I got my 02 Outback back from having an engine rebuild. (I know, shame on me for not watching the oil). It runs great now, but the coolant is not flowing back from the overflow after running for 30 minutes (my commute home)

 

I noted the tank level, topped off the coolant and drove home. When I get there, the overflow level is 8 oz higher than when I left. That makes sense as the coolant heats and expands. I leave the car overnight and the next morning check the levels. The overflow level hasn't changed and when I pull the radiator cap, there is a vacuum. At this point the radiator is 8 oz short of coolant.

 

If I don't open and refill the radiator, when I get to work, the overflow has 8 more oz in it and the radiator is 16 oz down.

 

What the heck is going on here?

 

We have replaced the cap, the radiator and the thermostat with original Subie parts.

 

Any ideas?

The rad cap is malfunctioning , If you take off the rad cap and examine the centre piece ( metal disc in the centre ) it will be stuck closed . You should be able to move it with you finger nail . you can usually free them up by gently prying on the disc and rinsing it under water , If you can't free it up , time for a new cap

 

Hope this helps

 

SEA#3

The rad cap as sea#3 suggested but I see you had already changed that. The line that runs from the rad to the over flow tank (And the part in the tank) can also effect coolants ability to return to rad. Check it for cracks, blockages or loose fittings. If the system has a vaccum when opening the cap the afore mentioned should be the only possibilities for failure to function properly that I can think of.

Stumpy

The rad cap as sea#3 suggested but I see you had already changed that. The line that runs from the rad to the over flow tank (And the part in the tank) can also effect coolants ability to return to rad. Check it for cracks, blockages or loose fittings. If the system has a vaccum when opening the cap the afore mentioned should be the only possibilities for failure to function properly that I can think of.

Stumpy

 

Just because the cap is new doesn't mean it works properly. If it's an OEM Subaru cap, I'd be less suspicious, but that could still be the problem. I'd try replacing the cap again, and double checking that the hose to the overflow tank has no issues. You could try pouring coolant into the overflow tank with the radiator cap off and making sure that the coolant flows through the hose.

The system needs to draw a vacuum to pull the coolant from the recovery tank, a leak anywhere in the system even a very small one will act as a vent and pull in air. Check carefully for any leaks.

 

Just another thought

Gary

If it is under vacuum when you remove the cap, either the 'new' cap is bad or the line/dip hose associated with the o'flow tank is clogged. If the line were cracked, you may very well collect coolant in the o'flow that does not return, but there would not be a vacuum as air would likely be sucked in through the crack.

Or, if everything appears normal otherwise, you still have air pockets in the cooling system!

 

Try this: Drive, let cool for about an hour so the pressure drops, remove the rad cap, idle the engine until the rad fan starts running, then while the engine and fan are still running top off the coolant and screw the cap back in.

 

While you are waiting coolant will overspill as the engine heats, try to collect it in a cloth or pan beneath the car. Driving beforehand ensures that the engine isn't so cold that it takes ages for the fan to switch on.

Check the rubber gasket on the cap for splits or cracks. If there are any they will allow the coolant to flow out of the radiator when the coolant expands, but will not cause a seal and allow the coolant to be sucked back through the center of the cap.

Great advice here guys.

 

I have verified that the overflow tube is not clogged, and because I do have vacuum, I've got to suspect the new cap. I can't imagine how an air bubble would cause the symptoms I've got. Can anyone enlighen me?

 

If it stops raining tonight, I'll use setright's procedure to top the coolant again & see what happens from there. After that it's back to the Subie dealer for a (another) new cap.

 

Thanks to everyone!

 

-l

The expansion ratio for air is much larger than for water. Therefore a smallish bubble can cause very erratic level readings in the overflow tank, all according to ambient temperature.

I have a brand-new Fuji Subaru cap sitting here that's defective ... it happens.

 

The little button in the center wasn't sealing - so pressure wouldn't build and the system would boil.

 

Sounds like you have just the opposite problem (the little button isn't allowing any coolant return).

Had a burnt exhaust valve and when they did the fix i experienced the same problem..coolant filling the resevoir and not flowing back in... replaced rad cap no luck..turns out they used wrong head bolts and had a head gasget leak..they redid the gasket with the correct bolts and everything has been fine ever since.