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on here and elsewhere I have seen freshly added coolant conditioner coat the inside of the rad cap. no big deal to rinse / wipe it off while it is still wet. I got it in my H4 car right now, but when I have looked every once in a green moon at the rad cap saw no residue ever.
 

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Seems doubtful to me that the Conditioner would seal the return vent valve; if anything, I wouldn't be surprised by the opposite -- the vent valve being held away from the gasket.

Some other possibilities: Is there accumulated "gunk" at the bottom of the reservoir? Is the bottom of the hose long enough to press flat against the bottom, effectively sealing it against drawing fluid back up as the engine cools? Is the hose clear inside all the way to the bottom?
 

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It sticks shut...

The small vacuum valve is held tightly closed by pressure during the operation of a hot running vehicle. When the engine cools it is supposed to allow coolant to siphon back to the radiator due to coolant contraction. The conditioner allowed the valve to get stuck to the mating surface and not open. I had to pry it open. The valve may get dry and allow the conditioner to cure (not sure). After adding the conditioner the coolant that expanded into the overflow tank didn't return to the radiator like it had the 10 months before. I checked it about twice a week since I felt the engine may have a head gasket leak (into the coolant, a few bubbles seen once).
I bought the Subaru cap about 6 months ago. I used a Stant cap originally when I drained and refilled the car, after I bought it used about a year ago. I have a Stant and 2 Subaru caps. The stuck one was a Subaru. However, the Stant is visually identical in all ways to the Subaru, even the forming tool marks from the sheet metal top are the same (other than "ND" on the Subaru and an "S" on the Stant). My force gauge shows the springs are the same force curve.
Doing a coolant drain/refill tonight.

Trig
 

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Works...

The conditioner appears to work but I believe can cause unintended problems. Like sticking a coolant return (to radiator) valve. It probably needs to cure by coming in contact with air. So perhaps the cooling system is sealed now from it. It's coming out (drain/fill) because I can't have the radiator not being full all the time.

Trig

P.S. I had a GM car with the LS2 V8 engine. It not only contained Dexcool (bad reputation) but also recommended sealant tablets. The tablets may have been Ginger Root fiber powder like the Holts/Subaru coolant conditioner contains. There was a tan colored residue that would be on the coolant dipstick.
 

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Drained...

Drained and refilled last night (2 gallons of coolant, proper dilution with distilled water). New "ND" Subaru radiator cap, new coolant.
The local Subi dealership parts counter person asked me what I was
doing with all the caps. I've bought 3 the past year. They come
in a box almost as big as a shoebox. Old coolant, with the conditioner,
was green with a red tint and had a smell of diesel fuel, odd. The radiator overflow tank had a gray coating in it. Where it dried it wouldn't come off.

Trig
 

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Drained and refilled last night (2 gallons of coolant, proper dilution with distilled water). New "ND" Subaru radiator cap, new coolant.
The local Subi dealership parts counter person asked me what I was
doing with all the caps. I've bought 3 the past year. They come
in a box almost as big as a shoebox. Old coolant, with the conditioner,
was green with a red tint and had a smell of diesel fuel, odd.
The radiator overflow tank had a gray coating in it. Where it dried it wouldn't come off.

Trig



I wonder if you are somehow getting fuel into the coolant system and it is melting some of the hoses / gaskets, and the residue from those is working its way into the cooler areas to dry again.


@grossgary @1 Lucky Texan @rasterman @lockmedic (anyone) do you got a guess for that?
 

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Coolant Smell...

Good point about fuel migration. All 3 coolant hoses are less than a year old as is the thermostat and new cap. The overflow tube isn't. I'll check that.

BTW: the car got 29.89 MPG (gas pump and odometer method) over a 400 mile mountainous round trip. I expected less.
Daily driving is 18 - 19 MPG.

Trig
 

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This is what my Coolant looks right now like OPs coolant.

I just bought 01 Beans and got scared that i may have HG problem since its has 189+ miles on.

Looks like i need to do cool flush.
 

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automatics 'could' have a leak in the trans cooling coil in the radiator. I think it's a very rare type of failure though. Maybe someone in the past was just confused and added some incorrect fluid to the rad or overflow?
 

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I have a small external coolant leak on my 03 H6 head gaskets (on the bottom).
At 150 or 170k or so I noticed the green crunge.
Sometimes it would be damp.

Since changing the coolant and adding one can of the magic goo they seem to have been dry.
Up a bit over 250,000 miles now.
 
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