Figure out which pipe is the inlet and which is the outlet.
Remove the hoses from the heater core pipes - on most British cars you can do this from under the hood - then connect a garden hose to the pipe that is the outlet or return line and run water through the heater core. This reverse water flow will often dislodge plugged passages.
Let the water run and gradually increase the volume by opening the faucet. Donít start with the faucet on wide open since you could damage the heater core if you encountered a blockage.
Once the water is flowing, gradually increase the flow and when you have good pressure, leave it run for 15 or 20 minutes.
Reconnect the hoses and be sure to check hose and hose clamp condition.
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