Should My Expansion Tank Not Have a Return Line?

[Doc76]

Ok so I am new to owning a Mk4 but I noticed a few drops of coolant below my expansion tank area after the last drive.
What I discovered I think is pretty serious so looking for feedback here.

It’s hard to see here but the guy who built this car installed the OEM rad and tank so that the tank lid cannot be removed. You read that right. It’s jammed up under the frame.
What’s more is there does not appear to be a return line so the tank is filling, I hope due to a bad cap, but the fluid can’t return to the rad out of the tank.

Does this make sense?

I noted when I bought the car that the rad had leaked at one time and he sprayed silver paint to hide it and very likely dumped stop leak in the was. It’s rough looking. 204372204373
Perhaps it’s a very good thing I have a new FFR rad, Breeze shroud and Flexlite Loboy fan on its way.

[Norm B]

The expansion tank should only have one line on it that goes to the side of the filler neck. The radiator cap acts as a two way valve. It allows coolant out to the expansion tank when the pressure in the system exceeds the pressure rating of the cap. It also has a small valve in the centre of it that allows coolant to be drawn back into the radiator when the system cools and a vacuum is created.
Can’t see any leaks in your pictures but, agree that the expansion tank location is less than ideal.

Norm

[CraigS]

That tank needs to be moved asap. This is my old mkII. The top rad hose is different from yours but you should have a similar T-filler. You can see the hose coming from the T-filler which goes into the bottom of the tank.
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Click to enlarge the image.

[Doc76]

The expansion tank should only have one line on it that goes to the side of the filler neck. The radiator cap acts as a two way valve. It allows coolant out to the expansion tank when the pressure in the system exceeds the pressure rating of the cap. It also has a small valve in the centre of it that allows coolant to be drawn back into the radiator when the system cools and a vacuum is created.
Can’t see any leaks in your pictures but, agree that the expansion tank location is less than ideal.

Norm

Yes there’s only one line in from the filler up at the motor down to the tank. The overflow line runs into the bottom of the tank but no way for the coolant to return back from the tank ?. What’s more I think my head gasket is gone :( (misfire for a while, mild white smoke, etc)

Looks like my decision of 347 or 351w-based 427 swap is going to come much sooner than anticipated.

FFR said the 351w based 427 will align with the mounts in my 302 based chassis. Only need to change or modify my headers.

[Norm B]

Here is a short YouTube video of how the system is supposed to work https://www.google.ca/search?q=how+a+radiator+cap+works+with+an+expansio n+tank&client=safari&sca_esv=2bd1bbaced4e221a&channel=ipad_bm&sxsrf=ADLYWILmdXS7z1haXi9B6G6f0YO-OVv3SQ%3A1726967589108&source=hp&ei=JW_vZu6CBL6X0_gPxMDt6As&iflsig=AL9hbdgAAAAAZu99NbA8tgvyeKsbraEau56zW-SBK0gw&oq=how+a+radiator+cap+works+with+an+ex&gs_lp=Egdnd3Mtd2l6IiNob3cgYSByYWRpYXRvciBjYXAgd29y a3Mgd2l0aCBhbiBleCoCCAAyBRAhGKABMgUQIRigATIFECEYoA EyBRAhGKABMgUQIRigATIFECEYnwUyBRAhGJ8FMgUQIRifBUiT uwFQ2QlYmK4BcAF4AJABAJgBwgKgAZEhqgEIMjMuNi41LjG4AQ HIAQD4AQGYAiSgAvsiqAIKwgIHECMYJxjqAsICChAjGIAEGCcY igXCAhEQLhiABBixAxjRAxiDARjHAcICCxAAGIAEGLEDGIMBwg IOEC4YgAQYsQMY0QMYxwHCAgUQABiABMICBRAuGIAEwgILEC4Y gAQY0QMYxwHCAgQQIxgnwgIIEC4YgAQYsQPCAgsQABiABBixAx iKBcICDhAuGIAEGLEDGIMBGIoFwgIEEAAYA8ICCBAAGIAEGLED wgIIEC4YgAQY1ALCAggQABgWGB4YD8ICBhAAGBYYHsICCxAAGI AEGIYDGIoFwgIIEAAYgAQYogTCAggQABiiBBiJBcICBxAhGKAB GArCAgQQIRgVmAMKkgcIMjIuNy42LjGgB4iOAg&sclient=gws-wiz#fpstate=ive&vld=cid:29b83571,vid:QhLUwsCdM08,st:0
Get a test kit from your local auto supply store to test for a blown head gasket. Simple to use and could save you spending a bunch of money sooner than necessary. This time of year in Canada it can look like the exhaust is putting out a lot more steam. The short exhaust on the roadster makes this more noticeable than most cars.

Norm

[MPTech]

As Norm said, one line from the Filler T to the bottom of the overflow tank. When the pressure builds, coolant is forced to the Overflow tank, when it cools back down, the vacuum pulls the coolant back into the Filler T from the Coolant tank. That's why the hose goes into the bottom of the overflow, to maintain the Closed-Loop system. There should also be a hose at the top of the overflow tank, in case there is too much coolant in the system, it will overflow to the ground.
If the system is setup properly and burped, it will equalize after a few cycles, keep an eye on the level in the Overflow Tank while cycling (may need to add coolant a little at a time).

[Doc76]

Here is a short YouTube video of how the system is supposed to work https://www.google.ca/search?q=how+a+radiator+cap+works+with+an+expansio n+tank&client=safari&sca_esv=2bd1bbaced4e221a&channel=ipad_bm&sxsrf=ADLYWILmdXS7z1haXi9B6G6f0YO-OVv3SQ%3A1726967589108&source=hp&ei=JW_vZu6CBL6X0_gPxMDt6As&iflsig=AL9hbdgAAAAAZu99NbA8tgvyeKsbraEau56zW-SBK0gw&oq=how+a+radiator+cap+works+with+an+ex&gs_lp=Egdnd3Mtd2l6IiNob3cgYSByYWRpYXRvciBjYXAgd29y a3Mgd2l0aCBhbiBleCoCCAAyBRAhGKABMgUQIRigATIFECEYoA EyBRAhGKABMgUQIRigATIFECEYnwUyBRAhGJ8FMgUQIRifBUiT uwFQ2QlYmK4BcAF4AJABAJgBwgKgAZEhqgEIMjMuNi41LjG4AQ HIAQD4AQGYAiSgAvsiqAIKwgIHECMYJxjqAsICChAjGIAEGCcY igXCAhEQLhiABBixAxjRAxiDARjHAcICCxAAGIAEGLEDGIMBwg IOEC4YgAQYsQMY0QMYxwHCAgUQABiABMICBRAuGIAEwgILEC4Y gAQY0QMYxwHCAgQQIxgnwgIIEC4YgAQYsQPCAgsQABiABBixAx iKBcICDhAuGIAEGLEDGIMBGIoFwgIEEAAYA8ICCBAAGIAEGLED wgIIEC4YgAQY1ALCAggQABgWGB4YD8ICBhAAGBYYHsICCxAAGI AEGIYDGIoFwgIIEAAYgAQYogTCAggQABiiBBiJBcICBxAhGKAB GArCAgQQIRgVmAMKkgcIMjIuNy42LjGgB4iOAg&sclient=gws-wiz#fpstate=ive&vld=cid:29b83571,vid:QhLUwsCdM08,st:0
Get a test kit from your local auto supply store to test for a blown head gasket. Simple to use and could save you spending a bunch of money sooner than necessary. This time of year in Canada it can look like the exhaust is putting out a lot more steam. The short exhaust on the roadster makes this more noticeable than most cars.

Norm

Thanks for this Norm.
What can cause the coolant to not be drawn back into the system?
I noticed my overflow was full when cold where it hadn’t been before.
Was at the “cold” marker line on the tank and now sits at the “hot” marker line now when cold.

Maybe worth noting my 16lb rad cap was tested at 14lbs so I replaced it.

[Doc76]

As Norm said, one line from the Filler T to the bottom of the overflow tank. When the pressure builds, coolant is forced to the Overflow tank, when it cools back down, the vacuum pulls the coolant back into the Filler T from the Coolant tank. That's why the hose goes into the bottom of the overflow, to maintain the Closed-Loop system. There should also be a hose at the top of the overflow tank, in case there is too much coolant in the system, it will overflow to the ground.
If the system is setup properly and burped, it will equalize after a few cycles, keep an eye on the level in the Overflow Tank while cycling (may need to add coolant a little at a time).

What might cause the coolant to not return to the system when cooled?

[CraigS]

Wrong type of cap? There are 2 types. I hope someone chimes in w/ a pic of the right style since I couldn't find one quickly.

[Norm B]

Was the engine overheating? What caused you to test the cap? If there were no issues with the cooling system prior to changing the cap then, put the old cap back on! The cap working at 14 psi is fine. Less strain on the rad and hoses.

[Doc76]

Was the engine overheating? What caused you to test the cap? If there were no issues with the cooling system prior to changing the cap then, put the old cap back on! The cap working at 14 psi is fine. Less strain on the rad and hoses.

No it wasn’t overheating. In fact I am surprised how cool this car runs,
The reason I tested the cap was as mentioned above. The coolant is leaving the system out to the expansion tank and does not appear to be returning.
I tested again today.
I marked the expansion tank, topped off the rad cap inlet, ran it for 15 min and noted the rise in the tank level with another mark. Let the car totally cool (I mean hours) and the coolant didn’t drop.
I’ve put about 150miles in the car since getting it however this just started.

[Norm B]

Thoroughly check the line from the filler neck to the expansion tank. The hose should be in good shape and the connections must be tight. Pay particular attention to the connection at the filler neck. Also check that the top sealing flange on the filler neck is not damaged.
Are there any coolant leaks that suddenly appeared around the time it stopped drawing the coolant back?

Norm

[Doc76]

Thoroughly check the line from the filler neck to the expansion tank. The hose should be in good shape and the connections must be tight. Pay particular attention to the connection at the filler neck. Also check that the top sealing flange on the filler neck is not damaged.
Are there any coolant leaks that suddenly appeared around the time it stopped drawing the coolant back?

Norm
The return line (exp to t-filler) is like new.
I just checked my reference pictures (already at the office for the day) There’s no clamp on the return tube. So I guess that’s the next try by adding the missing clamp?
Should the expansion tank be mounted higher than the t-filler?

[cob427sc]

If possible the expansion tank should be higher than the T-filler although this is not always possible.

[MPTech]

If possible the expansion tank should be higher than the T-filler although this is not always possible.

I don't think this is correct, T-filler should be highest point and it should be a closed system.

[MPTech]

Can you attach pictures of your T-filler, overflow tank, and overflow hose connecting them?
Something is not making sense.
If the system is up to temp and expands into the overflow tank, when it cools it would create a vacuum and should pull the coolant back into the Coolant System. You're losing the vacuum somewhere (if the coolant remains in the overflow) and when you open the Thermostat cap the Coolant System would be low and need to be refilled(?)

[rich grsc]

I don't think this is correct, T-filler should be highest point and it should be a closed system.
You're right Mark, doesn't matter if the tank is lower, but filler needs to be higher.

[Doc76]

Ya that’s why I wondered about the head gasket. It’s a new issue too. The car wasn’t like this for the first 3months of ownership.
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[MPTech]

Your hose from the T-Filler needs to go to the bottom of the Overflow (Recovery) Tank (I couldn't tell from the pictures). This will maintain the vacuum (Closed system). And the Overflow Tank cap needs to be vented.
If the hose is near the top of the Recovery tank, coolant will flow there from the T-Filler, but UNABLE to be pulled back by the vacuuum.

[Doc76]

Your hose from the T-Filler needs to go to the bottom of the Overflow (Recovery) Tank (I couldn't tell from the pictures). This will maintain the vacuum (Closed system). And the Overflow Tank cap needs to be vented.
If the hose is near the top of the Recovery tank, coolant will flow there from the T-Filler, but UNABLE to be pulled back by the vacuuum.
Ya sorry not the best picture
The small hose from the t-filler is going from the t-filler to the bottom of the tank. Maybe the issue is the cap on the tank is not vented. The tank is also installed, wedged up under the upper frame rail so not like I can even replace the cap.
I also noticed last night the tank mount points are cracked. You can see both noted issues in this picture.
So there’s two very good reasons to replace the tank.


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