Here's an excerpt from the thread I was thinking of on burping. I would try this, then maybe a good reverse flush, and finally a radiator "rodding."

Don't see much point in putting in a used radiator that may have the same problems.

Here's the excerpt:

The 4.5 is notorious for getting air pockets if the cooling system is opened. So, what you do when refilling the coolant is to disconnect the upper radiator hose from the radiator and pour the 50/50 mix through the hose. Try to hold the hose as high as possible (you'll hear it burping) and gently rock the car to "shake" it a little to further purge it. Just keep adding coolant until it flows back out of the upper hose. Reattach hose and top up, start car and put the defrost on. You'll probably see the auxiliary fan kick on if there is still an air pocket. Just keep an eye on the temperature gauge and feel the air coming out of the vents to make sure the t-stat has opened. One other note, the older sixes are also known for air pockets, the thing to do with them is to fill the coolant in, run the car and if and when the temp gauge begins to rise above the normal mark, shut it off and let it bleed itself for 15-20 minutes. Check the level in the radiator and top up if necessary. Start it back up and make sure the thermostat opens. Repeat again if still no heat.



__________________
Rgds,
Aaron Greenberg
owner, MB Classic, Inc.
Service and Restoration of Vintage Mercedes-Benz Automobiles

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe