Question about 1990 Subaru Legacy

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Heatere core replacement

Fan works, no heat. car is up to temp on gauge, no heat from heater core lines that enter firewall.

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Be careful you dont over heat soon. I posted this in a different web sight I hope it helps you.

Re: Overheating and blowing cold air
I have been a mechanic for 20 years. Your problem is the heater core is plugged. The cold water from the radiator blows directly at the thermostat keeping it closed even when you are overheating! BAD DESIGN!! If you look at the water flow charts in AllData you will understand. If you rev the motor to 3,000 rpm's the turbulent HOT water at the water pump impeller (near the thermostat) will reach the thermostat to open it and the car's temp will fall very fast to the correct temperature. If you remove the thermostat it will not overheat but will run way to cold. If you bypass (loop) the heater core it will not over heat.

The hot water that leaves the heater core returns to the engine at the thermostat. It is the only hot water introduced to the thermostat to keep it open and allow the water to flow from and to the radiator. I normally just back wash the heater core and get large chunks of rust flake out then its fixed. Back wash then froward wash repeatedly many times. Also flush out the motor so you don't re plug the heater core. I hope you have now blown the head gaskets or cracked a head yet. Make sure to run the proper coolant in the car so you don't promote more rust flakes. And bleed the air out as mentioned in detail by other posters.

Nobody knows this little secret. Not even Subaru mechanics at the dealer, no TSB (Technical Service Bulletins a special note after the fact to mechanics). After days and days of trouble shooting a 1993 Subaru Impreza (2.0?) I looked at the water flow chart and then it was clear what was happening. This phantom overheating problem stumps the best mechanics and sends many Subarus to the crusher.

Posted on Jan 26, 2010

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