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Premium Member
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Discussion Starter · #1 ·
Most of my trips are pretty short and so I haven't noticed this before. I recently undid and replaced my radiator hoses and to check them, I idled the car until the it was good and hot.

I turned the heater on part way through as well to try and remove any air.

Once heated, coolant started flowing (a thin but steady stream) from the reservoir cap. Hoses were tight, cap was on tight. I had filled everything so I expected some overflow but not this much. After letting it go for 5 - 10 minutes, I let it cool. When I came back my reservoir was empty and the coolant was very low. There was a LOT more coolant lost than what can be explained by thermal expansion.

I'm trying to think of what can cause this. My radiator cap (rated at 16 lbs) is new (from the complete kit) looks to be in good condition and it was tight. Anyone have to go to a higher pressure cap?

My other thought is that there is a block somewhere and the pump is pushing the coolant out. Not sure how to test for this if the block is in the engine block.

Any ideas on causes or tests I can do?
 

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Snake Farmer
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10,394 Posts
It may be the reservoir tank isn't large enough, how big is it?
 

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187 Posts
Discussion Starter · #3 ·
I don't see the capacity anywhere on it. Nor is there a brand or model number. All words are in English and German and it came with the complete kit (Mark 3.1). It looks less than a half gallon to me.

I'm going to guess it's 1 liter.
 

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Premium Member
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Discussion Starter · #6 ·
Thanks Shawn! Guess I'll be looking for a shiny new tank and mount it higher.
 

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1,987 Posts
You still have air in the system although a 2 quart tank would help.

Jeff
 

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Discussion Starter · #8 ·
After looking at my cap closer, it seems the return valve in the center of the cap is not spring loaded. It depends on water pressure to close it but it can't build up pressure unless it is closed first. It seems there is not enough flow to overcome gravity and push the valve closed.

Should there be a spring on this valve? My new cars use a diff system so I can't compare to them. Off to the auto parts store I guess.....
 

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Mark 3.1 (Sold)
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894 Posts
I doubt the problem is the cap if it is new. I went thru this issue 2 years ago on my 4.6 complete kit.
First, the supplied BMW tank is a joke. It is too small and is improperly configured. You would assume the upper hose connection would be the correct place to attach the hose from the tstat overflow. But it has a small vent hole to the vented side of the tank so it will draw air when the system cools. The large 1" (?) bottom connection would be the correct place to attach to but what would you use for an adapter.
Second, the BMW tank is pretty but about half the required capacity. I used a Dorman 603-001 which is inexpensive, configured correctly with the hose barb on the bottom, relatively easy to mount, and works correctly. But it's not pretty.
The third issue I had was the pretty billet T-filler which has MINIMAL capacity to separate air from water to de-gas the water system. I replaced it with a Forte de-gas tank, and have had NO issues with air in the system.
DD
 

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Another cheap tank is the 2 quart plastic one offered by Breeze and also sold by NAPA. Since it's plastic it's not pretty like the $100 Canton unit, but for about $10 it works great.
If you have the FF radiator; open the bleed port at the top of the radiator to get rid of some air. I had the same problem as you. I fixed mine by replacing the thermastat with a Ford unit, installing the 2 quart tank and bleeding air at the radiator. Three heat cycles later and it wasn't overflowing anymore.

Jeff
 

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Premium Member
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241 Posts
I used the FF radiator's uppper bleed port to attach to the degas hose of the ford coolant tank. got the air out from both the crossover and the tank and problem solved. I used the Mustang 2 speed fan and found I needed to have it turn on 5-10 degrees sooner than factory to keep it from overflowing when sitting at idle. Never a problem when moving. HTH
 

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Discussion Starter · #12 ·
I just checked a couple of radiator caps at the auto parts store and found both had spings on the coolant return valve. I definitely have a bad cap - I'll get something nicer looking while I'm at it.

I still plan get a nicer/larger reservoir and mount it as high as I can.

Thanks all for the help!
 

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Premium Member
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1,177 Posts
I used the Moroso Expansion tank, no problems so far. It also uses the stock radiator cap. Plus it matches my valve covers.




 
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