Thanks for the replies. I saw another 'couple of posts about experiencing the same thing and one turned out to be the thermostat gasket and other was the plastic fitting on top of the radiator for the hose to the expansion tank. I'll have to check after a long run.

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1993 320TE M104
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past:

1983 230E W123 M102
1994 E300D S124 OM606 (x2)
1967 250SE W108 M129
1972 280se 3.5 W108 M116
1980 280SE W116 M110
1980 350SE W116 M116
1992 300E W124 M103
1994 E280 W124 M104
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"music and women I cannot but give way to, whatever my business" -Pepys