everything into a basin to pour into a white bucket to see the condition. If no significant "muddy" water (just momentary flashes of solids pouring out), then just flush every hose each direction until water is clear, then fill with a 50% premixed coolant.

 

If the level under the cap keeps going down without leaks, I'd suspect the head gasket is leaking internally to the exhaust.

If the overflow level never goes down, I suspect the radiator cap outer sealing ring is torn or missing or head gasket.

 

 

 

[San Augustine]

Good to know about detecting head gasket leaks. I have already sourced a radiator cap, thermostat, and water pump. 

[MV8]

https://www.summitracing.com/parts/sum-tc001?seid=srese1&ppckw=pmax-oils-fluids-sealer&gclid=EAIaIQobChMI9-3SlsKmkQMVpZtaBR1KHiIMEAQYBCABEgJqfvD_BwE

 

https://www.evapo-rust.com/thermocure-cooling-system-radiator-rust-remover/

 

Video is just an example of using it and not concurrence with the kids assumptions.

Edited December 5, 2025 by MV8