Try Back flushing the heater core.
- Get a 5/8 inch female garden hose adapter. This can be purchased at just about any hardware store. Make sure it is the kind that slips inside the garden hose.
- With the engine cool disconnect the outlet hose of the heater core from the engine. You need to do this at the water pump connection.
- Insert the garden hose adapter into the end of the hose.
- Disconnect the heater core inlet hose from the engine connection. This splice is often where you normally connect to flush out the coolant system.
- Connect your garden hose up to the outlet hose and turn on the water. Have someone help you turn on and off the water as you flush out the heater core.
- Collect atleast the first 2 gallons of stuff you flush out and dispose of this according to your local EPA laws. After the first 2 gallons all you should get out of your heater core is dirty water.
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