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Senior Charter Member
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6,569 Posts
Discussion Starter #1
As many know, I've been having a problem with my 5.0 where coolant migrates into the over flow tank but it does not all go back into the block on cool down. I usually have to transfer about half a cup from the tank to the block after a trip to San Luis Obispo, about 35 miles round trip. I have been living with the issue while getting my '67 Cougar back on the road.

I started the engine yesterday, found some fuel leaks in one old rubber line, and one float bowl (the accelerator pump), ordered a carb kit, should arrive tomorrow.

The last four trips I've taken to SLO I didn't have to mess with the coolant. Just as I'm getting close to starting the tear down it is getting to where finding the problem may be more difficult. I still have to get into the engine as I need to do a ring job anyway.

Regards, Rick.
 

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Senior Charter Member
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1,771 Posts
Hey Rick, I sort of remember reading about your issue a while ago but not the specifics. Are you sure the system is not just trying to find, we'll say for the lack of a better term, a happy medium. Normally most systems leave a little/minimum liquid in the tank to keep from pulling air back in the radiator/motor. A half cup doesn't seem like a lot to be left behind if thats the total amount. Most OEM tanks have a minimum cold level mark on them.
 

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Official OLD GUY
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3,811 Posts
Rick,

FWIW, mine does the same thing sorta. If I fill the "T" filler to the top and replace the cap (16#), I get the tank filled after a short run and up to temperature, and then when I stop the engine, I get a small amount of overflow to the ground - my tank is more-than-likely just a bit too little. But, when the engine cools down it doesn't pull back all the fluid in the tank either, leaves about a cup. But when I look in the "T" filler, it needs about that much to fill to the top again. Why doesn't it just pull all of it?? Hmmmmmm.
I've learned to live with my engines little quirks and just add a cup every 3 weeks or so.

My thought on this little anomaly has to do with vacuum and pressure, in this case, water pressure. 1# of pressure isn't the same as 1" of vacuum and it's that difference that I think makes our cars act this way. Go figure . . .

Doc
 

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Senior Charter Member
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Discussion Starter #5
Thanks for the replies. I have 25k miles on the car now and didn't experience this particular problem until I took a road trip to Idaho to visit my Brother, about 900 miles one way (this was my third road trip of about the same distance). While there I went to my BIL's garage and we did all the pressure checks for the radiator and block as well as the cap. We also tested for hydrocarbon in the coolant. I also took the water pump off to check for proper rotation and operation, flushed the block and radiator, replaced the thermostat and checked all the hose clamps. I replaced the T-filler cap with a new 16 lb, I'm currently using a zero pressure cap.

Since that trip, if I drove more than about 70 miles it will push enough water into the over flow tank that it will spill out the vent, then the resulting void in the block allows the coolant to start to boil at which time the cap will be blown off the over flow tank and lots of coolant goes everywhere. The overflow tank is aftermarket sized to hold the same volume as the original Mustang tank.

It has been frustrating to have checked the system over and find nothing that makes a difference, and now, to have it start to function as a normal system, just when I'm getting ready to tear into it.

I'll come back later and share what ever I find as I go through the tear down.

Regards, Rick.
 

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1st RFM/FFR Legacy Winner
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22,589 Posts
Never mind I missed you have the vented cap already
 
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