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1991 F-150 losing coolant

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Old 10-19-2009, 12:29 PM
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when you were filling it up each week did you add coolant to the overflow bottle or the radiator itself???

maybe the radiator was low and it was just sucking it up from the overflow????

i am so stumped as to how you could lose sooo much coolant over such a short period of time with no puddles or a tremendous amount of white smoke.......
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Old 10-20-2009, 04:12 AM
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It did not have an overflow bottle when I filled it the first two times. It is possible that the first time the radiator was only a quart low, because when I was refilling it, I tried to fill it to the rim and of course some of that ran onto the ground through the hose that was supposed to go into the bottle.
I got a bottle from the salvage yard and installed it. I filled the radiator completely full with the engine cold, purged the upper hose until no more air bubbles appeared where the radiator cap goes, then filled the bottle to just above the cold fill line.
The bottle is now about two inches above the line, but the radiator is not full. I have found this the last two nights when I've checked both levels with the engine cold. I'd say the level in the radiator is about three inches below the radiator cap. There's a long vertical tube in there, and it's just at the top of that tube.

I just wonder why the radiator doesn't stay full on these engines even when they're not leaking or losing coolant. Maybe it's not supposed to... I don't know.

Perhaps when it had that massive coolant leak a day or two before we purchased it, there may have been air trapped in the system somewhere. All I did that day was crack the cap open to see if coolant flowed out, and it did. I didn't check it again until about 400 miles later, so if there were air pockets in there, they may have had something to do with it.

The area under the stat is a little drier tonight than it was earlier. When I pulled the housing off Saturday, I thought I saw a spot where part of the old gasket was missing.

Hmm, I know another way it might have lost some coolant and not been noticed. The master brake cylinder was leaking when we got it. It's entirely possible it could have leaked a small amount of coolant in the first few days we had it and that I may have mistaken that for brake fluid.

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Old 10-20-2009, 11:42 AM
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Check your radiator cap, there should be the rubber gasket around the sealing lip and one on the pressure valve. The sealing lip is the part on the outside by where the top is and the pressure valve is the one that sticks down into the radiator. Without these gaskets it will fill the overflow tank but when cooling down it might just pull air into the system instead of pulling coolant through the hose.
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Old 10-20-2009, 10:20 PM
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My cooling system is all good, and the fluid level in the rad is right in the same place you describe
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Old 10-21-2009, 07:04 PM
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Originally Posted by mustangGT90210 View Post
My cooling system is all good, and the fluid level in the rad is right in the same place you describe
That's what I needed to know.
After some engine work, my '88 T bird held about that same level. Also, my '8 E-159 van held about that level despite not having an overflow bottle. both vehicles had 5.0 liter engines.

@mitch: I had meant to change the radiator cap anyway, but I simply forgot to purchase one when I was buying parts. I think it was a bad cap that led to my Thunderbird drawing air into the system. The level in the bottle never dropped until one 110 degree day when the whole thing overheated severaly on a road trip. The temp gauge didn't work, either. That's why it needed a lot of work done to the engine...
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Old 10-21-2009, 07:42 PM
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nvm
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Old 10-21-2009, 08:14 PM
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My 5.0 keeps the radiator filled to the top. The only time it was down is when I started springing leaks from old hoses. After replacing the hoses I refiled the radiator and the overflow, today when I check my overflow was at full cold and my radiator was filled to the bottom of the cap.
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Old 10-23-2009, 01:37 AM
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My radiator is full this evening, and it's definitely cold out. There's still a dark brown area under the stat housing. I cannot tell if it is wet or not, and can't get my hand to it. The new coolant, though, is bright green.

I think I've got it solved. I guess when is doubt, one should fix it like it's supposed to be before going and troubleshooting it. So I did.
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