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I bought a 2008 impreza wrx hatch about a month ago and just recently started having over heating problems. I’m not not mechanically challenged but this one baffles me. When ever I am driving the car it will randomly (within about 10 mins usually) it will start over heating. At first it seemed like a faulty thermostat or that the system was not bled correctly so I replaced the thermostat and bled/burped the system correctly and sure enough it still overheated the fans kick on so I crossed that out but then the car started blowing off coolant all over the engine bay from right around the Reservoir I then noticed that there was white smoke coming from the reservoir when this happened and it would also come out of the radiator I felt both of the lines and the top one was very hot but the bottom one was cold so that led me to believe there was air in the system still because I had just replaced the thermostat. I decided to run a test and run the car without a thermostat and sure enough there was no heating issues even after running for a long time it stayed right at running temp nothing less nothing more. So that scratched off the radiator being faulty because if it was the car would over heat with or without the thermostat. I have also notice that the reservoir with the thermostat in overflows like all the way to the top. My thought is that I have a minor head gasket leak, I believe that it is releasing a very small amount of exhaust gasses which will gather around the thermostat stopping it from opening like if air was in the system. I’m not having white smoke coming out of the exhaust or milky oil so I think it is very minor but still causing over heating with the thermostat in could anyone give me their input and tell me if I’m diagnosing this wrong or if they have heard of this same problem?
 

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Consider examining the spark plugs. If your suspicion in correct then by default one of your chambers is the incorrect effective volume, meaning you have an incorrect mixture escaping into the cooling system. Have you pressure tested the cooling system? Have you considered paying to get exhaust gasses detected in the coolant, or vice-versa?
 

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Pressure testing the cooling system is a really good step.

Are you making sure the weep hole on the thermostat is at the top most position to let air pass? How long are you burping the system? When i did my wrx after doing some work on it, i was probably nearing a half hour before all the bubbling stopped. Would have been quicker if I used the reservoir (no idea if I spelled that properly, what a stupid word) near the turbo.

You can also drain the oil, look around the cap, and even compression and leakdown test the cylinders to see if it's a possible head gasket.
 

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I bought a 2008 impreza wrx hatch about a month ago and just recently started having over heating problems. I’m not not mechanically challenged but this one baffles me. When ever I am driving the car it will randomly (within about 10 mins usually) it will start over heating. At first it seemed like a faulty thermostat or that the system was not bled correctly so I replaced the thermostat and bled/burped the system correctly and sure enough it still overheated the fans kick on so I crossed that out but then the car started blowing off coolant all over the engine bay from right around the Reservoir I then noticed that there was white smoke coming from the reservoir when this happened and it would also come out of the radiator I felt both of the lines and the top one was very hot but the bottom one was cold so that led me to believe there was air in the system still because I had just replaced the thermostat. I decided to run a test and run the car without a thermostat and sure enough there was no heating issues even after running for a long time it stayed right at running temp nothing less nothing more. So that scratched off the radiator being faulty because if it was the car would over heat with or without the thermostat. I have also notice that the reservoir with the thermostat in overflows like all the way to the top. My thought is that I have a minor head gasket leak, I believe that it is releasing a very small amount of exhaust gasses which will gather around the thermostat stopping it from opening like if air was in the system. I’m not having white smoke coming out of the exhaust or milky oil so I think it is very minor but still causing over heating with the thermostat in could anyone give me their input and tell me if I’m diagnosing this wrong or if they have heard of this same problem?
Bear in mind fans are there only to cool the water at idle...at driving speed the forced air does the cooling. That coolant fills the reservoir... or overflows it sounds more a cooling system issue than head or cracked block issues. Is your oil pressure good....it also cools the engine. (On high overheat the oil can bubble.)

Your car may have had overheating problems and had some sort of crack filler put in even blocking some water passages can you contact the vendor's service people (if serviced and ask?)...however ...better than asking us at a distance is taking it to radiator specialist (e.g. "Red Devil" if in Qld) rather than a mechanic.

I had similar, intermittent overheating problems recently with an L300 van even after changing thermostat, hoses and coolant. Without cap, temperature ran high, but did not overheat. I noted radiator core heating not same top and bottom.

The new radiator fixed the problem....

{Radiator itself and hoses are my first port of call.A partly blocked radiator may not have whole sections completely blocked but reduced to inadequate flow. Suggestions such as pressure testing and reading the spark plugs are sensible. I don't know where you are but my first port of call would be radiator specialist.

Radiator should be a fairly even temperature throughout with car up to temperature. . I doubt generated gas could block the thermostat from opening but if water is not getting around the thermostat it will not open reliably if at all. The issue of air in the system is one aspect two others are radiator hoses which "suck in" during driving then blocking or reducing water flow. Cylinder temperature rises from faulty timing, lean fuel, exhaust blockage, overheating water-cooled turbo...but check radiator 'number one'...but have it done by a professional.)
 
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