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This is a much later post, nearly 10 years later, but did any light come of this?
I'm having the same problem, top pipe is hot, bottom pipe is cold. Thermostat is OK, and a good drive gets the engine up to operating temps, but the bottom part (like bottom 3/4) of the radiator is still cold, and the bottom pipe is cold.
Coolant doesn't seem to be flowing through it. Car isn't overheating. Wondering about airlocks etc.
Bewildered, and going to pull out the water pump to inspect.
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that sounds about normal . top hose has hot coolant from motor to radiator , bottom hose has cooled coolant from radiator going to the motor , just keep an eye on the over flow bottle and make sure it is filled up to to the minimum mark when the car is cold . the pressurised bottle beside it should be full .
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If in doubt park uphill some where steep ,cut the bottom off a 2lrt Coke bottle
tape that to the top of the radiator outlet and fill with coolant, well a bit of coolant
youll see any or all air disappear.
My mechanic told me they were self bleeding