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Old 06-17-2012, 02:02 PM
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Coolant bottle overflows; also losing coolant

I've had an on going overheating/cooling system problem since my engine overheated 2 years ago. This on a custom car with a 1975 Chevy 350 engine. The problems solved include:
1. radiator core supposedly plugged and was replaced.
2. both heads found to be cracked and replaced.
3. a high efficiency water pump installed.
4. new 180 degree thermostat and new 16 lb radiator cap.
5. new hoses between engine and radiator; new hose between rad and overflow tank.
6. pressure test ok; test for combustion gasses in rad and coolant in exhaust ok.
Thats about it on the problems found and fixed. Note that this is a second post on this problem - I got some much appreciated help on the problems above.

The current problems are:
1. when the engine warms up, about a quart and a half or so of coolant gushes out to the overflow bottle. Typically, this happens over a period of 10-15 minutes on the road. I see the coolant bottle go up a few inches and then it goes back and forth between the rad and bottle and then goes up to within an inch of the very top of the overflow bottle - the level will have gone back down some by the time I get back home after another half hour on the road. Note that I start out with the overflow bottle filled to about an inch above the add line (in the morning, cool engine). If I go any higher, the bottle will overflow and coolant will run out the hose attached to the top of the overflow tank. Note that I have a second bottle attached to the overflow hose so that I can catch any coolant that exits the bottle.
2. The second problem is that I am loosing some coolant, even when I keep the level of the coolant in the overflow tank down. Starting with a cool engine and carefully measured coolant level in the tank, I did a 35 mile cruise, chugging up the freeway, a lot of it on a gradual incline. I then let the engine cool overnight - the next morning the coolant level was down about 3/4 cup.
These are the two problems I am currently dealing with.

Otherwise, the car runs pretty good, no back firing, no run on. The temp gauge reads about 210 - 220 on the freeway or chugging through traffic. But when I check with a temp gun it appears that the temp gauge is reading 10-15 degrees high. The temp gauge sending unit is mounted on the intake manifold near the rad hose connection (originally it was mounted in the drivers side head).

Any thoughts or suggestions will be appreciated.

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Old 06-17-2012, 03:13 PM
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Sounds like a head gasket and you're putting compression into the cooling system.
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Old 06-17-2012, 03:37 PM
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head gasket

I did have a test for combustion gas leak to radiator. That should flag a head gasket leak - shouldn't it?
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Old 06-17-2012, 07:29 PM
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It sounds close to normal. It will "burp" a little coolant into the recovery tank when warming up and pull some back in when any air present is purged.
You may have some air trapped inside somewhere and when it expands, it expands more than liquid and pushes out coolant.
It sounds like the cap isnt doing its job. It sounds like it is able to recover coolant from the expansion/overflow tank, but isnt holding pressure soon enough on warmup.
Try a new cap see what happens.If the top of the radiator is lower than the highest point in the cooling system, some air can be trapped.
I have jacked up the front of a car quite high on occasion to help purge air.
I have also unhooked heater hoses at the rear of the system when filling to allow escapement of any air.just some tips.
Fact is stranger than Fiction
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Old 06-19-2012, 07:58 AM
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You might check to see if the hose from the radiator cap to the overflow tank is on good and tight at the radiator cap. This can be a source of air inleakeage as the engine cools. I have a clear hose on my street rod so I can see if there are air bubbles in this line.

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Old 06-19-2012, 08:48 AM
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burp the baby

On old mustangs the easy way since the heater was higher was to loosen a heater hose, slide it back and drill a 1/8 hole, start the engine and when the air came out, then fluid, slide the hose on and tighten it , get all the air out. on my 85 ford truck I use to have to put a cardboard in front of the radiator, let it get hot and push out coolant into the bottle, then spray cold wate on the radiator and it would suck in coolant, repeat several times untill it burped out all the trapped air.
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