Question about 1993 Subaru Impreza
It sounds very much like you might have an air lock in the coolant system. If this is the case then you need to bleed the heater matrix. Lift the bonnet and look at the back of the engine . You will see two rubber hoses going into the bulkhead which is the metal panel that separates the engine compartment from the passenger compartment. Both rubber hoses will have arrows printed on them. One pointing into and one pointing out of the passenger compartment ( i.e in and out). These are the hoses that attach to the heater matrix which is like a little radiator within the car behind the dash which receives air from the heater blower . The blower forces air through the matrix fins heating the air up as it passes through. Start the car with the heater temperature switched to full on hot. Loosen the hose clip on the hose with the arrow pointing into the engine bay and NOT pointing into the passenger compartment. Loosen the rubber hose. It will be tight on there so it will need a good wiggle to loosen it. Now this bit is important. DO NOT completely disconnect this hose. You need to partially disconnect it so when the engine is running the water spurts out at high pressure from a small area of the metal pipe and hose. It is the same principle as squeezing a garden hose pipe increases the pressure in the hose and the water spurts out further. This is what is needed on the heater hose so the flow is increased and pushes any air in the system out that may be locked in the coolant system and or heater matrix. If air is trapped in the heater matrix you will see or feel it rush out of the restricted hole you make by taking the hose partially off. Once you feel a steady flow of water coming out push the hose straight back on the metal pipe as quick as you can . Turn the engine off and do up the hose clip nice and tight. You should now have a proper water flow through the heater matrix and got rid of any air in the coolant system. It is important to get rid of any air in the system . If air is trapped then this part of the system is not getting a proper coolant flow and will cause hotspots which can blow a head gasket if the air lock is in the heads or engine block. You can also sqeeze the radiator top hose with the radiator cap removed to clear an air lock at the top of the engine. If air is present you will see it bubble up in the radiator when you compress the top hose. This is best done when the engine coolant is not hot. You may scald yourself otherwise so be careful. Following the steps I have described above should stop the boiling over in the expansion tank as well and you should have a working heater.
If you still find problems with boiling over ect then it may be you have a blown head gasket increasing pressure in the cooling system and sucking in air as well. Even if there is no sign of oil in the coolant the head gasket could still be blown between a cylinder and water channel around the engine block. That is an engine out job and is not too be attempted unless you have experience .
Posted on Mar 20, 2009
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