I've flushed & filled, temporarly removed the thermostat. It cools OK at idle, but at highway speed, I need to turn on the defrost to keep it cool enough. The car has 204K miles, and no service records. Could the water pump be worn. I've noticed no foam in the oil, rad, etc.

dpoole@ce.mediaone.net

The cause is a faulty radiator. Mercedes radiators have square tubes in them that are not smooth inside. They will build up an area of blockage if the coolant isn't flushed aprox every 2-3 years. Once blocked, it has to be replaced. YOU ALSO SHOULD REPLACE THE THERMOSTAT WITH A NEW ONE WHEN YOU INSTALL THE RADIATOR.
We sell the radiators on the partshop!!
SEE BELOW

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