Cooling Issues / Puzzle | Subaru Outback Forums

Hello everyone, I’ve lurked here for some time and I tried to find the answer to my particular situation, but I have reached the point where I need to ask for help from those more knowledgeable than myself.

Back in August I bought a 2008 Outback 2.5 SOHC auto (147,000mi) that was having cooling and overheat issues. The owners thought it might be the head gaskets. Then they parked it for 6 months before selling and never had a garage look at it. Anyway, the price was definitely right considering the condition of the vehicle.

After I towed it home I did a cold cooling system pressure test and found that the water pump was leaking. I decided to do the whole timing belt while I was at it since I didn’t have any maintenance history and replaced it with an Aisin one. While I was in there I also swapped the thermostat to a Subaru OEM and replaced the radiator cap with OEM. I filled the cooling system from the upper radiator hose, burped it properly (lots of 4k rpm revving) with the overflow funnel and everything was great. I then pressure tested the system and it went from 15 psi down to 12psi over the course of 12 hours, which I thought was pretty good.

After replacing some other problems with it, ball joints, a CV axle, etc. I got it out on the road and didn’t have any problems… until I did. It does great at highway speeds and driving at 30+, but stop and go for traffic lights or 15 MPH in my driveway and the needle goes straight up towards the red. While it is doing this the heat is either weak or not working, which indicates to me poor coolant flow to the heater core. And I’ve learned firsthand with this car that if coolant isn’t reaching the heater core the thermostat can’t open. If I put it in neutral and rev it up high I can feel the heat suddenly get hotter and the temp drops quickly down from the red. If I stop and let it idle it will go back up again. However, this only happens after a long (20+ minute drive). If I start it up in the driveway I can let it idle forever and it won’t be an issue.

So, what is happening? If the head gasket was bad, then why would this problem be intermittent and why would it go away if I revved the motor? To me it seems like an issue of inadequate coolant flow when idling or at low engine speeds, probably because of a blocked radiator or heater core. I took a garden hose and flushed the heater core and radiator for a long while and that didn’t help. It also seemed like it had decent flow, too. My cooling system can hold pressure overnight, which doesn’t indicate head gasket and I didn’t detect any exhaust gases in the radiator. But I do think the radiator is probably OEM. Would a heavily sedimented radiator with poor flow cause this? I don’t want to start throwing parts at it if I should just be pulling the engine instead. Also, I made a heater core bypass and it didn’t have issues during that time so maybe the combination of that and the radiator are the culprit? I recently removed the bypass because I live in eastern Maine and a functioning heater is a necessity.

Thanks for reading my rambling at this point. I guess what I am looking for is advice on whether or not I should continue replacing cooling system components because it could cause these symptoms. Head gasket replacement is always at the back of my mind but I don’t want to do it unless I really have to. In the meantime, I have still been driving it to work and just avoiding places where there is stop and go traffic.

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