What Causes Coolant Not To Suck Back Into The Overflow Tank?

j0rd4n · May 4, 2017

I tried replacing the hose going to over flow tank as well as the cap for the radiator but still after about 45 mins of driving, the car overheats and i check the overflow tank and it's damn near full. The only thing I can think is that there appears to be a small leak at the thermostat neck piece and it's losing vacuum pressure in the coolant system so when it cools down it may suck a little bit from the overflow tank and some air thru the thermostat neck piece

j0rd4n · May 4, 2017

Sorry I meant to title it, what causes not to suck back into the radiator from the overflow tank

mikestang63 · May 4, 2017

bad radiator cap
overflow hose crimped or leaking
leak in radiator

j0rd4n · May 4, 2017

mikestang63 said:
bad radiator cap
overflow hose crimped or leaking
leak in radiator

I thought that could be the issue so I replaced the cap and the hose with new and I still have the issue. My mechanic specializes in mustangs and more specifically foxs and he said my Intake might be warped at the thermostat inlet, he says they all do this so he belt sands them flat but I installed the intake myself so he couldn't make sure it was flat. He said it could be a huge air bubble in the system or the thermostat isn't opening but if the stat wasn't opening I don't think it would stay cool for the 45 mins that it does. Plus the thermostat is brand new.

Their is a lot things it could be.. this car really just needs the whole motor pulled and rebuilt but that's out of my budget. I didn't have this issue a month ago before I did the exploder intake tho so it has to be something I did or an untimely failure of a part

jrichker · May 4, 2017

Start with the radiator cap and the radiator filler neck. There are 2 seals on the radiator cap.

One seal is for the coolant inside the radiator and It is spring loaded closed and remains closed until the pressure inside the radiator reaches the pressure marked on the cap. Then it vents coolant to the overflow side of the cap

The other seal is the filler neck seal that insures that the coolant that overflows the radiator goes to the overflow tank. It is also responsible for maintaining the vacuum so that the coolant gets sucked out of the overflow tank and back into the radiator when the engine cools down..

Check both the seals and the metal surfaces they seat against for damage or evidence of leaks.

If you have an air compressor, there are a couple of other things to try. If you don't find anything wrong with the radiator cap or the radiator filler neck, repost and we'll try some of them.

5.0Droptop · May 4, 2017

Check over flow take for holes.

RaggedGT · May 4, 2017

How many Lbs is your radiator cap?
Have you pressure tested the cooling system?

5.0Droptop · May 4, 2017

RaggedGT said:
How many Lbs is your radiator cap?
Have you pressure tested the cooling system?

About 8ozs they are pretty light

General karthief · May 4, 2017

I got a 16 pounder, heavy sucker but it works.

j0rd4n · May 4, 2017

karthief said:
I got a 16 pounder, heavy sucker but it works.

Mine also says 16 lbs .. so did the new one I bought. The overflow tank is also new (about 3 months old) and it was working fine a couple weeks ago. I found that the thermostat neck was leaking because the gasket set and the bolts had loosened up. I gave them about a 1/4 turn and now they're snugged down. I'll test it tomorrow and go from there. Every piece I have on the worked fine a couple weeks ago and the car has sat since then. The only other thing I can think is the that the radiator is leaking because it doesn't look like it's in good shape.

If this doesn't work I'm gonna have the shop pressure test it

General karthief · May 5, 2017

I feel compelled to mention that when working on aluminum parts that have a bolt holding them together, such as the cooling system, that has liquid pumped through it, it is important to coat the threads of the fasteners with a generous amount of anti-size. I also slather the bolts that hold the water pump/timing cover to the block,

Mustang5L5 · May 5, 2017

The overflow will fill as the coolant heats up and expands. The radiator cap's job is to keep a certain pressure inside the system, and if this is failing will allow too much coolant to escape.

The coolant won't "suck" back into the system until it fully cools. So you'll want to mark where the coolant level is when hot, and then come back when the car cools down and see if it's dropped. It should drop, and if you take the cap off the coolant should be filled to the top. If you have a leak somewhere it will draw in air instead as the coolant cools.

Get a system pressure test unit and do a test by pressuring the cooling system

5.0specialist · May 5, 2017

Did you replace the radiator cap with a factory replacement? I replaced my stock one with a new ford racing radiator cap, it caused the same exact problem. I reinstalled the stock one, and it was working nicely again.

Dan02gt · May 6, 2017

You're not going to want to hear this, but I've seen that before and it was a blown head gasket. It was letting combustion gasses in the cooling system, blowing out the coolant, and causing the car to overheat. This was on my brother's '95 GT. We chased the problem for a while and ended having to pulling the heads as a last resort and sure enough and small leak in the passenger head gasket.

You could use a leak down test to determine if that's the problem or not.

j0rd4n · May 7, 2017

Dan02gt said:
You're not going to want to hear this, but I've seen that before and it was a blown head gasket. It was letting combustion gasses in the cooling system, blowing out the coolant, and causing the car to overheat. This was on my brother's '95 GT. We chased the problem for a while and ended having to pulling the heads as a last resort and sure enough and small leak in the passenger head gasket.

You could use a leak down test to determine if that's the problem or not.

Ya I have read that but the shop I use determined my head gaskets are fine. Since last posting after I tightened the bolts holding on the thermostat neck, this has eliminated my issue since then. If anything the coolant level has dropped slightly after cooling down which I assume were air pockets leaving the system. If the coolant level continues to go down I know that could be a sign that I'm burning coolant or it's mixing with the oi, but before I would assume that's a head gasket I think it would be the lower intake I just installed leaking somewhere which can cause the same issues. My mechanic has been working on these cars since they were made and he says the lower intake thermostat inlets always warp so he belts sands them flat before install , but I installed it myself and wasn't aware of that issue so if it does leak again that is probably why.

j0rd4n · May 7, 2017

5.0specialist said:
Did you replace the radiator cap with a factory replacement? I replaced my stock one with a new ford racing radiator cap, it caused the same exact problem. I reinstalled the stock one, and it was working nicely again.

I tried replacing it with a similar style cap from autozone but that didn't help so I put the other one back on which is probably from autozone too but I don't know because it was on there when I bought the car

j0rd4n · May 7, 2017

Mustang5L5 said:
The overflow will fill as the coolant heats up and expands. The radiator cap's job is to keep a certain pressure inside the system, and if this is failing will allow too much coolant to escape.

The coolant won't "suck" back into the system until it fully cools. So you'll want to mark where the coolant level is when hot, and then come back when the car cools down and see if it's dropped. It should drop, and if you take the cap off the coolant should be filled to the top. If you have a leak somewhere it will draw in air instead as the coolant cools.

Get a system pressure test unit and do a test by pressuring the cooling system

Ya I might have worded something improperly but I do understand that, how you describe it, is how it works.

j0rd4n · May 13, 2017

so ... i am still having issues. it took a little longer for it to happen this time but i havent really drove the car in over a week because i needed to conserve gas.

so far i have done these things
- pressure tested coolant system up to 20lbs , system held pressure
- pressure tested radiator cap, cap held pressure right up to 16 lbs , which the cap is rated for
- replaced the line going from the rad to the overflow tank and secured all the connections.
- tightened up the thermostat inlet after the gasket set
- checked the mating surface that the radiator cap seals too, there a was a little blemish that almost looks like a very small hole, and some crusty dried coolant. but if this was an issue wouldn't the coolant pressure test have failed?

Dan02gt said:
You're not going to want to hear this, but I've seen that before and it was a blown head gasket. It was letting combustion gasses in the cooling system, blowing out the coolant, and causing the car to overheat. This was on my brother's '95 GT. We chased the problem for a while and ended having to pulling the heads as a last resort and sure enough and small leak in the passenger head gasket.

You could use a leak down test to determine if that's the problem or not.

looks like im going to have to explore this option

5.0specialist said:
Did you replace the radiator cap with a factory replacement? I replaced my stock one with a new ford racing radiator cap, it caused the same exact problem. I reinstalled the stock one, and it was working nicely again.

i got some cheesy replacement from autozone that had a weird pressure releasing valve on it, which a tech from work said was definitely the wrong one. im going to try to get one that looks like mine and try that even tho it passed the pressure test

jrichker said:
Start with the radiator cap and the radiator filler neck. There are 2 seals on the radiator cap.

One seal is for the coolant inside the radiator and It is spring loaded closed and remains closed until the pressure inside the radiator reaches the pressure marked on the cap. Then it vents coolant to the overflow side of the cap

The other seal is the filler neck seal that insures that the coolant that overflows the radiator goes to the overflow tank. It is also responsible for maintaining the vacuum so that the coolant gets sucked out of the overflow tank and back into the radiator when the engine cools down..

Check both the seals and the metal surfaces they seat against for damage or evidence of leaks.

If you have an air compressor, there are a couple of other things to try. If you don't find anything wrong with the radiator cap or the radiator filler neck, repost and we'll try some of them.

I checked this out and noticed something... but it passed the pressure test so im confused. ive included a picture of the spot where it almost looks like there is a pinhole sized hole and some coolant dried up and i scraped it away (the white stuff was coolant)

.

General karthief · May 13, 2017

Let us know when you get the proper cap, I can't see that spot in the pic as a problem if it passed the pressure test.

j0rd4n · May 13, 2017

karthief said:
Let us know when you get the proper cap, I can't see that spot in the pic as a problem if it passed the pressure test.

Will do. Two techs from work said that even tho it pressure tested fine it could still be failing. He explained why but I didn't quite understand

What Causes Coolant Not To Suck Back Into The Overflow Tank?

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