This is a 97 Subaru Impreza OBS with a 2.2l engine.  Long story short, I've been chasing a case of excess coolant going into the overflow container and ultimately being lost followed by mild overheating if I don't top off the radiator each morning.  I first tried replacing the radiator cap with a new genuine Subaru cap but no change.  I tried flushing the cooling system but when I removed the lower hose to drain the old coolant/flush I found a large "dent" in the lower outlet of the radiator which was restricting flow.  I replaced the radiator but used a Autozone/Stant thermostat (shame on me) and the problem remained.  I tried running with the thermostat removed while waiting for a genuine Subi thermostat to arrive and that seemed good for a couple of days but now the problem is back (and it has been warmer lately).  At this point I started thinking head gasket.  I know the 2.2 isn't known for this but mine does have 230k miles on it.  Today I borrowed a block test kit (the "turkey baster" with the blue fluid) from Autozone and followed the directions.  I tested for 3+ minutes with the engine getting over 190 degrees and saw no change in the fluid suggesting that it's not a head gasket leak.  One other piece of evidence is that I've been noticing a buildup of "black crud" in the overflow tank but no obvious signs of oil in the coolant or coolant in the oil. 

 

So, my question is what is going on?  Is it possible that the head gasket could only be leaking under actual driving conditions but staying sealed at idle?  Could I really have gotten THAT much air into the system otherwise?  Any ideas?

 

 

Pushing coolant into the overflow is a pretty good sign of a blown Head gasket.

 

Take it for a drive and when it starts getting warm get out and look for bubbles in the overflow.

Pushing coolant into the overflow is a pretty good sign of a blown Head gasket.

 

Take it for a drive and when it starts getting warm get out and look for bubbles in the overflow.

 

When I warmed it up before the block test (got the temp up to around 170) I did get a few bubbles but not many.

Those chemical block tests don't work on these. Don't know exactly why, they just don't.

 

Head gaskets it is.

 

Do a leakdown test and you'll see bubbles flying out of the radiator when you pressurize the cylinder with the bad seal.

I tried some of the Blue Devil and "water glass" chemical based products which may be worth researching for mild issues.  One of them bought me some time, fall to summer.  Keep the receipts and get the refunds, or double refunds in the case of the K and W products.

One other possibilty, a failing water pump.

With 230 on the clock, it's due.

Do the leakdown test. If it passes, then it's the pump.

 

O.