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overheating puzzler/ 2007 forester


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I have a new radiator, replaced thermostat twice (thinking 1st one was sticking), fans are working. No evidence of head gasket leakage. Will run fine for 8 to 15 miles; temp will start rising- if on a road where I can floor it, temp will lower to normal. If I have to pull over, shut off and leave fans on, coolant bubbling in reservoir. Does not happen when idling. Pretty sure water pump was replaced w/ timing belt. Any ideas?

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How many miles on it? How long has it been overheating? 

 

That engine isn't known to have internal head gasket failures but its possible. 

 

What are outside temps when it does this? Should be getting a little chilly up in MA? 

Edited by Fairtax4me
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Probably headgaskets - but you can hope it needs an OEM tstat and radiator cap, and absolutely properly burped (no air at all, zero, none, in the cooling system).

 

Fairtax is right these internal headgasket failure symptoms aren't normal - but these engines will ocassionally have this issue if the headgaskets were previously replaced - so tell us the history of the car? 
How long have you owned it?
How long did you own it before symptoms arose?

Were the HG's ever replaced? 
Where did you buy it from?

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If the water pump and timing belt job was done fairly recently, agreed with above posters it may just be air in the system. These can be very tricky to purge the air out as the heater core is the highest point of the cooling system.

 

Though, if the work wasn't recently done I've run into an overheating issue (very very similar to the symptoms you've described)

 

In my cases (it happened on two different subarus) it turned out being a restricted heater core.

What happens is because the heater core is the by pass for the cooling system (by that I mean it's the only way coolant can circulate if the thermostat is closed) if the heater core comes partially restricted when the car is at low RPMs the water pump isn't spinning fast enough to pull enough warm coolant through the heater core. And then what happens is the thermostat will close because the lack of warm coolant coming from the heater core. 

 

When the thermostat closes exactly what your describing will happen. The temperature will rising up. And then if you floor it (or really just need to have higher RPMs) the temp will return to normal because the thermostat is now opening when floored.

 

Few ways to tell if the heater core is partially restricted.

1) Ideally, if you had a male to male hose adapter. Disconnect the heater core from the system completely. If it fixes your issue. There you go.

2) Disconnect on of the hoses from the heater core, (preferably when the coolant is warm) and blow (physically like with your mouth) through the core. It should have very minimal resistance blowing through it.

Make sure you have the know-hows to properly purge the air from the system before doing these tests though

 

If you have access to a scan tool. Monitor the coolant temperature when driving. If after driving for a bit the temperature suddenly skyrockets like 15 degrees in 4-5 seconds, your thermostat is closing. That's the only way the coolant temp can rise that quickly.

You can also see if keeping the car in 2nd or 3rd helps. For example, keep the RPMs in the 4k range. This might help prevent the thermostat from closing. 

 

As for the bubbling of the coolant. I agree, bubbling does usually mean HGs, but first of all this car isn't known for internal HG failures. Just oil seeps.

 

And also, bubbling in the reservoir is hard to tell. I had "bubbling" of coolant on one of the cars I worked on with the bad heater core. It kept burping air from the system. I could never seem to get all the air out of the system when it had a bad heater core. I've done it dozens of times before but there would always be small bubbling coming out when purging the air.

(And for the record, it was not a HG issue because I had just replaced the HG. The HG actually failed because the heater core plugged up, over heated the car a little and blew the HGs externally)

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